1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
|
<html>
<head>
<title>pcre2pattern specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcre2pattern man page</h1>
<p>
Return to the <a href="index.html">PCRE2 index page</a>.
</p>
<p>
This page is part of the PCRE2 HTML documentation. It was generated
automatically from the original man page. If there is any nonsense in it,
please consult the man page, in case the conversion went wrong.
<br>
<ul>
<li><a name="TOC1" href="#SEC1">PCRE2 REGULAR EXPRESSION DETAILS</a>
<li><a name="TOC2" href="#SEC2">SPECIAL START-OF-PATTERN ITEMS</a>
<li><a name="TOC3" href="#SEC3">EBCDIC CHARACTER CODES</a>
<li><a name="TOC4" href="#SEC4">CHARACTERS AND METACHARACTERS</a>
<li><a name="TOC5" href="#SEC5">BACKSLASH</a>
<li><a name="TOC6" href="#SEC6">CIRCUMFLEX AND DOLLAR</a>
<li><a name="TOC7" href="#SEC7">FULL STOP (PERIOD, DOT) AND \N</a>
<li><a name="TOC8" href="#SEC8">MATCHING A SINGLE CODE UNIT</a>
<li><a name="TOC9" href="#SEC9">SQUARE BRACKETS AND CHARACTER CLASSES</a>
<li><a name="TOC10" href="#SEC10">POSIX CHARACTER CLASSES</a>
<li><a name="TOC11" href="#SEC11">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a>
<li><a name="TOC12" href="#SEC12">VERTICAL BAR</a>
<li><a name="TOC13" href="#SEC13">INTERNAL OPTION SETTING</a>
<li><a name="TOC14" href="#SEC14">GROUPS</a>
<li><a name="TOC15" href="#SEC15">DUPLICATE GROUP NUMBERS</a>
<li><a name="TOC16" href="#SEC16">NAMED CAPTURE GROUPS</a>
<li><a name="TOC17" href="#SEC17">REPETITION</a>
<li><a name="TOC18" href="#SEC18">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a>
<li><a name="TOC19" href="#SEC19">BACKREFERENCES</a>
<li><a name="TOC20" href="#SEC20">ASSERTIONS</a>
<li><a name="TOC21" href="#SEC21">NON-ATOMIC ASSERTIONS</a>
<li><a name="TOC22" href="#SEC22">SCRIPT RUNS</a>
<li><a name="TOC23" href="#SEC23">CONDITIONAL GROUPS</a>
<li><a name="TOC24" href="#SEC24">COMMENTS</a>
<li><a name="TOC25" href="#SEC25">RECURSIVE PATTERNS</a>
<li><a name="TOC26" href="#SEC26">GROUPS AS SUBROUTINES</a>
<li><a name="TOC27" href="#SEC27">ONIGURUMA SUBROUTINE SYNTAX</a>
<li><a name="TOC28" href="#SEC28">CALLOUTS</a>
<li><a name="TOC29" href="#SEC29">BACKTRACKING CONTROL</a>
<li><a name="TOC30" href="#SEC30">SEE ALSO</a>
<li><a name="TOC31" href="#SEC31">AUTHOR</a>
<li><a name="TOC32" href="#SEC32">REVISION</a>
</ul>
<br><a name="SEC1" href="#TOC1">PCRE2 REGULAR EXPRESSION DETAILS</a><br>
<P>
The syntax and semantics of the regular expressions that are supported by PCRE2
are described in detail below. There is a quick-reference syntax summary in the
<a href="pcre2syntax.html"><b>pcre2syntax</b></a>
page. PCRE2 tries to match Perl syntax and semantics as closely as it can.
PCRE2 also supports some alternative regular expression syntax (which does not
conflict with the Perl syntax) in order to provide some compatibility with
regular expressions in Python, .NET, and Oniguruma.
</P>
<P>
Perl's regular expressions are described in its own documentation, and regular
expressions in general are covered in a number of books, some of which have
copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published
by O'Reilly, covers regular expressions in great detail. This description of
PCRE2's regular expressions is intended as reference material.
</P>
<P>
This document discusses the regular expression patterns that are supported by
PCRE2 when its main matching function, <b>pcre2_match()</b>, is used. PCRE2 also
has an alternative matching function, <b>pcre2_dfa_match()</b>, which matches
using a different algorithm that is not Perl-compatible. Some of the features
discussed below are not available when DFA matching is used. The advantages and
disadvantages of the alternative function, and how it differs from the normal
function, are discussed in the
<a href="pcre2matching.html"><b>pcre2matching</b></a>
page.
</P>
<br><a name="SEC2" href="#TOC1">SPECIAL START-OF-PATTERN ITEMS</a><br>
<P>
A number of options that can be passed to <b>pcre2_compile()</b> can also be set
by special items at the start of a pattern. These are not Perl-compatible, but
are provided to make these options accessible to pattern writers who are not
able to change the program that processes the pattern. Any number of these
items may appear, but they must all be together right at the start of the
pattern string, and the letters must be in upper case.
</P>
<br><b>
UTF support
</b><br>
<P>
In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as
single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be
specified for the 32-bit library, in which case it constrains the character
values to valid Unicode code points. To process UTF strings, PCRE2 must be
built to include Unicode support (which is the default). When using UTF strings
you must either call the compiling function with one or both of the PCRE2_UTF
or PCRE2_MATCH_INVALID_UTF options, or the pattern must start with the special
sequence (*UTF), which is equivalent to setting the relevant PCRE2_UTF. How
setting a UTF mode affects pattern matching is mentioned in several places
below. There is also a summary of features in the
<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
page.
</P>
<P>
Some applications that allow their users to supply patterns may wish to
restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF
option is passed to <b>pcre2_compile()</b>, (*UTF) is not allowed, and its
appearance in a pattern causes an error.
</P>
<br><b>
Unicode property support
</b><br>
<P>
Another special sequence that may appear at the start of a pattern is (*UCP).
This has the same effect as setting the PCRE2_UCP option: it causes sequences
such as \d and \w to use Unicode properties to determine character types,
instead of recognizing only characters with codes less than 256 via a lookup
table. If also causes upper/lower casing operations to use Unicode properties
for characters with code points greater than 127, even when UTF is not set.
</P>
<P>
Some applications that allow their users to supply patterns may wish to
restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to
<b>pcre2_compile()</b>, (*UCP) is not allowed, and its appearance in a pattern
causes an error.
</P>
<br><b>
Locking out empty string matching
</b><br>
<P>
Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect
as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever
matching function is subsequently called to match the pattern. These options
lock out the matching of empty strings, either entirely, or only at the start
of the subject.
</P>
<br><b>
Disabling auto-possessification
</b><br>
<P>
If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers
possessive when what follows cannot match the repeated item. For example, by
default a+b is treated as a++b. For more details, see the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
<br><b>
Disabling start-up optimizations
</b><br>
<P>
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly
reaching "no match" results. For more details, see the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
<br><b>
Disabling automatic anchoring
</b><br>
<P>
If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as
setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that
apply to patterns whose top-level branches all start with .* (match any number
of arbitrary characters). For more details, see the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
<br><b>
Disabling JIT compilation
</b><br>
<P>
If a pattern that starts with (*NO_JIT) is successfully compiled, an attempt by
the application to apply the JIT optimization by calling
<b>pcre2_jit_compile()</b> is ignored.
</P>
<br><b>
Setting match resource limits
</b><br>
<P>
The <b>pcre2_match()</b> function contains a counter that is incremented every
time it goes round its main loop. The caller of <b>pcre2_match()</b> can set a
limit on this counter, which therefore limits the amount of computing resource
used for a match. The maximum depth of nested backtracking can also be limited;
this indirectly restricts the amount of heap memory that is used, but there is
also an explicit memory limit that can be set.
</P>
<P>
These facilities are provided to catch runaway matches that are provoked by
patterns with huge matching trees. A common example is a pattern with nested
unlimited repeats applied to a long string that does not match. When one of
these limits is reached, <b>pcre2_match()</b> gives an error return. The limits
can also be set by items at the start of the pattern of the form
<pre>
(*LIMIT_HEAP=d)
(*LIMIT_MATCH=d)
(*LIMIT_DEPTH=d)
</pre>
where d is any number of decimal digits. However, the value of the setting must
be less than the value set (or defaulted) by the caller of <b>pcre2_match()</b>
for it to have any effect. In other words, the pattern writer can lower the
limits set by the programmer, but not raise them. If there is more than one
setting of one of these limits, the lower value is used. The heap limit is
specified in kibibytes (units of 1024 bytes).
</P>
<P>
Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This name is
still recognized for backwards compatibility.
</P>
<P>
The heap limit applies only when the <b>pcre2_match()</b> or
<b>pcre2_dfa_match()</b> interpreters are used for matching. It does not apply
to JIT. The match limit is used (but in a different way) when JIT is being
used, or when <b>pcre2_dfa_match()</b> is called, to limit computing resource
usage by those matching functions. The depth limit is ignored by JIT but is
relevant for DFA matching, which uses function recursion for recursions within
the pattern and for lookaround assertions and atomic groups. In this case, the
depth limit controls the depth of such recursion.
<a name="newlines"></a></P>
<br><b>
Newline conventions
</b><br>
<P>
PCRE2 supports six different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, any
Unicode newline sequence, or the NUL character (binary zero). The
<a href="pcre2api.html"><b>pcre2api</b></a>
page has
<a href="pcre2api.html#newlines">further discussion</a>
about newlines, and shows how to set the newline convention when calling
<b>pcre2_compile()</b>.
</P>
<P>
It is also possible to specify a newline convention by starting a pattern
string with one of the following sequences:
<pre>
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
(*NUL) the NUL character (binary zero)
</pre>
These override the default and the options given to the compiling function. For
example, on a Unix system where LF is the default newline sequence, the pattern
<pre>
(*CR)a.b
</pre>
changes the convention to CR. That pattern matches "a\nb" because LF is no
longer a newline. If more than one of these settings is present, the last one
is used.
</P>
<P>
The newline convention affects where the circumflex and dollar assertions are
true. It also affects the interpretation of the dot metacharacter when
PCRE2_DOTALL is not set, and the behaviour of \N when not followed by an
opening brace. However, it does not affect what the \R escape sequence
matches. By default, this is any Unicode newline sequence, for Perl
compatibility. However, this can be changed; see the next section and the
description of \R in the section entitled
<a href="#newlineseq">"Newline sequences"</a>
below. A change of \R setting can be combined with a change of newline
convention.
</P>
<br><b>
Specifying what \R matches
</b><br>
<P>
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
at compile time. This effect can also be achieved by starting a pattern with
(*BSR_ANYCRLF). For completeness, (*BSR_UNICODE) is also recognized,
corresponding to PCRE2_BSR_UNICODE.
</P>
<br><a name="SEC3" href="#TOC1">EBCDIC CHARACTER CODES</a><br>
<P>
PCRE2 can be compiled to run in an environment that uses EBCDIC as its
character code instead of ASCII or Unicode (typically a mainframe system). In
the sections below, character code values are ASCII or Unicode; in an EBCDIC
environment these characters may have different code values, and there are no
code points greater than 255.
</P>
<br><a name="SEC4" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
<P>
A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern
<pre>
The quick brown fox
</pre>
matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE2_CASELESS option or (?i) within the
pattern), letters are matched independently of case. Note that there are two
ASCII characters, K and S, that, in addition to their lower case ASCII
equivalents, are case-equivalent with Unicode U+212A (Kelvin sign) and U+017F
(long S) respectively when either PCRE2_UTF or PCRE2_UCP is set.
</P>
<P>
The power of regular expressions comes from the ability to include wild cards,
character classes, alternatives, and repetitions in the pattern. These are
encoded in the pattern by the use of <i>metacharacters</i>, which do not stand
for themselves but instead are interpreted in some special way.
</P>
<P>
There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the metacharacters
are as follows:
<pre>
\ general escape character with several uses
^ assert start of string (or line, in multiline mode)
$ assert end of string (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start group or control verb
) end group or control verb
* 0 or more quantifier
+ 1 or more quantifier; also "possessive quantifier"
? 0 or 1 quantifier; also quantifier minimizer
{ start min/max quantifier
</pre>
Part of a pattern that is in square brackets is called a "character class". In
a character class the only metacharacters are:
<pre>
\ general escape character
^ negate the class, but only if the first character
- indicates character range
[ POSIX character class (if followed by POSIX syntax)
] terminates the character class
</pre>
If a pattern is compiled with the PCRE2_EXTENDED option, most white space in
the pattern, other than in a character class, and characters between a #
outside a character class and the next newline, inclusive, are ignored. An
escaping backslash can be used to include a white space or a # character as
part of the pattern. If the PCRE2_EXTENDED_MORE option is set, the same
applies, but in addition unescaped space and horizontal tab characters are
ignored inside a character class. Note: only these two characters are ignored,
not the full set of pattern white space characters that are ignored outside a
character class. Option settings can be changed within a pattern; see the
section entitled
<a href="#internaloptions">"Internal Option Setting"</a>
below.
</P>
<P>
The following sections describe the use of each of the metacharacters.
</P>
<br><a name="SEC5" href="#TOC1">BACKSLASH</a><br>
<P>
The backslash character has several uses. Firstly, if it is followed by a
character that is not a digit or a letter, it takes away any special meaning
that character may have. This use of backslash as an escape character applies
both inside and outside character classes.
</P>
<P>
For example, if you want to match a * character, you must write \* in the
pattern. This escaping action applies whether or not the following character
would otherwise be interpreted as a metacharacter, so it is always safe to
precede a non-alphanumeric with backslash to specify that it stands for itself.
In particular, if you want to match a backslash, you write \\.
</P>
<P>
Only ASCII digits and letters have any special meaning after a backslash. All
other characters (in particular, those whose code points are greater than 127)
are treated as literals.
</P>
<P>
If you want to treat all characters in a sequence as literals, you can do so by
putting them between \Q and \E. This is different from Perl in that $ and @
are handled as literals in \Q...\E sequences in PCRE2, whereas in Perl, $ and
@ cause variable interpolation. Also, Perl does "double-quotish backslash
interpolation" on any backslashes between \Q and \E which, its documentation
says, "may lead to confusing results". PCRE2 treats a backslash between \Q and
\E just like any other character. Note the following examples:
<pre>
Pattern PCRE2 matches Perl matches
\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
\Qabc\$xyz\E abc\$xyz abc\$xyz
\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
\QA\B\E A\B A\B
\Q\\E \ \\E
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
by \E later in the pattern, the literal interpretation continues to the end of
the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
a character class, this causes an error, because the character class is not
terminated by a closing square bracket.
<a name="digitsafterbackslash"></a></P>
<br><b>
Non-printing characters
</b><br>
<P>
A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters in a pattern, but when a pattern is being prepared by
text editing, it is often easier to use one of the following escape sequences
instead of the binary character it represents. In an ASCII or Unicode
environment, these escapes are as follows:
<pre>
\a alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any printable ASCII character
\e escape (hex 1B)
\f form feed (hex 0C)
\n linefeed (hex 0A)
\r carriage return (hex 0D) (but see below)
\t tab (hex 09)
\0dd character with octal code 0dd
\ddd character with octal code ddd, or backreference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
\N{U+hhh..} character with Unicode hex code point hhh..
</pre>
By default, after \x that is not followed by {, from zero to two hexadecimal
digits are read (letters can be in upper or lower case). Any number of
hexadecimal digits may appear between \x{ and }. If a character other than a
hexadecimal digit appears between \x{ and }, or if there is no terminating },
an error occurs.
</P>
<P>
Characters whose code points are less than 256 can be defined by either of the
two syntaxes for \x or by an octal sequence. There is no difference in the way
they are handled. For example, \xdc is exactly the same as \x{dc} or \334.
However, using the braced versions does make such sequences easier to read.
</P>
<P>
Support is available for some ECMAScript (aka JavaScript) escape sequences via
two compile-time options. If PCRE2_ALT_BSUX is set, the sequence \x followed
by { is not recognized. Only if \x is followed by two hexadecimal digits is it
recognized as a character escape. Otherwise it is interpreted as a literal "x"
character. In this mode, support for code points greater than 256 is provided
by \u, which must be followed by four hexadecimal digits; otherwise it is
interpreted as a literal "u" character.
</P>
<P>
PCRE2_EXTRA_ALT_BSUX has the same effect as PCRE2_ALT_BSUX and, in addition,
\u{hhh..} is recognized as the character specified by hexadecimal code point.
There may be any number of hexadecimal digits. This syntax is from ECMAScript
6.
</P>
<P>
The \N{U+hhh..} escape sequence is recognized only when PCRE2 is operating in
UTF mode. Perl also uses \N{name} to specify characters by Unicode name; PCRE2
does not support this. Note that when \N is not followed by an opening brace
(curly bracket) it has an entirely different meaning, matching any character
that is not a newline.
</P>
<P>
There are some legacy applications where the escape sequence \r is expected to
match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option is set, \r in a
pattern is converted to \n so that it matches a LF (linefeed) instead of a CR
(carriage return) character.
</P>
<P>
The precise effect of \cx on ASCII characters is as follows: if x is a lower
case letter, it is converted to upper case. Then bit 6 of the character (hex
40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A (A is 41, Z is 5A),
but \c{ becomes hex 3B ({ is 7B), and \c; becomes hex 7B (; is 3B). If the
code unit following \c has a value less than 32 or greater than 126, a
compile-time error occurs.
</P>
<P>
When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..} is not supported. \a, \e,
\f, \n, \r, and \t generate the appropriate EBCDIC code values. The \c
escape is processed as specified for Perl in the <b>perlebcdic</b> document. The
only characters that are allowed after \c are A-Z, a-z, or one of @, [, \, ],
^, _, or ?. Any other character provokes a compile-time error. The sequence
\c@ encodes character code 0; after \c the letters (in either case) encode
characters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31
(hex 1B to hex 1F), and \c? becomes either 255 (hex FF) or 95 (hex 5F).
</P>
<P>
Thus, apart from \c?, these escapes generate the same character code values as
they do in an ASCII environment, though the meanings of the values mostly
differ. For example, \cG always generates code value 7, which is BEL in ASCII
but DEL in EBCDIC.
</P>
<P>
The sequence \c? generates DEL (127, hex 7F) in an ASCII environment, but
because 127 is not a control character in EBCDIC, Perl makes it generate the
APC character. Unfortunately, there are several variants of EBCDIC. In most of
them the APC character has the value 255 (hex FF), but in the one Perl calls
POSIX-BC its value is 95 (hex 5F). If certain other characters have POSIX-BC
values, PCRE2 makes \c? generate 95; otherwise it generates 255.
</P>
<P>
After \0 up to two further octal digits are read. If there are fewer than two
digits, just those that are present are used. Thus the sequence \0\x\015
specifies two binary zeros followed by a CR character (code value 13). Make
sure you supply two digits after the initial zero if the pattern character that
follows is itself an octal digit.
</P>
<P>
The escape \o must be followed by a sequence of octal digits, enclosed in
braces. An error occurs if this is not the case. This escape is a recent
addition to Perl; it provides way of specifying character code points as octal
numbers greater than 0777, and it also allows octal numbers and backreferences
to be unambiguously specified.
</P>
<P>
For greater clarity and unambiguity, it is best to avoid following \ by a
digit greater than zero. Instead, use \o{} or \x{} to specify numerical
character code points, and \g{} to specify backreferences. The following
paragraphs describe the old, ambiguous syntax.
</P>
<P>
The handling of a backslash followed by a digit other than 0 is complicated,
and Perl has changed over time, causing PCRE2 also to change.
</P>
<P>
Outside a character class, PCRE2 reads the digit and any following digits as a
decimal number. If the number is less than 10, begins with the digit 8 or 9, or
if there are at least that many previous capture groups in the expression, the
entire sequence is taken as a <i>backreference</i>. A description of how this
works is given
<a href="#backreferences">later,</a>
following the discussion of
<a href="#group">parenthesized groups.</a>
Otherwise, up to three octal digits are read to form a character code.
</P>
<P>
Inside a character class, PCRE2 handles \8 and \9 as the literal characters
"8" and "9", and otherwise reads up to three octal digits following the
backslash, using them to generate a data character. Any subsequent digits stand
for themselves. For example, outside a character class:
<pre>
\040 is another way of writing an ASCII space
\40 is the same, provided there are fewer than 40 previous capture groups
\7 is always a backreference
\11 might be a backreference, or another way of writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 might be a backreference, otherwise the character with octal code 113
\377 might be a backreference, otherwise the value 255 (decimal)
\81 is always a backreference .sp
</pre>
Note that octal values of 100 or greater that are specified using this syntax
must not be introduced by a leading zero, because no more than three octal
digits are ever read.
</P>
<br><b>
Constraints on character values
</b><br>
<P>
Characters that are specified using octal or hexadecimal numbers are
limited to certain values, as follows:
<pre>
8-bit non-UTF mode no greater than 0xff
16-bit non-UTF mode no greater than 0xffff
32-bit non-UTF mode no greater than 0xffffffff
All UTF modes no greater than 0x10ffff and a valid code point
</pre>
Invalid Unicode code points are all those in the range 0xd800 to 0xdfff (the
so-called "surrogate" code points). The check for these can be disabled by the
caller of <b>pcre2_compile()</b> by setting the option
PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES. However, this is possible only in UTF-8
and UTF-32 modes, because these values are not representable in UTF-16.
</P>
<br><b>
Escape sequences in character classes
</b><br>
<P>
All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, \b is
interpreted as the backspace character (hex 08).
</P>
<P>
When not followed by an opening brace, \N is not allowed in a character class.
\B, \R, and \X are not special inside a character class. Like other
unrecognized alphabetic escape sequences, they cause an error. Outside a
character class, these sequences have different meanings.
</P>
<br><b>
Unsupported escape sequences
</b><br>
<P>
In Perl, the sequences \F, \l, \L, \u, and \U are recognized by its string
handler and used to modify the case of following characters. By default, PCRE2
does not support these escape sequences in patterns. However, if either of the
PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX options is set, \U matches a "U"
character, and \u can be used to define a character by code point, as
described above.
</P>
<br><b>
Absolute and relative backreferences
</b><br>
<P>
The sequence \g followed by a signed or unsigned number, optionally enclosed
in braces, is an absolute or relative backreference. A named backreference
can be coded as \g{name}. Backreferences are discussed
<a href="#backreferences">later,</a>
following the discussion of
<a href="#group">parenthesized groups.</a>
</P>
<br><b>
Absolute and relative subroutine calls
</b><br>
<P>
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a capture group as a subroutine. Details are discussed
<a href="#onigurumasubroutines">later.</a>
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
synonymous. The former is a backreference; the latter is a
<a href="#groupsassubroutines">subroutine</a>
call.
<a name="genericchartypes"></a></P>
<br><b>
Generic character types
</b><br>
<P>
Another use of backslash is for specifying generic character types:
<pre>
\d any decimal digit
\D any character that is not a decimal digit
\h any horizontal white space character
\H any character that is not a horizontal white space character
\N any character that is not a newline
\s any white space character
\S any character that is not a white space character
\v any vertical white space character
\V any character that is not a vertical white space character
\w any "word" character
\W any "non-word" character
</pre>
The \N escape sequence has the same meaning as
<a href="#fullstopdot">the "." metacharacter</a>
when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not change the
meaning of \N. Note that when \N is followed by an opening brace it has a
different meaning. See the section entitled
<a href="#digitsafterbackslash">"Non-printing characters"</a>
above for details. Perl also uses \N{name} to specify characters by Unicode
name; PCRE2 does not support this.
</P>
<P>
Each pair of lower and upper case escape sequences partitions the complete set
of characters into two disjoint sets. Any given character matches one, and only
one, of each pair. The sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, because
there is no character to match.
</P>
<P>
The default \s characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32), which are defined as white space in the "C" locale. This list may
vary if locale-specific matching is taking place. For example, in some locales
the "non-breaking space" character (\xA0) is recognized as white space, and in
others the VT character is not.
</P>
<P>
A "word" character is an underscore or any character that is a letter or digit.
By default, the definition of letters and digits is controlled by PCRE2's
low-valued character tables, and may vary if locale-specific matching is taking
place (see
<a href="pcre2api.html#localesupport">"Locale support"</a>
in the
<a href="pcre2api.html"><b>pcre2api</b></a>
page). For example, in a French locale such as "fr_FR" in Unix-like systems,
or "french" in Windows, some character codes greater than 127 are used for
accented letters, and these are then matched by \w. The use of locales with
Unicode is discouraged.
</P>
<P>
By default, characters whose code points are greater than 127 never match \d,
\s, or \w, and always match \D, \S, and \W, although this may be different
for characters in the range 128-255 when locale-specific matching is happening.
These escape sequences retain their original meanings from before Unicode
support was available, mainly for efficiency reasons. If the PCRE2_UCP option
is set, the behaviour is changed so that Unicode properties are used to
determine character types, as follows:
<pre>
\d any character that matches \p{Nd} (decimal digit)
\s any character that matches \p{Z} or \h or \v
\w any character that matches \p{L} or \p{N}, plus underscore
</pre>
The upper case escapes match the inverse sets of characters. Note that \d
matches only decimal digits, whereas \w matches any Unicode digit, as well as
any Unicode letter, and underscore. Note also that PCRE2_UCP affects \b, and
\B because they are defined in terms of \w and \W. Matching these sequences
is noticeably slower when PCRE2_UCP is set.
</P>
<P>
The sequences \h, \H, \v, and \V, in contrast to the other sequences, which
match only ASCII characters by default, always match a specific list of code
points, whether or not PCRE2_UCP is set. The horizontal space characters are:
<pre>
U+0009 Horizontal tab (HT)
U+0020 Space
U+00A0 Non-break space
U+1680 Ogham space mark
U+180E Mongolian vowel separator
U+2000 En quad
U+2001 Em quad
U+2002 En space
U+2003 Em space
U+2004 Three-per-em space
U+2005 Four-per-em space
U+2006 Six-per-em space
U+2007 Figure space
U+2008 Punctuation space
U+2009 Thin space
U+200A Hair space
U+202F Narrow no-break space
U+205F Medium mathematical space
U+3000 Ideographic space
</pre>
The vertical space characters are:
<pre>
U+000A Linefeed (LF)
U+000B Vertical tab (VT)
U+000C Form feed (FF)
U+000D Carriage return (CR)
U+0085 Next line (NEL)
U+2028 Line separator
U+2029 Paragraph separator
</pre>
In 8-bit, non-UTF-8 mode, only the characters with code points less than 256
are relevant.
<a name="newlineseq"></a></P>
<br><b>
Newline sequences
</b><br>
<P>
Outside a character class, by default, the escape sequence \R matches any
Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent to the
following:
<pre>
(?>\r\n|\n|\x0b|\f|\r|\x85)
</pre>
This is an example of an "atomic group", details of which are given
<a href="#atomicgroup">below.</a>
This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
line, U+0085). Because this is an atomic group, the two-character sequence is
treated as a single unit that cannot be split.
</P>
<P>
In other modes, two additional characters whose code points are greater than 255
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
Unicode support is not needed for these characters to be recognized.
</P>
<P>
It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
at compile time. (BSR is an abbreviation for "backslash R".) This can be made
the default when PCRE2 is built; if this is the case, the other behaviour can
be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify
these settings by starting a pattern string with one of the following
sequences:
<pre>
(*BSR_ANYCRLF) CR, LF, or CRLF only
(*BSR_UNICODE) any Unicode newline sequence
</pre>
These override the default and the options given to the compiling function.
Note that these special settings, which are not Perl-compatible, are recognized
only at the very start of a pattern, and that they must be in upper case. If
more than one of them is present, the last one is used. They can be combined
with a change of newline convention; for example, a pattern can start with:
<pre>
(*ANY)(*BSR_ANYCRLF)
</pre>
They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a
character class, \R is treated as an unrecognized escape sequence, and causes
an error.
<a name="uniextseq"></a></P>
<br><b>
Unicode character properties
</b><br>
<P>
When PCRE2 is built with Unicode support (the default), three additional escape
sequences that match characters with specific properties are available. They
can be used in any mode, though in 8-bit and 16-bit non-UTF modes these
sequences are of course limited to testing characters whose code points are
less than U+0100 and U+10000, respectively. In 32-bit non-UTF mode, code points
greater than 0x10ffff (the Unicode limit) may be encountered. These are all
treated as being in the Unknown script and with an unassigned type. The extra
escape sequences are:
<pre>
\p{<i>xx</i>} a character with the <i>xx</i> property
\P{<i>xx</i>} a character without the <i>xx</i> property
\X a Unicode extended grapheme cluster
</pre>
The property names represented by <i>xx</i> above are case-sensitive. There is
support for Unicode script names, Unicode general category properties, "Any",
which matches any character (including newline), and some special PCRE2
properties (described in the
<a href="#extraprops">next section).</a>
Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2.
Note that \P{Any} does not match any characters, so always causes a match
failure.
</P>
<P>
Sets of Unicode characters are defined as belonging to certain scripts. A
character from one of these sets can be matched using a script name. For
example:
<pre>
\p{Greek}
\P{Han}
</pre>
Unassigned characters (and in non-UTF 32-bit mode, characters with code points
greater than 0x10FFFF) are assigned the "Unknown" script. Others that are not
part of an identified script are lumped together as "Common". The current list
of scripts is:
</P>
<P>
Adlam,
Ahom,
Anatolian_Hieroglyphs,
Arabic,
Armenian,
Avestan,
Balinese,
Bamum,
Bassa_Vah,
Batak,
Bengali,
Bhaiksuki,
Bopomofo,
Brahmi,
Braille,
Buginese,
Buhid,
Canadian_Aboriginal,
Carian,
Caucasian_Albanian,
Chakma,
Cham,
Cherokee,
Chorasmian,
Common,
Coptic,
Cuneiform,
Cypriot,
Cyrillic,
Deseret,
Devanagari,
Dives_Akuru,
Dogra,
Duployan,
Egyptian_Hieroglyphs,
Elbasan,
Elymaic,
Ethiopic,
Georgian,
Glagolitic,
Gothic,
Grantha,
Greek,
Gujarati,
Gunjala_Gondi,
Gurmukhi,
Han,
Hangul,
Hanifi_Rohingya,
Hanunoo,
Hatran,
Hebrew,
Hiragana,
Imperial_Aramaic,
Inherited,
Inscriptional_Pahlavi,
Inscriptional_Parthian,
Javanese,
Kaithi,
Kannada,
Katakana,
Kayah_Li,
Kharoshthi,
Khitan_Small_Script,
Khmer,
Khojki,
Khudawadi,
Lao,
Latin,
Lepcha,
Limbu,
Linear_A,
Linear_B,
Lisu,
Lycian,
Lydian,
Mahajani,
Makasar,
Malayalam,
Mandaic,
Manichaean,
Marchen,
Masaram_Gondi,
Medefaidrin,
Meetei_Mayek,
Mende_Kikakui,
Meroitic_Cursive,
Meroitic_Hieroglyphs,
Miao,
Modi,
Mongolian,
Mro,
Multani,
Myanmar,
Nabataean,
Nandinagari,
New_Tai_Lue,
Newa,
Nko,
Nushu,
Nyakeng_Puachue_Hmong,
Ogham,
Ol_Chiki,
Old_Hungarian,
Old_Italic,
Old_North_Arabian,
Old_Permic,
Old_Persian,
Old_Sogdian,
Old_South_Arabian,
Old_Turkic,
Oriya,
Osage,
Osmanya,
Pahawh_Hmong,
Palmyrene,
Pau_Cin_Hau,
Phags_Pa,
Phoenician,
Psalter_Pahlavi,
Rejang,
Runic,
Samaritan,
Saurashtra,
Sharada,
Shavian,
Siddham,
SignWriting,
Sinhala,
Sogdian,
Sora_Sompeng,
Soyombo,
Sundanese,
Syloti_Nagri,
Syriac,
Tagalog,
Tagbanwa,
Tai_Le,
Tai_Tham,
Tai_Viet,
Takri,
Tamil,
Tangut,
Telugu,
Thaana,
Thai,
Tibetan,
Tifinagh,
Tirhuta,
Ugaritic,
Unknown,
Vai,
Wancho,
Warang_Citi,
Yezidi,
Yi,
Zanabazar_Square.
</P>
<P>
Each character has exactly one Unicode general category property, specified by
a two-letter abbreviation. For compatibility with Perl, negation can be
specified by including a circumflex between the opening brace and the property
name. For example, \p{^Lu} is the same as \P{Lu}.
</P>
<P>
If only one letter is specified with \p or \P, it includes all the general
category properties that start with that letter. In this case, in the absence
of negation, the curly brackets in the escape sequence are optional; these two
examples have the same effect:
<pre>
\p{L}
\pL
</pre>
The following general category property codes are supported:
<pre>
C Other
Cc Control
Cf Format
Cn Unassigned
Co Private use
Cs Surrogate
L Letter
Ll Lower case letter
Lm Modifier letter
Lo Other letter
Lt Title case letter
Lu Upper case letter
M Mark
Mc Spacing mark
Me Enclosing mark
Mn Non-spacing mark
N Number
Nd Decimal number
Nl Letter number
No Other number
P Punctuation
Pc Connector punctuation
Pd Dash punctuation
Pe Close punctuation
Pf Final punctuation
Pi Initial punctuation
Po Other punctuation
Ps Open punctuation
S Symbol
Sc Currency symbol
Sk Modifier symbol
Sm Mathematical symbol
So Other symbol
Z Separator
Zl Line separator
Zp Paragraph separator
Zs Space separator
</pre>
The special property L& is also supported: it matches a character that has
the Lu, Ll, or Lt property, in other words, a letter that is not classified as
a modifier or "other".
</P>
<P>
The Cs (Surrogate) property applies only to characters whose code points are in
the range U+D800 to U+DFFF. These characters are no different to any other
character when PCRE2 is not in UTF mode (using the 16-bit or 32-bit library).
However, they are not valid in Unicode strings and so cannot be tested by PCRE2
in UTF mode, unless UTF validity checking has been turned off (see the
discussion of PCRE2_NO_UTF_CHECK in the
<a href="pcre2api.html"><b>pcre2api</b></a>
page).
</P>
<P>
The long synonyms for property names that Perl supports (such as \p{Letter})
are not supported by PCRE2, nor is it permitted to prefix any of these
properties with "Is".
</P>
<P>
No character that is in the Unicode table has the Cn (unassigned) property.
Instead, this property is assumed for any code point that is not in the
Unicode table.
</P>
<P>
Specifying caseless matching does not affect these escape sequences. For
example, \p{Lu} always matches only upper case letters. This is different from
the behaviour of current versions of Perl.
</P>
<P>
Matching characters by Unicode property is not fast, because PCRE2 has to do a
multistage table lookup in order to find a character's property. That is why
the traditional escape sequences such as \d and \w do not use Unicode
properties in PCRE2 by default, though you can make them do so by setting the
PCRE2_UCP option or by starting the pattern with (*UCP).
</P>
<br><b>
Extended grapheme clusters
</b><br>
<P>
The \X escape matches any number of Unicode characters that form an "extended
grapheme cluster", and treats the sequence as an atomic group
<a href="#atomicgroup">(see below).</a>
Unicode supports various kinds of composite character by giving each character
a grapheme breaking property, and having rules that use these properties to
define the boundaries of extended grapheme clusters. The rules are defined in
Unicode Standard Annex 29, "Unicode Text Segmentation". Unicode 11.0.0
abandoned the use of some previous properties that had been used for emojis.
Instead it introduced various emoji-specific properties. PCRE2 uses only the
Extended Pictographic property.
</P>
<P>
\X always matches at least one character. Then it decides whether to add
additional characters according to the following rules for ending a cluster:
</P>
<P>
1. End at the end of the subject string.
</P>
<P>
2. Do not end between CR and LF; otherwise end after any control character.
</P>
<P>
3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
are of five types: L, V, T, LV, and LVT. An L character may be followed by an
L, V, LV, or LVT character; an LV or V character may be followed by a V or T
character; an LVT or T character may be followed only by a T character.
</P>
<P>
4. Do not end before extending characters or spacing marks or the "zero-width
joiner" character. Characters with the "mark" property always have the
"extend" grapheme breaking property.
</P>
<P>
5. Do not end after prepend characters.
</P>
<P>
6. Do not break within emoji modifier sequences or emoji zwj sequences. That
is, do not break between characters with the Extended_Pictographic property.
Extend and ZWJ characters are allowed between the characters.
</P>
<P>
7. Do not break within emoji flag sequences. That is, do not break between
regional indicator (RI) characters if there are an odd number of RI characters
before the break point.
</P>
<P>
8. Otherwise, end the cluster.
<a name="extraprops"></a></P>
<br><b>
PCRE2's additional properties
</b><br>
<P>
As well as the standard Unicode properties described above, PCRE2 supports four
more that make it possible to convert traditional escape sequences such as \w
and \s to use Unicode properties. PCRE2 uses these non-standard, non-Perl
properties internally when PCRE2_UCP is set. However, they may also be used
explicitly. These properties are:
<pre>
Xan Any alphanumeric character
Xps Any POSIX space character
Xsp Any Perl space character
Xwd Any Perl "word" character
</pre>
Xan matches characters that have either the L (letter) or the N (number)
property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
carriage return, and any other character that has the Z (separator) property.
Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl
compatibility, but Perl changed. Xwd matches the same characters as Xan, plus
underscore.
</P>
<P>
There is another non-standard property, Xuc, which matches any character that
can be represented by a Universal Character Name in C++ and other programming
languages. These are the characters $, @, ` (grave accent), and all characters
with Unicode code points greater than or equal to U+00A0, except for the
surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH
where H is a hexadecimal digit. Note that the Xuc property does not match these
sequences but the characters that they represent.)
<a name="resetmatchstart"></a></P>
<br><b>
Resetting the match start
</b><br>
<P>
In normal use, the escape sequence \K causes any previously matched characters
not to be included in the final matched sequence that is returned. For example,
the pattern:
<pre>
foo\Kbar
</pre>
matches "foobar", but reports that it has matched "bar". \K does not interact
with anchoring in any way. The pattern:
<pre>
^foo\Kbar
</pre>
matches only when the subject begins with "foobar" (in single line mode),
though it again reports the matched string as "bar". This feature is similar to
a lookbehind assertion
<a href="#lookbehind">(described below).</a>
However, in this case, the part of the subject before the real match does not
have to be of fixed length, as lookbehind assertions do. The use of \K does
not interfere with the setting of
<a href="#group">captured substrings.</a>
For example, when the pattern
<pre>
(foo)\Kbar
</pre>
matches "foobar", the first substring is still set to "foo".
</P>
<P>
Perl used to document that the use of \K within lookaround assertions is "not
well defined", but from version 5.32.0 Perl does not support this usage at all.
In PCRE2, \K is acted upon when it occurs inside positive assertions, but is
ignored in negative assertions. Note that when a pattern such as (?=ab\K)
matches, the reported start of the match can be greater than the end of the
match. Using \K in a lookbehind assertion at the start of a pattern can also
lead to odd effects. For example, consider this pattern:
<pre>
(?<=\Kfoo)bar
</pre>
If the subject is "foobar", a call to <b>pcre2_match()</b> with a starting
offset of 3 succeeds and reports the matching string as "foobar", that is, the
start of the reported match is earlier than where the match started.
<a name="smallassertions"></a></P>
<br><b>
Simple assertions
</b><br>
<P>
The final use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
groups for more complicated assertions is described
<a href="#bigassertions">below.</a>
The backslashed assertions are:
<pre>
\b matches at a word boundary
\B matches when not at a word boundary
\A matches at the start of the subject
\Z matches at the end of the subject
also matches before a newline at the end of the subject
\z matches only at the end of the subject
\G matches at the first matching position in the subject
</pre>
Inside a character class, \b has a different meaning; it matches the backspace
character. If any other of these assertions appears in a character class, an
"invalid escape sequence" error is generated.
</P>
<P>
A word boundary is a position in the subject string where the current character
and the previous character do not both match \w or \W (i.e. one matches
\w and the other matches \W), or the start or end of the string if the
first or last character matches \w, respectively. When PCRE2 is built with
Unicode support, the meanings of \w and \W can be changed by setting the
PCRE2_UCP option. When this is done, it also affects \b and \B. Neither PCRE2
nor Perl has a separate "start of word" or "end of word" metasequence. However,
whatever follows \b normally determines which it is. For example, the fragment
\ba matches "a" at the start of a word.
</P>
<P>
The \A, \Z, and \z assertions differ from the traditional circumflex and
dollar (described in the next section) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus, they are
independent of multiline mode. These three assertions are not affected by the
PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the
circumflex and dollar metacharacters. However, if the <i>startoffset</i>
argument of <b>pcre2_match()</b> is non-zero, indicating that matching is to
start at a point other than the beginning of the subject, \A can never match.
The difference between \Z and \z is that \Z matches before a newline at the
end of the string as well as at the very end, whereas \z matches only at the
end.
</P>
<P>
The \G assertion is true only when the current matching position is at the
start point of the matching process, as specified by the <i>startoffset</i>
argument of <b>pcre2_match()</b>. It differs from \A when the value of
<i>startoffset</i> is non-zero. By calling <b>pcre2_match()</b> multiple times
with appropriate arguments, you can mimic Perl's /g option, and it is in this
kind of implementation where \G can be useful.
</P>
<P>
Note, however, that PCRE2's implementation of \G, being true at the starting
character of the matching process, is subtly different from Perl's, which
defines it as true at the end of the previous match. In Perl, these can be
different when the previously matched string was empty. Because PCRE2 does just
one match at a time, it cannot reproduce this behaviour.
</P>
<P>
If all the alternatives of a pattern begin with \G, the expression is anchored
to the starting match position, and the "anchored" flag is set in the compiled
regular expression.
</P>
<br><a name="SEC6" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
<P>
The circumflex and dollar metacharacters are zero-width assertions. That is,
they test for a particular condition being true without consuming any
characters from the subject string. These two metacharacters are concerned with
matching the starts and ends of lines. If the newline convention is set so that
only the two-character sequence CRLF is recognized as a newline, isolated CR
and LF characters are treated as ordinary data characters, and are not
recognized as newlines.
</P>
<P>
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is at
the start of the subject string. If the <i>startoffset</i> argument of
<b>pcre2_match()</b> is non-zero, or if PCRE2_NOTBOL is set, circumflex can
never match if the PCRE2_MULTILINE option is unset. Inside a character class,
circumflex has an entirely different meaning
<a href="#characterclass">(see below).</a>
</P>
<P>
Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)
</P>
<P>
The dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline at
the end of the string (by default), unless PCRE2_NOTEOL is set. Note, however,
that it does not actually match the newline. Dollar need not be the last
character of the pattern if a number of alternatives are involved, but it
should be the last item in any branch in which it appears. Dollar has no
special meaning in a character class.
</P>
<P>
The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This
does not affect the \Z assertion.
</P>
<P>
The meanings of the circumflex and dollar metacharacters are changed if the
PCRE2_MULTILINE option is set. When this is the case, a dollar character
matches before any newlines in the string, as well as at the very end, and a
circumflex matches immediately after internal newlines as well as at the start
of the subject string. It does not match after a newline that ends the string,
for compatibility with Perl. However, this can be changed by setting the
PCRE2_ALT_CIRCUMFLEX option.
</P>
<P>
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
\n represents a newline) in multiline mode, but not otherwise. Consequently,
patterns that are anchored in single line mode because all branches start with
^ are not anchored in multiline mode, and a match for circumflex is possible
when the <i>startoffset</i> argument of <b>pcre2_match()</b> is non-zero. The
PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.
</P>
<P>
When the newline convention (see
<a href="#newlines">"Newline conventions"</a>
below) recognizes the two-character sequence CRLF as a newline, this is
preferred, even if the single characters CR and LF are also recognized as
newlines. For example, if the newline convention is "any", a multiline mode
circumflex matches before "xyz" in the string "abc\r\nxyz" rather than after
CR, even though CR on its own is a valid newline. (It also matches at the very
start of the string, of course.)
</P>
<P>
Note that the sequences \A, \Z, and \z can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\A it is always anchored, whether or not PCRE2_MULTILINE is set.
<a name="fullstopdot"></a></P>
<br><a name="SEC7" href="#TOC1">FULL STOP (PERIOD, DOT) AND \N</a><br>
<P>
Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a
line.
</P>
<P>
When a line ending is defined as a single character, dot never matches that
character; when the two-character sequence CRLF is used, dot does not match CR
if it is immediately followed by LF, but otherwise it matches all characters
(including isolated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other line ending
characters.
</P>
<P>
The behaviour of dot with regard to newlines can be changed. If the
PCRE2_DOTALL option is set, a dot matches any one character, without exception.
If the two-character sequence CRLF is present in the subject string, it takes
two dots to match it.
</P>
<P>
The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.
</P>
<P>
The escape sequence \N when not followed by an opening brace behaves like a
dot, except that it is not affected by the PCRE2_DOTALL option. In other words,
it matches any character except one that signifies the end of a line.
</P>
<P>
When \N is followed by an opening brace it has a different meaning. See the
section entitled
<a href="digitsafterbackslash">"Non-printing characters"</a>
above for details. Perl also uses \N{name} to specify characters by Unicode
name; PCRE2 does not support this.
</P>
<br><a name="SEC8" href="#TOC1">MATCHING A SINGLE CODE UNIT</a><br>
<P>
Outside a character class, the escape sequence \C matches any one code unit,
whether or not a UTF mode is set. In the 8-bit library, one code unit is one
byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a
32-bit unit. Unlike a dot, \C always matches line-ending characters. The
feature is provided in Perl in order to match individual bytes in UTF-8 mode,
but it is unclear how it can usefully be used.
</P>
<P>
Because \C breaks up characters into individual code units, matching one unit
with \C in UTF-8 or UTF-16 mode means that the rest of the string may start
with a malformed UTF character. This has undefined results, because PCRE2
assumes that it is matching character by character in a valid UTF string (by
default it checks the subject string's validity at the start of processing
unless the PCRE2_NO_UTF_CHECK or PCRE2_MATCH_INVALID_UTF option is used).
</P>
<P>
An application can lock out the use of \C by setting the
PCRE2_NEVER_BACKSLASH_C option when compiling a pattern. It is also possible to
build PCRE2 with the use of \C permanently disabled.
</P>
<P>
PCRE2 does not allow \C to appear in lookbehind assertions
<a href="#lookbehind">(described below)</a>
in UTF-8 or UTF-16 modes, because this would make it impossible to calculate
the length of the lookbehind. Neither the alternative matching function
<b>pcre2_dfa_match()</b> nor the JIT optimizer support \C in these UTF modes.
The former gives a match-time error; the latter fails to optimize and so the
match is always run using the interpreter.
</P>
<P>
In the 32-bit library, however, \C is always supported (when not explicitly
locked out) because it always matches a single code unit, whether or not UTF-32
is specified.
</P>
<P>
In general, the \C escape sequence is best avoided. However, one way of using
it that avoids the problem of malformed UTF-8 or UTF-16 characters is to use a
lookahead to check the length of the next character, as in this pattern, which
could be used with a UTF-8 string (ignore white space and line breaks):
<pre>
(?| (?=[\x00-\x7f])(\C) |
(?=[\x80-\x{7ff}])(\C)(\C) |
(?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
(?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
</pre>
In this example, a group that starts with (?| resets the capturing parentheses
numbers in each alternative (see
<a href="#dupgroupnumber">"Duplicate Group Numbers"</a>
below). The assertions at the start of each branch check the next UTF-8
character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the appropriate number of
\C groups.
<a name="characterclass"></a></P>
<br><a name="SEC9" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
<P>
An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special by default.
If a closing square bracket is required as a member of the class, it should be
the first data character in the class (after an initial circumflex, if present)
or escaped with a backslash. This means that, by default, an empty class cannot
be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing
square bracket at the start does end the (empty) class.
</P>
<P>
A character class matches a single character in the subject. A matched
character must be in the set of characters defined by the class, unless the
first character in the class definition is a circumflex, in which case the
subject character must not be in the set defined by the class. If a circumflex
is actually required as a member of the class, ensure it is not the first
character, or escape it with a backslash.
</P>
<P>
For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters that
are in the class by enumerating those that are not. A class that starts with a
circumflex is not an assertion; it still consumes a character from the subject
string, and therefore it fails if the current pointer is at the end of the
string.
</P>
<P>
Characters in a class may be specified by their code points using \o, \x, or
\N{U+hh..} in the usual way. When caseless matching is set, any letters in a
class represent both their upper case and lower case versions, so for example,
a caseless [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
match "A", whereas a caseful version would. Note that there are two ASCII
characters, K and S, that, in addition to their lower case ASCII equivalents,
are case-equivalent with Unicode U+212A (Kelvin sign) and U+017F (long S)
respectively when either PCRE2_UTF or PCRE2_UCP is set.
</P>
<P>
Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use, and
whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A
class such as [^a] always matches one of these characters.
</P>
<P>
The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
\S, \v, \V, \w, and \W may appear in a character class, and add the
characters that they match to the class. For example, [\dABCDEF] matches any
hexadecimal digit. In UTF modes, the PCRE2_UCP option affects the meanings of
\d, \s, \w and their upper case partners, just as it does when they appear
outside a character class, as described in the section entitled
<a href="#genericchartypes">"Generic character types"</a>
above. The escape sequence \b has a different meaning inside a character
class; it matches the backspace character. The sequences \B, \R, and \X are
not special inside a character class. Like any other unrecognized escape
sequences, they cause an error. The same is true for \N when not followed by
an opening brace.
</P>
<P>
The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [d-m] matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class,
or immediately after a range. For example, [b-d-z] matches letters in the range
b to d, a hyphen character, or z.
</P>
<P>
Perl treats a hyphen as a literal if it appears before or after a POSIX class
(see below) or before or after a character type escape such as as \d or \H.
However, unless the hyphen is the last character in the class, Perl outputs a
warning in its warning mode, as this is most likely a user error. As PCRE2 has
no facility for warning, an error is given in these cases.
</P>
<P>
It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\]46] is interpreted as a class containing a range
followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.
</P>
<P>
Ranges normally include all code points between the start and end characters,
inclusive. They can also be used for code points specified numerically, for
example [\000-\037]. Ranges can include any characters that are valid for the
current mode. In any UTF mode, the so-called "surrogate" characters (those
whose code points lie between 0xd800 and 0xdfff inclusive) may not be specified
explicitly by default (the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables
this check). However, ranges such as [\x{d7ff}-\x{e000}], which include the
surrogates, are always permitted.
</P>
<P>
There is a special case in EBCDIC environments for ranges whose end points are
both specified as literal letters in the same case. For compatibility with
Perl, EBCDIC code points within the range that are not letters are omitted. For
example, [h-k] matches only four characters, even though the codes for h and k
are 0x88 and 0x92, a range of 11 code points. However, if the range is
specified numerically, for example, [\x88-\x92] or [h-\x92], all code points
are included.
</P>
<P>
If a range that includes letters is used when caseless matching is set, it
matches the letters in either case. For example, [W-c] is equivalent to
[][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
tables for a French locale are in use, [\xc8-\xcb] matches accented E
characters in both cases.
</P>
<P>
A circumflex can conveniently be used with the upper case character types to
specify a more restricted set of characters than the matching lower case type.
For example, the class [^\W_] matches any letter or digit, but not underscore,
whereas [\w] includes underscore. A positive character class should be read as
"something OR something OR ..." and a negative class as "NOT something AND NOT
something AND NOT ...".
</P>
<P>
The only metacharacters that are recognized in character classes are backslash,
hyphen (only where it can be interpreted as specifying a range), circumflex
(only at the start), opening square bracket (only when it can be interpreted as
introducing a POSIX class name, or for a special compatibility feature - see
the next two sections), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
</P>
<br><a name="SEC10" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
<P>
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports
this notation. For example,
<pre>
[01[:alpha:]%]
</pre>
matches "0", "1", any alphabetic character, or "%". The supported class names
are:
<pre>
alnum letters and digits
alpha letters
ascii character codes 0 - 127
blank space or tab only
cntrl control characters
digit decimal digits (same as \d)
graph printing characters, excluding space
lower lower case letters
print printing characters, including space
punct printing characters, excluding letters and digits and space
space white space (the same as \s from PCRE2 8.34)
upper upper case letters
word "word" characters (same as \w)
xdigit hexadecimal digits
</pre>
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). If locale-specific matching is taking place, the list of space
characters may be different; there may be fewer or more of them. "Space" and
\s match the same set of characters.
</P>
<P>
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,
<pre>
[12[:^digit:]]
</pre>
matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.
</P>
<P>
By default, characters with values greater than 127 do not match any of the
POSIX character classes, although this may be different for characters in the
range 128-255 when locale-specific matching is happening. However, if the
PCRE2_UCP option is passed to <b>pcre2_compile()</b>, some of the classes are
changed so that Unicode character properties are used. This is achieved by
replacing certain POSIX classes with other sequences, as follows:
<pre>
[:alnum:] becomes \p{Xan}
[:alpha:] becomes \p{L}
[:blank:] becomes \h
[:cntrl:] becomes \p{Cc}
[:digit:] becomes \p{Nd}
[:lower:] becomes \p{Ll}
[:space:] becomes \p{Xps}
[:upper:] becomes \p{Lu}
[:word:] becomes \p{Xwd}
</pre>
Negated versions, such as [:^alpha:] use \P instead of \p. Three other POSIX
classes are handled specially in UCP mode:
</P>
<P>
[:graph:]
This matches characters that have glyphs that mark the page when printed. In
Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
properties, except for:
<pre>
U+061C Arabic Letter Mark
U+180E Mongolian Vowel Separator
U+2066 - U+2069 Various "isolate"s
</PRE>
</P>
<P>
[:print:]
This matches the same characters as [:graph:] plus space characters that are
not controls, that is, characters with the Zs property.
</P>
<P>
[:punct:]
This matches all characters that have the Unicode P (punctuation) property,
plus those characters with code points less than 256 that have the S (Symbol)
property.
</P>
<P>
The other POSIX classes are unchanged, and match only characters with code
points less than 256.
</P>
<br><a name="SEC11" href="#TOC1">COMPATIBILITY FEATURE FOR WORD BOUNDARIES</a><br>
<P>
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
word". PCRE2 treats these items as follows:
<pre>
[[:<:]] is converted to \b(?=\w)
[[:>:]] is converted to \b(?<=\w)
</pre>
Only these exact character sequences are recognized. A sequence such as
[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
not compatible with Perl. It is provided to help migrations from other
environments, and is best not used in any new patterns. Note that \b matches
at the start and the end of a word (see
<a href="#smallassertions">"Simple assertions"</a>
above), and in a Perl-style pattern the preceding or following character
normally shows which is wanted, without the need for the assertions that are
used above in order to give exactly the POSIX behaviour.
</P>
<br><a name="SEC12" href="#TOC1">VERTICAL BAR</a><br>
<P>
Vertical bar characters are used to separate alternative patterns. For example,
the pattern
<pre>
gilbert|sullivan
</pre>
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string). The matching
process tries each alternative in turn, from left to right, and the first one
that succeeds is used. If the alternatives are within a group
<a href="#group">(defined below),</a>
"succeeds" means matching the rest of the main pattern as well as the
alternative in the group.
<a name="internaloptions"></a></P>
<br><a name="SEC13" href="#TOC1">INTERNAL OPTION SETTING</a><br>
<P>
The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL,
PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE options can be
changed from within the pattern by a sequence of letters enclosed between "(?"
and ")". These options are Perl-compatible, and are described in detail in the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation. The option letters are:
<pre>
i for PCRE2_CASELESS
m for PCRE2_MULTILINE
n for PCRE2_NO_AUTO_CAPTURE
s for PCRE2_DOTALL
x for PCRE2_EXTENDED
xx for PCRE2_EXTENDED_MORE
</pre>
For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the relevant letters with a hyphen, for
example (?-im). The two "extended" options are not independent; unsetting either
one cancels the effects of both of them.
</P>
<P>
A combined setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS
and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also
permitted. Only one hyphen may appear in the options string. If a letter
appears both before and after the hyphen, the option is unset. An empty options
setting "(?)" is allowed. Needless to say, it has no effect.
</P>
<P>
If the first character following (? is a circumflex, it causes all of the above
options to be unset. Thus, (?^) is equivalent to (?-imnsx). Letters may follow
the circumflex to cause some options to be re-instated, but a hyphen may not
appear.
</P>
<P>
The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in
the same way as the Perl-compatible options by using the characters J and U
respectively. However, these are not unset by (?^).
</P>
<P>
When one of these option changes occurs at top level (that is, not inside
group parentheses), the change applies to the remainder of the pattern
that follows. An option change within a group (see below for a description
of groups) affects only that part of the group that follows it, so
<pre>
(a(?i)b)c
</pre>
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same group. For example,
<pre>
(a(?i)b|c)
</pre>
matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.
</P>
<P>
As a convenient shorthand, if any option settings are required at the start of
a non-capturing group (see the next section), the option letters may
appear between the "?" and the ":". Thus the two patterns
<pre>
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
</pre>
match exactly the same set of strings.
</P>
<P>
<b>Note:</b> There are other PCRE2-specific options, applying to the whole
pattern, which can be set by the application when the compiling function is
called. In addition, the pattern can contain special leading sequences such as
(*CRLF) to override what the application has set or what has been defaulted.
Details are given in the section entitled
<a href="#newlineseq">"Newline sequences"</a>
above. There are also the (*UTF) and (*UCP) leading sequences that can be used
to set UTF and Unicode property modes; they are equivalent to setting the
PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set
the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the
(*UTF) and (*UCP) sequences.
<a name="group"></a></P>
<br><a name="SEC14" href="#TOC1">GROUPS</a><br>
<P>
Groups are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a group does two things:
<br>
<br>
1. It localizes a set of alternatives. For example, the pattern
<pre>
cat(aract|erpillar|)
</pre>
matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
match "cataract", "erpillar" or an empty string.
<br>
<br>
2. It creates a "capture group". This means that, when the whole pattern
matches, the portion of the subject string that matched the group is passed
back to the caller, separately from the portion that matched the whole pattern.
(This applies only to the traditional matching function; the DFA matching
function does not support capturing.)
</P>
<P>
Opening parentheses are counted from left to right (starting from 1) to obtain
numbers for capture groups. For example, if the string "the red king" is
matched against the pattern
<pre>
the ((red|white) (king|queen))
</pre>
the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3, respectively.
</P>
<P>
The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when grouping is required without capturing. If an
opening parenthesis is followed by a question mark and a colon, the group
does not do any capturing, and is not counted when computing the number of any
subsequent capture groups. For example, if the string "the white queen"
is matched against the pattern
<pre>
the ((?:red|white) (king|queen))
</pre>
the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of capture groups is 65535.
</P>
<P>
As a convenient shorthand, if any option settings are required at the start of
a non-capturing group, the option letters may appear between the "?" and the
":". Thus the two patterns
<pre>
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
</pre>
match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the group is
reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".
<a name="dupgroupnumber"></a></P>
<br><a name="SEC15" href="#TOC1">DUPLICATE GROUP NUMBERS</a><br>
<P>
Perl 5.10 introduced a feature whereby each alternative in a group uses the
same numbers for its capturing parentheses. Such a group starts with (?| and is
itself a non-capturing group. For example, consider this pattern:
<pre>
(?|(Sat)ur|(Sun))day
</pre>
Because the two alternatives are inside a (?| group, both sets of capturing
parentheses are numbered one. Thus, when the pattern matches, you can look
at captured substring number one, whichever alternative matched. This construct
is useful when you want to capture part, but not all, of one of a number of
alternatives. Inside a (?| group, parentheses are numbered as usual, but the
number is reset at the start of each branch. The numbers of any capturing
parentheses that follow the whole group start after the highest number used in
any branch. The following example is taken from the Perl documentation. The
numbers underneath show in which buffer the captured content will be stored.
<pre>
# before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
</pre>
A backreference to a capture group uses the most recent value that is set for
the group. The following pattern matches "abcabc" or "defdef":
<pre>
/(?|(abc)|(def))\1/
</pre>
In contrast, a subroutine call to a capture group always refers to the
first one in the pattern with the given number. The following pattern matches
"abcabc" or "defabc":
<pre>
/(?|(abc)|(def))(?1)/
</pre>
A relative reference such as (?-1) is no different: it is just a convenient way
of computing an absolute group number.
</P>
<P>
If a
<a href="#conditions">condition test</a>
for a group's having matched refers to a non-unique number, the test is
true if any group with that number has matched.
</P>
<P>
An alternative approach to using this "branch reset" feature is to use
duplicate named groups, as described in the next section.
</P>
<br><a name="SEC16" href="#TOC1">NAMED CAPTURE GROUPS</a><br>
<P>
Identifying capture groups by number is simple, but it can be very hard to keep
track of the numbers in complicated patterns. Furthermore, if an expression is
modified, the numbers may change. To help with this difficulty, PCRE2 supports
the naming of capture groups. This feature was not added to Perl until release
5.10. Python had the feature earlier, and PCRE1 introduced it at release 4.0,
using the Python syntax. PCRE2 supports both the Perl and the Python syntax.
</P>
<P>
In PCRE2, a capture group can be named in one of three ways: (?<name>...) or
(?'name'...) as in Perl, or (?P<name>...) as in Python. Names may be up to 32
code units long. When PCRE2_UTF is not set, they may contain only ASCII
alphanumeric characters and underscores, but must start with a non-digit. When
PCRE2_UTF is set, the syntax of group names is extended to allow any Unicode
letter or Unicode decimal digit. In other words, group names must match one of
these patterns:
<pre>
^[_A-Za-z][_A-Za-z0-9]*\z when PCRE2_UTF is not set
^[_\p{L}][_\p{L}\p{Nd}]*\z when PCRE2_UTF is set
</pre>
References to capture groups from other parts of the pattern, such as
<a href="#backreferences">backreferences,</a>
<a href="#recursion">recursion,</a>
and
<a href="#conditions">conditions,</a>
can all be made by name as well as by number.
</P>
<P>
Named capture groups are allocated numbers as well as names, exactly as
if the names were not present. In both PCRE2 and Perl, capture groups
are primarily identified by numbers; any names are just aliases for these
numbers. The PCRE2 API provides function calls for extracting the complete
name-to-number translation table from a compiled pattern, as well as
convenience functions for extracting captured substrings by name.
</P>
<P>
<b>Warning:</b> When more than one capture group has the same number, as
described in the previous section, a name given to one of them applies to all
of them. Perl allows identically numbered groups to have different names.
Consider this pattern, where there are two capture groups, both numbered 1:
<pre>
(?|(?<AA>aa)|(?<BB>bb))
</pre>
Perl allows this, with both names AA and BB as aliases of group 1. Thus, after
a successful match, both names yield the same value (either "aa" or "bb").
</P>
<P>
In an attempt to reduce confusion, PCRE2 does not allow the same group number
to be associated with more than one name. The example above provokes a
compile-time error. However, there is still scope for confusion. Consider this
pattern:
<pre>
(?|(?<AA>aa)|(bb))
</pre>
Although the second group number 1 is not explicitly named, the name AA is
still an alias for any group 1. Whether the pattern matches "aa" or "bb", a
reference by name to group AA yields the matched string.
</P>
<P>
By default, a name must be unique within a pattern, except that duplicate names
are permitted for groups with the same number, for example:
<pre>
(?|(?<AA>aa)|(?<AA>bb))
</pre>
The duplicate name constraint can be disabled by setting the PCRE2_DUPNAMES
option at compile time, or by the use of (?J) within the pattern, as described
in the section entitled
<a href="#internaloptions">"Internal Option Setting"</a>
above.
</P>
<P>
Duplicate names can be useful for patterns where only one instance of the named
capture group can match. Suppose you want to match the name of a weekday,
either as a 3-letter abbreviation or as the full name, and in both cases you
want to extract the abbreviation. This pattern (ignoring the line breaks) does
the job:
<pre>
(?J)
(?<DN>Mon|Fri|Sun)(?:day)?|
(?<DN>Tue)(?:sday)?|
(?<DN>Wed)(?:nesday)?|
(?<DN>Thu)(?:rsday)?|
(?<DN>Sat)(?:urday)?
</pre>
There are five capture groups, but only one is ever set after a match. The
convenience functions for extracting the data by name returns the substring for
the first (and in this example, the only) group of that name that matched. This
saves searching to find which numbered group it was. (An alternative way of
solving this problem is to use a "branch reset" group, as described in the
previous section.)
</P>
<P>
If you make a backreference to a non-unique named group from elsewhere in the
pattern, the groups to which the name refers are checked in the order in which
they appear in the overall pattern. The first one that is set is used for the
reference. For example, this pattern matches both "foofoo" and "barbar" but not
"foobar" or "barfoo":
<pre>
(?J)(?:(?<n>foo)|(?<n>bar))\k<n>
</PRE>
</P>
<P>
If you make a subroutine call to a non-unique named group, the one that
corresponds to the first occurrence of the name is used. In the absence of
duplicate numbers this is the one with the lowest number.
</P>
<P>
If you use a named reference in a condition
test (see the
<a href="#conditions">section about conditions</a>
below), either to check whether a capture group has matched, or to check for
recursion, all groups with the same name are tested. If the condition is true
for any one of them, the overall condition is true. This is the same behaviour
as testing by number. For further details of the interfaces for handling named
capture groups, see the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
<br><a name="SEC17" href="#TOC1">REPETITION</a><br>
<P>
Repetition is specified by quantifiers, which can follow any of the following
items:
<pre>
a literal data character
the dot metacharacter
the \C escape sequence
the \R escape sequence
the \X escape sequence
an escape such as \d or \pL that matches a single character
a character class
a backreference
a parenthesized group (including lookaround assertions)
a subroutine call (recursive or otherwise)
</pre>
The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example,
<pre>
z{2,4}
</pre>
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus
<pre>
[aeiou]{3,}
</pre>
matches at least 3 successive vowels, but may match many more, whereas
<pre>
\d{8}
</pre>
matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.
</P>
<P>
In UTF modes, quantifiers apply to characters rather than to individual code
units. Thus, for example, \x{100}{2} matches two characters, each of
which is represented by a two-byte sequence in a UTF-8 string. Similarly,
\X{3} matches three Unicode extended grapheme clusters, each of which may be
several code units long (and they may be of different lengths).
</P>
<P>
The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present. This may be useful for
capture groups that are referenced as
<a href="#groupsassubroutines">subroutines</a>
from elsewhere in the pattern (but see also the section entitled
<a href="#subdefine">"Defining capture groups for use by reference only"</a>
below). Except for parenthesized groups, items that have a {0} quantifier are
omitted from the compiled pattern.
</P>
<P>
For convenience, the three most common quantifiers have single-character
abbreviations:
<pre>
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
</pre>
It is possible to construct infinite loops by following a group that can match
no characters with a quantifier that has no upper limit, for example:
<pre>
(a?)*
</pre>
Earlier versions of Perl and PCRE1 used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but whenever an iteration of such a group matches no
characters, matching moves on to the next item in the pattern instead of
repeatedly matching an empty string. This does not prevent backtracking into
any of the iterations if a subsequent item fails to match.
</P>
<P>
By default, quantifiers are "greedy", that is, they match as much as possible
(up to the maximum number of permitted times), without causing the rest of the
pattern to fail. The classic example of where this gives problems is in trying
to match comments in C programs. These appear between /* and */ and within the
comment, individual * and / characters may appear. An attempt to match C
comments by applying the pattern
<pre>
/\*.*\*/
</pre>
to the string
<pre>
/* first comment */ not comment /* second comment */
</pre>
fails, because it matches the entire string owing to the greediness of the .*
item. However, if a quantifier is followed by a question mark, it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern
<pre>
/\*.*?\*/
</pre>
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in
<pre>
\d??\d
</pre>
which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.
</P>
<P>
If the PCRE2_UNGREEDY option is set (an option that is not available in Perl),
the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.
</P>
<P>
When a parenthesized group is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
</P>
<P>
If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE2 normally treats such a
pattern as though it were preceded by \A.
</P>
<P>
In cases where it is known that the subject string contains no newlines, it is
worth setting PCRE2_DOTALL in order to obtain this optimization, or
alternatively, using ^ to indicate anchoring explicitly.
</P>
<P>
However, there are some cases where the optimization cannot be used. When .*
is inside capturing parentheses that are the subject of a backreference
elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:
<pre>
(.*)abc\1
</pre>
If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.
</P>
<P>
Another case where implicit anchoring is not applied is when the leading .* is
inside an atomic group. Once again, a match at the start may fail where a later
one succeeds. Consider this pattern:
<pre>
(?>.*?a)b
</pre>
It matches "ab" in the subject "aab". The use of the backtracking control verbs
(*PRUNE) and (*SKIP) also disable this optimization, and there is an option,
PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
</P>
<P>
When a capture group is repeated, the value captured is the substring that
matched the final iteration. For example, after
<pre>
(tweedle[dume]{3}\s*)+
</pre>
has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capture groups, the corresponding
captured values may have been set in previous iterations. For example, after
<pre>
(a|(b))+
</pre>
matches "aba" the value of the second captured substring is "b".
<a name="atomicgroup"></a></P>
<br><a name="SEC18" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
<P>
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item to be
re-evaluated to see if a different number of repeats allows the rest of the
pattern to match. Sometimes it is useful to prevent this, either to change the
nature of the match, or to cause it fail earlier than it otherwise might, when
the author of the pattern knows there is no point in carrying on.
</P>
<P>
Consider, for example, the pattern \d+foo when applied to the subject line
<pre>
123456bar
</pre>
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \d+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl's book) provides the means for specifying
that once a group has matched, it is not to be re-evaluated in this way.
</P>
<P>
If we use atomic grouping for the previous example, the matcher gives up
immediately on failing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:
<pre>
(?>\d+)foo
</pre>
Perl 5.28 introduced an experimental alphabetic form starting with (* which may
be easier to remember:
<pre>
(*atomic:\d+)foo
</pre>
This kind of parenthesized group "locks up" the part of the pattern it
contains once it has matched, and a failure further into the pattern is
prevented from backtracking into it. Backtracking past it to previous items,
however, works as normal.
</P>
<P>
An alternative description is that a group of this type matches exactly the
string of characters that an identical standalone pattern would match, if
anchored at the current point in the subject string.
</P>
<P>
Atomic groups are not capture groups. Simple cases such as the above example
can be thought of as a maximizing repeat that must swallow everything it can.
So, while both \d+ and \d+? are prepared to adjust the number of digits they
match in order to make the rest of the pattern match, (?>\d+) can only match
an entire sequence of digits.
</P>
<P>
Atomic groups in general can of course contain arbitrarily complicated
expressions, and can be nested. However, when the contents of an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as
<pre>
\d++foo
</pre>
Note that a possessive quantifier can be used with an entire group, for
example:
<pre>
(abc|xyz){2,3}+
</pre>
Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.
</P>
<P>
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
Jeffrey Friedl originated the idea (and the name) in the first edition of his
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
package, and PCRE1 copied it from there. It found its way into Perl at release
5.10.
</P>
<P>
PCRE2 has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence A+B is treated as A++B because
there is no point in backtracking into a sequence of A's when B must follow.
This feature can be disabled by the PCRE2_NO_AUTOPOSSESS option, or starting
the pattern with (*NO_AUTO_POSSESS).
</P>
<P>
When a pattern contains an unlimited repeat inside a group that can itself be
repeated an unlimited number of times, the use of an atomic group is the only
way to avoid some failing matches taking a very long time indeed. The pattern
<pre>
(\D+|<\d+>)*[!?]
</pre>
matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to
<pre>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
</pre>
it takes a long time before reporting failure. This is because the string can
be divided between the internal \D+ repeat and the external * repeat in a
large number of ways, and all have to be tried. (The example uses [!?] rather
than a single character at the end, because both PCRE2 and Perl have an
optimization that allows for fast failure when a single character is used. They
remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses
an atomic group, like this:
<pre>
((?>\D+)|<\d+>)*[!?]
</pre>
sequences of non-digits cannot be broken, and failure happens quickly.
<a name="backreferences"></a></P>
<br><a name="SEC19" href="#TOC1">BACKREFERENCES</a><br>
<P>
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a backreference to a capture group earlier (that
is, to its left) in the pattern, provided there have been that many previous
capture groups.
</P>
<P>
However, if the decimal number following the backslash is less than 8, it is
always taken as a backreference, and causes an error only if there are not that
many capture groups in the entire pattern. In other words, the group that is
referenced need not be to the left of the reference for numbers less than 8. A
"forward backreference" of this type can make sense when a repetition is
involved and the group to the right has participated in an earlier iteration.
</P>
<P>
It is not possible to have a numerical "forward backreference" to a group whose
number is 8 or more using this syntax because a sequence such as \50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters"
<a href="#digitsafterbackslash">above</a>
for further details of the handling of digits following a backslash. Other
forms of backreferencing do not suffer from this restriction. In particular,
there is no problem when named capture groups are used (see below).
</P>
<P>
Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the \g escape sequence. This escape must be followed by a
signed or unsigned number, optionally enclosed in braces. These examples are
all identical:
<pre>
(ring), \1
(ring), \g1
(ring), \g{1}
</pre>
An unsigned number specifies an absolute reference without the ambiguity that
is present in the older syntax. It is also useful when literal digits follow
the reference. A signed number is a relative reference. Consider this example:
<pre>
(abc(def)ghi)\g{-1}
</pre>
The sequence \g{-1} is a reference to the most recently started capture group
before \g, that is, is it equivalent to \2 in this example. Similarly,
\g{-2} would be equivalent to \1. The use of relative references can be
helpful in long patterns, and also in patterns that are created by joining
together fragments that contain references within themselves.
</P>
<P>
The sequence \g{+1} is a reference to the next capture group. This kind of
forward reference can be useful in patterns that repeat. Perl does not support
the use of + in this way.
</P>
<P>
A backreference matches whatever actually most recently matched the capture
group in the current subject string, rather than anything at all that matches
the group (see
<a href="#groupsassubroutines">"Groups as subroutines"</a>
below for a way of doing that). So the pattern
<pre>
(sens|respons)e and \1ibility
</pre>
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
backreference, the case of letters is relevant. For example,
<pre>
((?i)rah)\s+\1
</pre>
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capture group is matched caselessly.
</P>
<P>
There are several different ways of writing backreferences to named capture
groups. The .NET syntax \k{name} and the Perl syntax \k<name> or \k'name'
are supported, as is the Python syntax (?P=name). Perl 5.10's unified
backreference syntax, in which \g can be used for both numeric and named
references, is also supported. We could rewrite the above example in any of the
following ways:
<pre>
(?<p1>(?i)rah)\s+\k<p1>
(?'p1'(?i)rah)\s+\k{p1}
(?P<p1>(?i)rah)\s+(?P=p1)
(?<p1>(?i)rah)\s+\g{p1}
</pre>
A capture group that is referenced by name may appear in the pattern before or
after the reference.
</P>
<P>
There may be more than one backreference to the same group. If a group has not
actually been used in a particular match, backreferences to it always fail by
default. For example, the pattern
<pre>
(a|(bc))\2
</pre>
always fails if it starts to match "a" rather than "bc". However, if the
PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backreference to an
unset value matches an empty string.
</P>
<P>
Because there may be many capture groups in a pattern, all digits following a
backslash are taken as part of a potential backreference number. If the pattern
continues with a digit character, some delimiter must be used to terminate the
backreference. If the PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, this
can be white space. Otherwise, the \g{} syntax or an empty comment (see
<a href="#comments">"Comments"</a>
below) can be used.
</P>
<br><b>
Recursive backreferences
</b><br>
<P>
A backreference that occurs inside the group to which it refers fails when the
group is first used, so, for example, (a\1) never matches. However, such
references can be useful inside repeated groups. For example, the pattern
<pre>
(a|b\1)+
</pre>
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
the group, the backreference matches the character string corresponding to the
previous iteration. In order for this to work, the pattern must be such that
the first iteration does not need to match the backreference. This can be done
using alternation, as in the example above, or by a quantifier with a minimum
of zero.
</P>
<P>
For versions of PCRE2 less than 10.25, backreferences of this type used to
cause the group that they reference to be treated as an
<a href="#atomicgroup">atomic group.</a>
This restriction no longer applies, and backtracking into such groups can occur
as normal.
<a name="bigassertions"></a></P>
<br><a name="SEC20" href="#TOC1">ASSERTIONS</a><br>
<P>
An assertion is a test on the characters following or preceding the current
matching point that does not consume any characters. The simple assertions
coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
<a href="#smallassertions">above.</a>
</P>
<P>
More complicated assertions are coded as parenthesized groups. There are two
kinds: those that look ahead of the current position in the subject string, and
those that look behind it, and in each case an assertion may be positive (must
match for the assertion to be true) or negative (must not match for the
assertion to be true). An assertion group is matched in the normal way,
and if it is true, matching continues after it, but with the matching position
in the subject string reset to what it was before the assertion was processed.
</P>
<P>
The Perl-compatible lookaround assertions are atomic. If an assertion is true,
but there is a subsequent matching failure, there is no backtracking into the
assertion. However, there are some cases where non-atomic assertions can be
useful. PCRE2 has some support for these, described in the section entitled
<a href="#nonatomicassertions">"Non-atomic assertions"</a>
below, but they are not Perl-compatible.
</P>
<P>
A lookaround assertion may appear as the condition in a
<a href="#conditions">conditional group</a>
(see below). In this case, the result of matching the assertion determines
which branch of the condition is followed.
</P>
<P>
Assertion groups are not capture groups. If an assertion contains capture
groups within it, these are counted for the purposes of numbering the capture
groups in the whole pattern. Within each branch of an assertion, locally
captured substrings may be referenced in the usual way. For example, a sequence
such as (.)\g{-1} can be used to check that two adjacent characters are the
same.
</P>
<P>
When a branch within an assertion fails to match, any substrings that were
captured are discarded (as happens with any pattern branch that fails to
match). A negative assertion is true only when all its branches fail to match;
this means that no captured substrings are ever retained after a successful
negative assertion. When an assertion contains a matching branch, what happens
depends on the type of assertion.
</P>
<P>
For a positive assertion, internally captured substrings in the successful
branch are retained, and matching continues with the next pattern item after
the assertion. For a negative assertion, a matching branch means that the
assertion is not true. If such an assertion is being used as a condition in a
<a href="#conditions">conditional group</a>
(see below), captured substrings are retained, because matching continues with
the "no" branch of the condition. For other failing negative assertions,
control passes to the previous backtracking point, thus discarding any captured
strings within the assertion.
</P>
<P>
Most assertion groups may be repeated; though it makes no sense to assert the
same thing several times, the side effect of capturing in positive assertions
may occasionally be useful. However, an assertion that forms the condition for
a conditional group may not be quantified. PCRE2 used to restrict the
repetition of assertions, but from release 10.35 the only restriction is that
an unlimited maximum repetition is changed to be one more than the minimum. For
example, {3,} is treated as {3,4}.
</P>
<br><b>
Alphabetic assertion names
</b><br>
<P>
Traditionally, symbolic sequences such as (?= and (?<= have been used to
specify lookaround assertions. Perl 5.28 introduced some experimental
alphabetic alternatives which might be easier to remember. They all start with
(* instead of (? and must be written using lower case letters. PCRE2 supports
the following synonyms:
<pre>
(*positive_lookahead: or (*pla: is the same as (?=
(*negative_lookahead: or (*nla: is the same as (?!
(*positive_lookbehind: or (*plb: is the same as (?<=
(*negative_lookbehind: or (*nlb: is the same as (?<!
</pre>
For example, (*pla:foo) is the same assertion as (?=foo). In the following
sections, the various assertions are described using the original symbolic
forms.
</P>
<br><b>
Lookahead assertions
</b><br>
<P>
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
<pre>
\w+(?=;)
</pre>
matches a word followed by a semicolon, but does not include the semicolon in
the match, and
<pre>
foo(?!bar)
</pre>
matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern
<pre>
(?!foo)bar
</pre>
does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve the other effect.
</P>
<P>
If you want to force a matching failure at some point in a pattern, the most
convenient way to do it is with (?!) because an empty string always matches, so
an assertion that requires there not to be an empty string must always fail.
The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
<a name="lookbehind"></a></P>
<br><b>
Lookbehind assertions
</b><br>
<P>
Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,
<pre>
(?<!foo)bar
</pre>
does find an occurrence of "bar" that is not preceded by "foo". The contents of
a lookbehind assertion are restricted such that all the strings it matches must
have a fixed length. However, if there are several top-level alternatives, they
do not all have to have the same fixed length. Thus
<pre>
(?<=bullock|donkey)
</pre>
is permitted, but
<pre>
(?<!dogs?|cats?)
</pre>
causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl, which requires all branches to match the same
length of string. An assertion such as
<pre>
(?<=ab(c|de))
</pre>
is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable to PCRE2 if rewritten to use two top-level
branches:
<pre>
(?<=abc|abde)
</pre>
In some cases, the escape sequence \K
<a href="#resetmatchstart">(see above)</a>
can be used instead of a lookbehind assertion to get round the fixed-length
restriction.
</P>
<P>
The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed length and then try to
match. If there are insufficient characters before the current position, the
assertion fails.
</P>
<P>
In UTF-8 and UTF-16 modes, PCRE2 does not allow the \C escape (which matches a
single code unit even in a UTF mode) to appear in lookbehind assertions,
because it makes it impossible to calculate the length of the lookbehind. The
\X and \R escapes, which can match different numbers of code units, are never
permitted in lookbehinds.
</P>
<P>
<a href="#groupsassubroutines">"Subroutine"</a>
calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
as the called capture group matches a fixed-length string. However,
<a href="#recursion">recursion,</a>
that is, a "subroutine" call into a group that is already active,
is not supported.
</P>
<P>
Perl does not support backreferences in lookbehinds. PCRE2 does support them,
but only if certain conditions are met. The PCRE2_MATCH_UNSET_BACKREF option
must not be set, there must be no use of (?| in the pattern (it creates
duplicate group numbers), and if the backreference is by name, the name
must be unique. Of course, the referenced group must itself match a fixed
length substring. The following pattern matches words containing at least two
characters that begin and end with the same character:
<pre>
\b(\w)\w++(?<=\1)
</PRE>
</P>
<P>
Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching of fixed-length strings at the end of subject
strings. Consider a simple pattern such as
<pre>
abcd$
</pre>
when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE2 will look for each "a" in the subject and then see if
what follows matches the rest of the pattern. If the pattern is specified as
<pre>
^.*abcd$
</pre>
the initial .* matches the entire string at first, but when this fails (because
there is no following "a"), it backtracks to match all but the last character,
then all but the last two characters, and so on. Once again the search for "a"
covers the entire string, from right to left, so we are no better off. However,
if the pattern is written as
<pre>
^.*+(?<=abcd)
</pre>
there can be no backtracking for the .*+ item because of the possessive
quantifier; it can match only the entire string. The subsequent lookbehind
assertion does a single test on the last four characters. If it fails, the
match fails immediately. For long strings, this approach makes a significant
difference to the processing time.
</P>
<br><b>
Using multiple assertions
</b><br>
<P>
Several assertions (of any sort) may occur in succession. For example,
<pre>
(?<=\d{3})(?<!999)foo
</pre>
matches "foo" preceded by three digits that are not "999". Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, and then there is a check that the same three characters are not "999".
This pattern does <i>not</i> match "foo" preceded by six characters, the first
of which are digits and the last three of which are not "999". For example, it
doesn't match "123abcfoo". A pattern to do that is
<pre>
(?<=\d{3}...)(?<!999)foo
</pre>
This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".
</P>
<P>
Assertions can be nested in any combination. For example,
<pre>
(?<=(?<!foo)bar)baz
</pre>
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while
<pre>
(?<=\d{3}(?!999)...)foo
</pre>
is another pattern that matches "foo" preceded by three digits and any three
characters that are not "999".
<a name="nonatomicassertions"></a></P>
<br><a name="SEC21" href="#TOC1">NON-ATOMIC ASSERTIONS</a><br>
<P>
The traditional Perl-compatible lookaround assertions are atomic. That is, if
an assertion is true, but there is a subsequent matching failure, there is no
backtracking into the assertion. However, there are some cases where non-atomic
positive assertions can be useful. PCRE2 provides these using the following
syntax:
<pre>
(*non_atomic_positive_lookahead: or (*napla: or (?*
(*non_atomic_positive_lookbehind: or (*naplb: or (?<*
</pre>
Consider the problem of finding the right-most word in a string that also
appears earlier in the string, that is, it must appear at least twice in total.
This pattern returns the required result as captured substring 1:
<pre>
^(?x)(*napla: .* \b(\w++)) (?> .*? \b\1\b ){2}
</pre>
For a subject such as "word1 word2 word3 word2 word3 word4" the result is
"word3". How does it work? At the start, ^(?x) anchors the pattern and sets the
"x" option, which causes white space (introduced for readability) to be
ignored. Inside the assertion, the greedy .* at first consumes the entire
string, but then has to backtrack until the rest of the assertion can match a
word, which is captured by group 1. In other words, when the assertion first
succeeds, it captures the right-most word in the string.
</P>
<P>
The current matching point is then reset to the start of the subject, and the
rest of the pattern match checks for two occurrences of the captured word,
using an ungreedy .*? to scan from the left. If this succeeds, we are done, but
if the last word in the string does not occur twice, this part of the pattern
fails. If a traditional atomic lookhead (?= or (*pla: had been used, the
assertion could not be re-entered, and the whole match would fail. The pattern
would succeed only if the very last word in the subject was found twice.
</P>
<P>
Using a non-atomic lookahead, however, means that when the last word does not
occur twice in the string, the lookahead can backtrack and find the second-last
word, and so on, until either the match succeeds, or all words have been
tested.
</P>
<P>
Two conditions must be met for a non-atomic assertion to be useful: the
contents of one or more capturing groups must change after a backtrack into the
assertion, and there must be a backreference to a changed group later in the
pattern. If this is not the case, the rest of the pattern match fails exactly
as before because nothing has changed, so using a non-atomic assertion just
wastes resources.
</P>
<P>
There is one exception to backtracking into a non-atomic assertion. If an
(*ACCEPT) control verb is triggered, the assertion succeeds atomically. That
is, a subsequent match failure cannot backtrack into the assertion.
</P>
<P>
Non-atomic assertions are not supported by the alternative matching function
<b>pcre2_dfa_match()</b>. They are supported by JIT, but only if they do not
contain any control verbs such as (*ACCEPT). (This may change in future). Note
that assertions that appear as conditions for
<a href="#conditions">conditional groups</a>
(see below) must be atomic.
</P>
<br><a name="SEC22" href="#TOC1">SCRIPT RUNS</a><br>
<P>
In concept, a script run is a sequence of characters that are all from the same
Unicode script such as Latin or Greek. However, because some scripts are
commonly used together, and because some diacritical and other marks are used
with multiple scripts, it is not that simple. There is a full description of
the rules that PCRE2 uses in the section entitled
<a href="pcre2unicode.html#scriptruns">"Script Runs"</a>
in the
<a href="pcre2unicode.html"><b>pcre2unicode</b></a>
documentation.
</P>
<P>
If part of a pattern is enclosed between (*script_run: or (*sr: and a closing
parenthesis, it fails if the sequence of characters that it matches are not a
script run. After a failure, normal backtracking occurs. Script runs can be
used to detect spoofing attacks using characters that look the same, but are
from different scripts. The string "paypal.com" is an infamous example, where
the letters could be a mixture of Latin and Cyrillic. This pattern ensures that
the matched characters in a sequence of non-spaces that follow white space are
a script run:
<pre>
\s+(*sr:\S+)
</pre>
To be sure that they are all from the Latin script (for example), a lookahead
can be used:
<pre>
\s+(?=\p{Latin})(*sr:\S+)
</pre>
This works as long as the first character is expected to be a character in that
script, and not (for example) punctuation, which is allowed with any script. If
this is not the case, a more creative lookahead is needed. For example, if
digits, underscore, and dots are permitted at the start:
<pre>
\s+(?=[0-9_.]*\p{Latin})(*sr:\S+)
</PRE>
</P>
<P>
In many cases, backtracking into a script run pattern fragment is not
desirable. The script run can employ an atomic group to prevent this. Because
this is a common requirement, a shorthand notation is provided by
(*atomic_script_run: or (*asr:
<pre>
(*asr:...) is the same as (*sr:(?>...))
</pre>
Note that the atomic group is inside the script run. Putting it outside would
not prevent backtracking into the script run pattern.
</P>
<P>
Support for script runs is not available if PCRE2 is compiled without Unicode
support. A compile-time error is given if any of the above constructs is
encountered. Script runs are not supported by the alternate matching function,
<b>pcre2_dfa_match()</b> because they use the same mechanism as capturing
parentheses.
</P>
<P>
<b>Warning:</b> The (*ACCEPT) control verb
<a href="#acceptverb">(see below)</a>
should not be used within a script run group, because it causes an immediate
exit from the group, bypassing the script run checking.
<a name="conditions"></a></P>
<br><a name="SEC23" href="#TOC1">CONDITIONAL GROUPS</a><br>
<P>
It is possible to cause the matching process to obey a pattern fragment
conditionally or to choose between two alternative fragments, depending on
the result of an assertion, or whether a specific capture group has
already been matched. The two possible forms of conditional group are:
<pre>
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
</pre>
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. An absent no-pattern is equivalent to an empty
string (it always matches). If there are more than two alternatives in the
group, a compile-time error occurs. Each of the two alternatives may itself
contain nested groups of any form, including conditional groups; the
restriction to two alternatives applies only at the level of the condition
itself. This pattern fragment is an example where the alternatives are complex:
<pre>
(?(1) (A|B|C) | (D | (?(2)E|F) | E) )
</PRE>
</P>
<P>
There are five kinds of condition: references to capture groups, references to
recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.
</P>
<br><b>
Checking for a used capture group by number
</b><br>
<P>
If the text between the parentheses consists of a sequence of digits, the
condition is true if a capture group of that number has previously matched. If
there is more than one capture group with the same number (see the earlier
<a href="#recursion">section about duplicate group numbers),</a>
the condition is true if any of them have matched. An alternative notation is
to precede the digits with a plus or minus sign. In this case, the group number
is relative rather than absolute. The most recently opened capture group can be
referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops
it can also make sense to refer to subsequent groups. The next capture group
can be referenced as (?(+1), and so on. (The value zero in any of these forms
is not used; it provokes a compile-time error.)
</P>
<P>
Consider the following pattern, which contains non-significant white space to
make it more readable (assume the PCRE2_EXTENDED option) and to divide it into
three parts for ease of discussion:
<pre>
( \( )? [^()]+ (?(1) \) )
</pre>
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional group that tests whether or not the first capture group
matched. If it did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
conditional group matches nothing. In other words, this pattern matches a
sequence of non-parentheses, optionally enclosed in parentheses.
</P>
<P>
If you were embedding this pattern in a larger one, you could use a relative
reference:
<pre>
...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
</pre>
This makes the fragment independent of the parentheses in the larger pattern.
</P>
<br><b>
Checking for a used capture group by name
</b><br>
<P>
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
capture group by name. For compatibility with earlier versions of PCRE1, which
had this facility before Perl, the syntax (?(name)...) is also recognized.
Note, however, that undelimited names consisting of the letter R followed by
digits are ambiguous (see the following section). Rewriting the above example
to use a named group gives this:
<pre>
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
</pre>
If the name used in a condition of this kind is a duplicate, the test is
applied to all groups of the same name, and is true if any one of them has
matched.
</P>
<br><b>
Checking for pattern recursion
</b><br>
<P>
"Recursion" in this sense refers to any subroutine-like call from one part of
the pattern to another, whether or not it is actually recursive. See the
sections entitled
<a href="#recursion">"Recursive patterns"</a>
and
<a href="#groupsassubroutines">"Groups as subroutines"</a>
below for details of recursion and subroutine calls.
</P>
<P>
If a condition is the string (R), and there is no capture group with the name
R, the condition is true if matching is currently in a recursion or subroutine
call to the whole pattern or any capture group. If digits follow the letter R,
and there is no group with that name, the condition is true if the most recent
call is into a group with the given number, which must exist somewhere in the
overall pattern. This is a contrived example that is equivalent to a+b:
<pre>
((?(R1)a+|(?1)b))
</pre>
However, in both cases, if there is a capture group with a matching name, the
condition tests for its being set, as described in the section above, instead
of testing for recursion. For example, creating a group with the name R1 by
adding (?<R1>) to the above pattern completely changes its meaning.
</P>
<P>
If a name preceded by ampersand follows the letter R, for example:
<pre>
(?(R&name)...)
</pre>
the condition is true if the most recent recursion is into a group of that name
(which must exist within the pattern).
</P>
<P>
This condition does not check the entire recursion stack. It tests only the
current level. If the name used in a condition of this kind is a duplicate, the
test is applied to all groups of the same name, and is true if any one of
them is the most recent recursion.
</P>
<P>
At "top level", all these recursion test conditions are false.
<a name="subdefine"></a></P>
<br><b>
Defining capture groups for use by reference only
</b><br>
<P>
If the condition is the string (DEFINE), the condition is always false, even if
there is a group with the name DEFINE. In this case, there may be only one
alternative in the rest of the conditional group. It is always skipped if
control reaches this point in the pattern; the idea of DEFINE is that it can be
used to define subroutines that can be referenced from elsewhere. (The use of
<a href="#groupsassubroutines">subroutines</a>
is described below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore white space and line
breaks):
<pre>
(?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
\b (?&byte) (\.(?&byte)){3} \b
</pre>
The first part of the pattern is a DEFINE group inside which a another group
named "byte" is defined. This matches an individual component of an IPv4
address (a number less than 256). When matching takes place, this part of the
pattern is skipped because DEFINE acts like a false condition. The rest of the
pattern uses references to the named group to match the four dot-separated
components of an IPv4 address, insisting on a word boundary at each end.
</P>
<br><b>
Checking the PCRE2 version
</b><br>
<P>
Programs that link with a PCRE2 library can check the version by calling
<b>pcre2_config()</b> with appropriate arguments. Users of applications that do
not have access to the underlying code cannot do this. A special "condition"
called VERSION exists to allow such users to discover which version of PCRE2
they are dealing with by using this condition to match a string such as
"yesno". VERSION must be followed either by "=" or ">=" and a version number.
For example:
<pre>
(?(VERSION>=10.4)yes|no)
</pre>
This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or
"no" otherwise. The fractional part of the version number may not contain more
than two digits.
</P>
<br><b>
Assertion conditions
</b><br>
<P>
If the condition is not in any of the above formats, it must be a parenthesized
assertion. This may be a positive or negative lookahead or lookbehind
assertion. However, it must be a traditional atomic assertion, not one of the
PCRE2-specific
<a href="#nonatomicassertions">non-atomic assertions.</a>
</P>
<P>
Consider this pattern, again containing non-significant white space, and with
the two alternatives on the second line:
<pre>
(?(?=[^a-z]*[a-z])
\d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
</pre>
The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
</P>
<P>
When an assertion that is a condition contains capture groups, any
capturing that occurs in a matching branch is retained afterwards, for both
positive and negative assertions, because matching always continues after the
assertion, whether it succeeds or fails. (Compare non-conditional assertions,
for which captures are retained only for positive assertions that succeed.)
<a name="comments"></a></P>
<br><a name="SEC24" href="#TOC1">COMMENTS</a><br>
<P>
There are two ways of including comments in patterns that are processed by
PCRE2. In both cases, the start of the comment must not be in a character
class, nor in the middle of any other sequence of related characters such as
(?: or a group name or number. The characters that make up a comment play
no part in the pattern matching.
</P>
<P>
The sequence (?# marks the start of a comment that continues up to the next
closing parenthesis. Nested parentheses are not permitted. If the
PCRE2_EXTENDED or PCRE2_EXTENDED_MORE option is set, an unescaped # character
also introduces a comment, which in this case continues to immediately after
the next newline character or character sequence in the pattern. Which
characters are interpreted as newlines is controlled by an option passed to the
compiling function or by a special sequence at the start of the pattern, as
described in the section entitled
<a href="#newlines">"Newline conventions"</a>
above. Note that the end of this type of comment is a literal newline sequence
in the pattern; escape sequences that happen to represent a newline do not
count. For example, consider this pattern when PCRE2_EXTENDED is set, and the
default newline convention (a single linefeed character) is in force:
<pre>
abc #comment \n still comment
</pre>
On encountering the # character, <b>pcre2_compile()</b> skips along, looking for
a newline in the pattern. The sequence \n is still literal at this stage, so
it does not terminate the comment. Only an actual character with the code value
0x0a (the default newline) does so.
<a name="recursion"></a></P>
<br><a name="SEC25" href="#TOC1">RECURSIVE PATTERNS</a><br>
<P>
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best that can
be done is to use a pattern that matches up to some fixed depth of nesting. It
is not possible to handle an arbitrary nesting depth.
</P>
<P>
For some time, Perl has provided a facility that allows regular expressions to
recurse (amongst other things). It does this by interpolating Perl code in the
expression at run time, and the code can refer to the expression itself. A Perl
pattern using code interpolation to solve the parentheses problem can be
created like this:
<pre>
$re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
</pre>
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.
</P>
<P>
Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual capture group recursion. After its introduction in PCRE1 and Python,
this kind of recursion was subsequently introduced into Perl at release 5.10.
</P>
<P>
A special item that consists of (? followed by a number greater than zero and a
closing parenthesis is a recursive subroutine call of the capture group of the
given number, provided that it occurs inside that group. (If not, it is a
<a href="#groupsassubroutines">non-recursive subroutine</a>
call, which is described in the next section.) The special item (?R) or (?0) is
a recursive call of the entire regular expression.
</P>
<P>
This PCRE2 pattern solves the nested parentheses problem (assume the
PCRE2_EXTENDED option is set so that white space is ignored):
<pre>
\( ( [^()]++ | (?R) )* \)
</pre>
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is, a correctly parenthesized substring).
Finally there is a closing parenthesis. Note the use of a possessive quantifier
to avoid backtracking into sequences of non-parentheses.
</P>
<P>
If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:
<pre>
( \( ( [^()]++ | (?1) )* \) )
</pre>
We have put the pattern into parentheses, and caused the recursion to refer to
them instead of the whole pattern.
</P>
<P>
In a larger pattern, keeping track of parenthesis numbers can be tricky. This
is made easier by the use of relative references. Instead of (?1) in the
pattern above you can write (?-2) to refer to the second most recently opened
parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered.
</P>
<P>
Be aware however, that if
<a href="#dupgroupnumber">duplicate capture group numbers</a>
are in use, relative references refer to the earliest group with the
appropriate number. Consider, for example:
<pre>
(?|(a)|(b)) (c) (?-2)
</pre>
The first two capture groups (a) and (b) are both numbered 1, and group (c)
is number 2. When the reference (?-2) is encountered, the second most recently
opened parentheses has the number 1, but it is the first such group (the (a)
group) to which the recursion refers. This would be the same if an absolute
reference (?1) was used. In other words, relative references are just a
shorthand for computing a group number.
</P>
<P>
It is also possible to refer to subsequent capture groups, by writing
references such as (?+2). However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always
<a href="#groupsassubroutines">non-recursive subroutine</a>
calls, as described in the next section.
</P>
<P>
An alternative approach is to use named parentheses. The Perl syntax for this
is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could
rewrite the above example as follows:
<pre>
(?<pn> \( ( [^()]++ | (?&pn) )* \) )
</pre>
If there is more than one group with the same name, the earliest one is
used.
</P>
<P>
The example pattern that we have been looking at contains nested unlimited
repeats, and so the use of a possessive quantifier for matching strings of
non-parentheses is important when applying the pattern to strings that do not
match. For example, when this pattern is applied to
<pre>
(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
</pre>
it yields "no match" quickly. However, if a possessive quantifier is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
before failure can be reported.
</P>
<P>
At the end of a match, the values of capturing parentheses are those from
the outermost level. If you want to obtain intermediate values, a callout
function can be used (see below and the
<a href="pcre2callout.html"><b>pcre2callout</b></a>
documentation). If the pattern above is matched against
<pre>
(ab(cd)ef)
</pre>
the value for the inner capturing parentheses (numbered 2) is "ef", which is
the last value taken on at the top level. If a capture group is not matched at
the top level, its final captured value is unset, even if it was (temporarily)
set at a deeper level during the matching process.
</P>
<P>
Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.
<pre>
< (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
</pre>
In this pattern, (?(R) is the start of a conditional group, with two different
alternatives for the recursive and non-recursive cases. The (?R) item is the
actual recursive call.
<a name="recursiondifference"></a></P>
<br><b>
Differences in recursion processing between PCRE2 and Perl
</b><br>
<P>
Some former differences between PCRE2 and Perl no longer exist.
</P>
<P>
Before release 10.30, recursion processing in PCRE2 differed from Perl in that
a recursive subroutine call was always treated as an atomic group. That is,
once it had matched some of the subject string, it was never re-entered, even
if it contained untried alternatives and there was a subsequent matching
failure. (Historical note: PCRE implemented recursion before Perl did.)
</P>
<P>
Starting with release 10.30, recursive subroutine calls are no longer treated
as atomic. That is, they can be re-entered to try unused alternatives if there
is a matching failure later in the pattern. This is now compatible with the way
Perl works. If you want a subroutine call to be atomic, you must explicitly
enclose it in an atomic group.
</P>
<P>
Supporting backtracking into recursions simplifies certain types of recursive
pattern. For example, this pattern matches palindromic strings:
<pre>
^((.)(?1)\2|.?)$
</pre>
The second branch in the group matches a single central character in the
palindrome when there are an odd number of characters, or nothing when there
are an even number of characters, but in order to work it has to be able to try
the second case when the rest of the pattern match fails. If you want to match
typical palindromic phrases, the pattern has to ignore all non-word characters,
which can be done like this:
<pre>
^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$
</pre>
If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A
man, a plan, a canal: Panama!". Note the use of the possessive quantifier *+ to
avoid backtracking into sequences of non-word characters. Without this, PCRE2
takes a great deal longer (ten times or more) to match typical phrases, and
Perl takes so long that you think it has gone into a loop.
</P>
<P>
Another way in which PCRE2 and Perl used to differ in their recursion
processing is in the handling of captured values. Formerly in Perl, when a
group was called recursively or as a subroutine (see the next section), it
had no access to any values that were captured outside the recursion, whereas
in PCRE2 these values can be referenced. Consider this pattern:
<pre>
^(.)(\1|a(?2))
</pre>
This pattern matches "bab". The first capturing parentheses match "b", then in
the second group, when the backreference \1 fails to match "b", the second
alternative matches "a" and then recurses. In the recursion, \1 does now match
"b" and so the whole match succeeds. This match used to fail in Perl, but in
later versions (I tried 5.024) it now works.
<a name="groupsassubroutines"></a></P>
<br><a name="SEC26" href="#TOC1">GROUPS AS SUBROUTINES</a><br>
<P>
If the syntax for a recursive group call (either by number or by name) is used
outside the parentheses to which it refers, it operates a bit like a subroutine
in a programming language. More accurately, PCRE2 treats the referenced group
as an independent subpattern which it tries to match at the current matching
position. The called group may be defined before or after the reference. A
numbered reference can be absolute or relative, as in these examples:
<pre>
(...(absolute)...)...(?2)...
(...(relative)...)...(?-1)...
(...(?+1)...(relative)...
</pre>
An earlier example pointed out that the pattern
<pre>
(sens|respons)e and \1ibility
</pre>
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern
<pre>
(sens|respons)e and (?1)ibility
</pre>
is used, it does match "sense and responsibility" as well as the other two
strings. Another example is given in the discussion of DEFINE above.
</P>
<P>
Like recursions, subroutine calls used to be treated as atomic, but this
changed at PCRE2 release 10.30, so backtracking into subroutine calls can now
occur. However, any capturing parentheses that are set during the subroutine
call revert to their previous values afterwards.
</P>
<P>
Processing options such as case-independence are fixed when a group is
defined, so if it is used as a subroutine, such options cannot be changed for
different calls. For example, consider this pattern:
<pre>
(abc)(?i:(?-1))
</pre>
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called group.
</P>
<P>
The behaviour of
<a href="#backtrackcontrol">backtracking control verbs</a>
in groups when called as subroutines is described in the section entitled
<a href="#btsub">"Backtracking verbs in subroutines"</a>
below.
<a name="onigurumasubroutines"></a></P>
<br><a name="SEC27" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
<P>
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for calling a group as a subroutine, possibly recursively. Here are two
of the examples used above, rewritten using this syntax:
<pre>
(?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
(sens|respons)e and \g'1'ibility
</pre>
PCRE2 supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
<pre>
(abc)(?i:\g<-1>)
</pre>
Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are <i>not</i>
synonymous. The former is a backreference; the latter is a subroutine call.
</P>
<br><a name="SEC28" href="#TOC1">CALLOUTS</a><br>
<P>
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes it
possible, amongst other things, to extract different substrings that match the
same pair of parentheses when there is a repetition.
</P>
<P>
PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE2 provides an external
function by putting its entry point in a match context using the function
<b>pcre2_set_callout()</b>, and then passing that context to <b>pcre2_match()</b>
or <b>pcre2_dfa_match()</b>. If no match context is passed, or if the callout
entry point is set to NULL, callouts are disabled.
</P>
<P>
Within a regular expression, (?C<arg>) indicates a point at which the external
function is to be called. There are two kinds of callout: those with a
numerical argument and those with a string argument. (?C) on its own with no
argument is treated as (?C0). A numerical argument allows the application to
distinguish between different callouts. String arguments were added for release
10.20 to make it possible for script languages that use PCRE2 to embed short
scripts within patterns in a similar way to Perl.
</P>
<P>
During matching, when PCRE2 reaches a callout point, the external function is
called. It is provided with the number or string argument of the callout, the
position in the pattern, and one item of data that is also set in the match
block. The callout function may cause matching to proceed, to backtrack, or to
fail.
</P>
<P>
By default, PCRE2 implements a number of optimizations at matching time, and
one side-effect is that sometimes callouts are skipped. If you need all
possible callouts to happen, you need to set options that disable the relevant
optimizations. More details, including a complete description of the
programming interface to the callout function, are given in the
<a href="pcre2callout.html"><b>pcre2callout</b></a>
documentation.
</P>
<br><b>
Callouts with numerical arguments
</b><br>
<P>
If you just want to have a means of identifying different callout points, put a
number less than 256 after the letter C. For example, this pattern has two
callout points:
<pre>
(?C1)abc(?C2)def
</pre>
If the PCRE2_AUTO_CALLOUT flag is passed to <b>pcre2_compile()</b>, numerical
callouts are automatically installed before each item in the pattern. They are
all numbered 255. If there is a conditional group in the pattern whose
condition is an assertion, an additional callout is inserted just before the
condition. An explicit callout may also be set at this position, as in this
example:
<pre>
(?(?C9)(?=a)abc|def)
</pre>
Note that this applies only to assertion conditions, not to other types of
condition.
</P>
<br><b>
Callouts with string arguments
</b><br>
<P>
A delimited string may be used instead of a number as a callout argument. The
starting delimiter must be one of ` ' " ^ % # $ { and the ending delimiter is
the same as the start, except for {, where the ending delimiter is }. If the
ending delimiter is needed within the string, it must be doubled. For
example:
<pre>
(?C'ab ''c'' d')xyz(?C{any text})pqr
</pre>
The doubling is removed before the string is passed to the callout function.
<a name="backtrackcontrol"></a></P>
<br><a name="SEC29" href="#TOC1">BACKTRACKING CONTROL</a><br>
<P>
There are a number of special "Backtracking Control Verbs" (to use Perl's
terminology) that modify the behaviour of backtracking during matching. They
are generally of the form (*VERB) or (*VERB:NAME). Some verbs take either form,
and may behave differently depending on whether or not a name argument is
present. The names are not required to be unique within the pattern.
</P>
<P>
By default, for compatibility with Perl, a name is any sequence of characters
that does not include a closing parenthesis. The name is not processed in
any way, and it is not possible to include a closing parenthesis in the name.
This can be changed by setting the PCRE2_ALT_VERBNAMES option, but the result
is no longer Perl-compatible.
</P>
<P>
When PCRE2_ALT_VERBNAMES is set, backslash processing is applied to verb names
and only an unescaped closing parenthesis terminates the name. However, the
only backslash items that are permitted are \Q, \E, and sequences such as
\x{100} that define character code points. Character type escapes such as \d
are faulted.
</P>
<P>
A closing parenthesis can be included in a name either as \) or between \Q
and \E. In addition to backslash processing, if the PCRE2_EXTENDED or
PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb names is
skipped, and #-comments are recognized, exactly as in the rest of the pattern.
PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do not affect verb names unless
PCRE2_ALT_VERBNAMES is also set.
</P>
<P>
The maximum length of a name is 255 in the 8-bit library and 65535 in the
16-bit and 32-bit libraries. If the name is empty, that is, if the closing
parenthesis immediately follows the colon, the effect is as if the colon were
not there. Any number of these verbs may occur in a pattern. Except for
(*ACCEPT), they may not be quantified.
</P>
<P>
Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using the traditional matching
function, because that uses a backtracking algorithm. With the exception of
(*FAIL), which behaves like a failing negative assertion, the backtracking
control verbs cause an error if encountered by the DFA matching function.
</P>
<P>
The behaviour of these verbs in
<a href="#btrepeat">repeated groups,</a>
<a href="#btassert">assertions,</a>
and in
<a href="#btsub">capture groups called as subroutines</a>
(whether or not recursively) is documented below.
<a name="nooptimize"></a></P>
<br><b>
Optimizations that affect backtracking verbs
</b><br>
<P>
PCRE2 contains some optimizations that are used to speed up matching by running
some checks at the start of each match attempt. For example, it may know the
minimum length of matching subject, or that a particular character must be
present. When one of these optimizations bypasses the running of a match, any
included backtracking verbs will not, of course, be processed. You can suppress
the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option
when calling <b>pcre2_compile()</b>, or by starting the pattern with
(*NO_START_OPT). There is more discussion of this option in the section
entitled
<a href="pcre2api.html#compiling">"Compiling a pattern"</a>
in the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation.
</P>
<P>
Experiments with Perl suggest that it too has similar optimizations, and like
PCRE2, turning them off can change the result of a match.
<a name="acceptverb"></a></P>
<br><b>
Verbs that act immediately
</b><br>
<P>
The following verbs act as soon as they are encountered.
<pre>
(*ACCEPT) or (*ACCEPT:NAME)
</pre>
This verb causes the match to end successfully, skipping the remainder of the
pattern. However, when it is inside a capture group that is called as a
subroutine, only that group is ended successfully. Matching then continues
at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
assertion succeeds; in a negative assertion, the assertion fails.
</P>
<P>
If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
example:
<pre>
A((?:A|B(*ACCEPT)|C)D)
</pre>
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
the outer parentheses.
</P>
<P>
(*ACCEPT) is the only backtracking verb that is allowed to be quantified
because an ungreedy quantification with a minimum of zero acts only when a
backtrack happens. Consider, for example,
<pre>
(A(*ACCEPT)??B)C
</pre>
where A, B, and C may be complex expressions. After matching "A", the matcher
processes "BC"; if that fails, causing a backtrack, (*ACCEPT) is triggered and
the match succeeds. In both cases, all but C is captured. Whereas (*COMMIT)
(see below) means "fail on backtrack", a repeated (*ACCEPT) of this type means
"succeed on backtrack".
</P>
<P>
<b>Warning:</b> (*ACCEPT) should not be used within a script run group, because
it causes an immediate exit from the group, bypassing the script run checking.
<pre>
(*FAIL) or (*FAIL:NAME)
</pre>
This verb causes a matching failure, forcing backtracking to occur. It may be
abbreviated to (*F). It is equivalent to (?!) but easier to read. The Perl
documentation notes that it is probably useful only when combined with (?{}) or
(??{}). Those are, of course, Perl features that are not present in PCRE2. The
nearest equivalent is the callout feature, as for example in this pattern:
<pre>
a+(?C)(*FAIL)
</pre>
A match with the string "aaaa" always fails, but the callout is taken before
each backtrack happens (in this example, 10 times).
</P>
<P>
(*ACCEPT:NAME) and (*FAIL:NAME) behave the same as (*MARK:NAME)(*ACCEPT) and
(*MARK:NAME)(*FAIL), respectively, that is, a (*MARK) is recorded just before
the verb acts.
</P>
<br><b>
Recording which path was taken
</b><br>
<P>
There is one verb whose main purpose is to track how a match was arrived at,
though it also has a secondary use in conjunction with advancing the match
starting point (see (*SKIP) below).
<pre>
(*MARK:NAME) or (*:NAME)
</pre>
A name is always required with this verb. For all the other backtracking
control verbs, a NAME argument is optional.
</P>
<P>
When a match succeeds, the name of the last-encountered mark name on the
matching path is passed back to the caller as described in the section entitled
<a href="pcre2api.html#matchotherdata">"Other information about the match"</a>
in the
<a href="pcre2api.html"><b>pcre2api</b></a>
documentation. This applies to all instances of (*MARK) and other verbs,
including those inside assertions and atomic groups. However, there are
differences in those cases when (*MARK) is used in conjunction with (*SKIP) as
described below.
</P>
<P>
The mark name that was last encountered on the matching path is passed back. A
verb without a NAME argument is ignored for this purpose. Here is an example of
<b>pcre2test</b> output, where the "mark" modifier requests the retrieval and
outputting of (*MARK) data:
<pre>
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XY
0: XY
MK: A
XZ
0: XZ
MK: B
</pre>
The (*MARK) name is tagged with "MK:" in this output, and in this example it
indicates which of the two alternatives matched. This is a more efficient way
of obtaining this information than putting each alternative in its own
capturing parentheses.
</P>
<P>
If a verb with a name is encountered in a positive assertion that is true, the
name is recorded and passed back if it is the last-encountered. This does not
happen for negative assertions or failing positive assertions.
</P>
<P>
After a partial match or a failed match, the last encountered name in the
entire match process is returned. For example:
<pre>
re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
data> XP
No match, mark = B
</pre>
Note that in this unanchored example the mark is retained from the match
attempt that started at the letter "X" in the subject. Subsequent match
attempts starting at "P" and then with an empty string do not get as far as the
(*MARK) item, but nevertheless do not reset it.
</P>
<P>
If you are interested in (*MARK) values after failed matches, you should
probably set the PCRE2_NO_START_OPTIMIZE option
<a href="#nooptimize">(see above)</a>
to ensure that the match is always attempted.
</P>
<br><b>
Verbs that act after backtracking
</b><br>
<P>
The following verbs do nothing when they are encountered. Matching continues
with what follows, but if there is a subsequent match failure, causing a
backtrack to the verb, a failure is forced. That is, backtracking cannot pass
to the left of the verb. However, when one of these verbs appears inside an
atomic group or in a lookaround assertion that is true, its effect is confined
to that group, because once the group has been matched, there is never any
backtracking into it. Backtracking from beyond an assertion or an atomic group
ignores the entire group, and seeks a preceding backtracking point.
</P>
<P>
These verbs differ in exactly what kind of failure occurs when backtracking
reaches them. The behaviour described below is what happens when the verb is
not in a subroutine or an assertion. Subsequent sections cover these special
cases.
<pre>
(*COMMIT) or (*COMMIT:NAME)
</pre>
This verb causes the whole match to fail outright if there is a later matching
failure that causes backtracking to reach it. Even if the pattern is
unanchored, no further attempts to find a match by advancing the starting point
take place. If (*COMMIT) is the only backtracking verb that is encountered,
once it has been passed <b>pcre2_match()</b> is committed to finding a match at
the current starting point, or not at all. For example:
<pre>
a+(*COMMIT)b
</pre>
This matches "xxaab" but not "aacaab". It can be thought of as a kind of
dynamic anchor, or "I've started, so I must finish."
</P>
<P>
The behaviour of (*COMMIT:NAME) is not the same as (*MARK:NAME)(*COMMIT). It is
like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names that are set with
(*MARK), ignoring those set by any of the other backtracking verbs.
</P>
<P>
If there is more than one backtracking verb in a pattern, a different one that
follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
match does not always guarantee that a match must be at this starting point.
</P>
<P>
Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
unless PCRE2's start-of-match optimizations are turned off, as shown in this
output from <b>pcre2test</b>:
<pre>
re> /(*COMMIT)abc/
data> xyzabc
0: abc
data>
re> /(*COMMIT)abc/no_start_optimize
data> xyzabc
No match
</pre>
For the first pattern, PCRE2 knows that any match must start with "a", so the
optimization skips along the subject to "a" before applying the pattern to the
first set of data. The match attempt then succeeds. The second pattern disables
the optimization that skips along to the first character. The pattern is now
applied starting at "x", and so the (*COMMIT) causes the match to fail without
trying any other starting points.
<pre>
(*PRUNE) or (*PRUNE:NAME)
</pre>
This verb causes the match to fail at the current starting position in the
subject if there is a later matching failure that causes backtracking to reach
it. If the pattern is unanchored, the normal "bumpalong" advance to the next
starting character then happens. Backtracking can occur as usual to the left of
(*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
if there is no match to the right, backtracking cannot cross (*PRUNE). In
simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
possessive quantifier, but there are some uses of (*PRUNE) that cannot be
expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
as (*COMMIT).
</P>
<P>
The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE). It is
like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names set with (*MARK),
ignoring those set by other backtracking verbs.
<pre>
(*SKIP)
</pre>
This verb, when given without a name, is like (*PRUNE), except that if the
pattern is unanchored, the "bumpalong" advance is not to the next character,
but to the position in the subject where (*SKIP) was encountered. (*SKIP)
signifies that whatever text was matched leading up to it cannot be part of a
successful match if there is a later mismatch. Consider:
<pre>
a+(*SKIP)b
</pre>
If the subject is "aaaac...", after the first match attempt fails (starting at
the first character in the string), the starting point skips on to start the
next attempt at "c". Note that a possessive quantifier does not have the same
effect as this example; although it would suppress backtracking during the
first match attempt, the second attempt would start at the second character
instead of skipping on to "c".
</P>
<P>
If (*SKIP) is used to specify a new starting position that is the same as the
starting position of the current match, or (by being inside a lookbehind)
earlier, the position specified by (*SKIP) is ignored, and instead the normal
"bumpalong" occurs.
<pre>
(*SKIP:NAME)
</pre>
When (*SKIP) has an associated name, its behaviour is modified. When such a
(*SKIP) is triggered, the previous path through the pattern is searched for the
most recent (*MARK) that has the same name. If one is found, the "bumpalong"
advance is to the subject position that corresponds to that (*MARK) instead of
to where (*SKIP) was encountered. If no (*MARK) with a matching name is found,
the (*SKIP) is ignored.
</P>
<P>
The search for a (*MARK) name uses the normal backtracking mechanism, which
means that it does not see (*MARK) settings that are inside atomic groups or
assertions, because they are never re-entered by backtracking. Compare the
following <b>pcre2test</b> examples:
<pre>
re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
data: abc
0: a
1: a
data:
re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
data: abc
0: b
1: b
</pre>
In the first example, the (*MARK) setting is in an atomic group, so it is not
seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored. This allows
the second branch of the pattern to be tried at the first character position.
In the second example, the (*MARK) setting is not in an atomic group. This
allows (*SKIP:X) to find the (*MARK) when it backtracks, and this causes a new
matching attempt to start at the second character. This time, the (*MARK) is
never seen because "a" does not match "b", so the matcher immediately jumps to
the second branch of the pattern.
</P>
<P>
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
names that are set by other backtracking verbs.
<pre>
(*THEN) or (*THEN:NAME)
</pre>
This verb causes a skip to the next innermost alternative when backtracking
reaches it. That is, it cancels any further backtracking within the current
alternative. Its name comes from the observation that it can be used for a
pattern-based if-then-else block:
<pre>
( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
</pre>
If the COND1 pattern matches, FOO is tried (and possibly further items after
the end of the group if FOO succeeds); on failure, the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. If that
succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
more alternatives, so there is a backtrack to whatever came before the entire
group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
</P>
<P>
The behaviour of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN). It is
like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names set with (*MARK),
ignoring those set by other backtracking verbs.
</P>
<P>
A group that does not contain a | character is just a part of the enclosing
alternative; it is not a nested alternation with only one alternative. The
effect of (*THEN) extends beyond such a group to the enclosing alternative.
Consider this pattern, where A, B, etc. are complex pattern fragments that do
not contain any | characters at this level:
<pre>
A (B(*THEN)C) | D
</pre>
If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
However, if the group containing (*THEN) is given an alternative, it
behaves differently:
<pre>
A (B(*THEN)C | (*FAIL)) | D
</pre>
The effect of (*THEN) is now confined to the inner group. After a failure in C,
matching moves to (*FAIL), which causes the whole group to fail because there
are no more alternatives to try. In this case, matching does backtrack into A.
</P>
<P>
Note that a conditional group is not considered as having two alternatives,
because only one is ever used. In other words, the | character in a conditional
group has a different meaning. Ignoring white space, consider:
<pre>
^.*? (?(?=a) a | b(*THEN)c )
</pre>
If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
it initially matches zero characters. The condition (?=a) then fails, the
character "b" is matched, but "c" is not. At this point, matching does not
backtrack to .*? as might perhaps be expected from the presence of the |
character. The conditional group is part of the single alternative that
comprises the whole pattern, and so the match fails. (If there was a backtrack
into .*?, allowing it to match "b", the match would succeed.)
</P>
<P>
The verbs just described provide four different "strengths" of control when
subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
next alternative. (*PRUNE) comes next, failing the match at the current
starting position, but allowing an advance to the next character (for an
unanchored pattern). (*SKIP) is similar, except that the advance may be more
than one character. (*COMMIT) is the strongest, causing the entire match to
fail.
</P>
<br><b>
More than one backtracking verb
</b><br>
<P>
If more than one backtracking verb is present in a pattern, the one that is
backtracked onto first acts. For example, consider this pattern, where A, B,
etc. are complex pattern fragments:
<pre>
(A(*COMMIT)B(*THEN)C|ABD)
</pre>
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
the next alternative (ABD) to be tried. This behaviour is consistent, but is
not always the same as Perl's. It means that if two or more backtracking verbs
appear in succession, all the the last of them has no effect. Consider this
example:
<pre>
...(*COMMIT)(*PRUNE)...
</pre>
If there is a matching failure to the right, backtracking onto (*PRUNE) causes
it to be triggered, and its action is taken. There can never be a backtrack
onto (*COMMIT).
<a name="btrepeat"></a></P>
<br><b>
Backtracking verbs in repeated groups
</b><br>
<P>
PCRE2 sometimes differs from Perl in its handling of backtracking verbs in
repeated groups. For example, consider:
<pre>
/(a(*COMMIT)b)+ac/
</pre>
If the subject is "abac", Perl matches unless its optimizations are disabled,
but PCRE2 always fails because the (*COMMIT) in the second repeat of the group
acts.
<a name="btassert"></a></P>
<br><b>
Backtracking verbs in assertions
</b><br>
<P>
(*FAIL) in any assertion has its normal effect: it forces an immediate
backtrack. The behaviour of the other backtracking verbs depends on whether or
not the assertion is standalone or acting as the condition in a conditional
group.
</P>
<P>
(*ACCEPT) in a standalone positive assertion causes the assertion to succeed
without any further processing; captured strings and a mark name (if set) are
retained. In a standalone negative assertion, (*ACCEPT) causes the assertion to
fail without any further processing; captured substrings and any mark name are
discarded.
</P>
<P>
If the assertion is a condition, (*ACCEPT) causes the condition to be true for
a positive assertion and false for a negative one; captured substrings are
retained in both cases.
</P>
<P>
The remaining verbs act only when a later failure causes a backtrack to
reach them. This means that, for the Perl-compatible assertions, their effect
is confined to the assertion, because Perl lookaround assertions are atomic. A
backtrack that occurs after such an assertion is complete does not jump back
into the assertion. Note in particular that a (*MARK) name that is set in an
assertion is not "seen" by an instance of (*SKIP:NAME) later in the pattern.
</P>
<P>
PCRE2 now supports non-atomic positive assertions, as described in the section
entitled
<a href="#nonatomicassertions">"Non-atomic assertions"</a>
above. These assertions must be standalone (not used as conditions). They are
not Perl-compatible. For these assertions, a later backtrack does jump back
into the assertion, and therefore verbs such as (*COMMIT) can be triggered by
backtracks from later in the pattern.
</P>
<P>
The effect of (*THEN) is not allowed to escape beyond an assertion. If there
are no more branches to try, (*THEN) causes a positive assertion to be false,
and a negative assertion to be true.
</P>
<P>
The other backtracking verbs are not treated specially if they appear in a
standalone positive assertion. In a conditional positive assertion,
backtracking (from within the assertion) into (*COMMIT), (*SKIP), or (*PRUNE)
causes the condition to be false. However, for both standalone and conditional
negative assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes
the assertion to be true, without considering any further alternative branches.
<a name="btsub"></a></P>
<br><b>
Backtracking verbs in subroutines
</b><br>
<P>
These behaviours occur whether or not the group is called recursively.
</P>
<P>
(*ACCEPT) in a group called as a subroutine causes the subroutine match to
succeed without any further processing. Matching then continues after the
subroutine call. Perl documents this behaviour. Perl's treatment of the other
verbs in subroutines is different in some cases.
</P>
<P>
(*FAIL) in a group called as a subroutine has its normal effect: it forces
an immediate backtrack.
</P>
<P>
(*COMMIT), (*SKIP), and (*PRUNE) cause the subroutine match to fail when
triggered by being backtracked to in a group called as a subroutine. There is
then a backtrack at the outer level.
</P>
<P>
(*THEN), when triggered, skips to the next alternative in the innermost
enclosing group that has alternatives (its normal behaviour). However, if there
is no such group within the subroutine's group, the subroutine match fails and
there is a backtrack at the outer level.
</P>
<br><a name="SEC30" href="#TOC1">SEE ALSO</a><br>
<P>
<b>pcre2api</b>(3), <b>pcre2callout</b>(3), <b>pcre2matching</b>(3),
<b>pcre2syntax</b>(3), <b>pcre2</b>(3).
</P>
<br><a name="SEC31" href="#TOC1">AUTHOR</a><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge, England.
<br>
</P>
<br><a name="SEC32" href="#TOC1">REVISION</a><br>
<P>
Last updated: 06 October 2020
<br>
Copyright © 1997-2020 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE2 index page</a>.
</p>
|
