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
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Perl Regular Expression Syntax</title>
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.77.1">
<link rel="home" href="../../index.html" title="Boost.Regex 5.0.1">
<link rel="up" href="../syntax.html" title="Regular Expression Syntax">
<link rel="prev" href="../syntax.html" title="Regular Expression Syntax">
<link rel="next" href="basic_extended.html" title="POSIX Extended Regular Expression Syntax">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../syntax.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../syntax.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="basic_extended.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_regex.syntax.perl_syntax"></a><a class="link" href="perl_syntax.html" title="Perl Regular Expression Syntax">Perl Regular Expression
Syntax</a>
</h3></div></div></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h0"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.synopsis"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.synopsis">Synopsis</a>
</h4>
<p>
The Perl regular expression syntax is based on that used by the programming
language Perl . Perl regular expressions are the default behavior in Boost.Regex
or you can pass the flag <code class="literal">perl</code> to the <a class="link" href="../ref/basic_regex.html" title="basic_regex"><code class="computeroutput"><span class="identifier">basic_regex</span></code></a> constructor, for example:
</p>
<pre class="programlisting"><span class="comment">// e1 is a case sensitive Perl regular expression: </span>
<span class="comment">// since Perl is the default option there's no need to explicitly specify the syntax used here:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">);</span>
<span class="comment">// e2 a case insensitive Perl regular expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e2</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">perl</span><span class="special">|</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">icase</span><span class="special">);</span>
</pre>
<h4>
<a name="boost_regex.syntax.perl_syntax.h1"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.perl_regular_expression_syntax"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.perl_regular_expression_syntax">Perl
Regular Expression Syntax</a>
</h4>
<p>
In Perl regular expressions, all characters match themselves except for the
following special characters:
</p>
<pre class="programlisting">.[{}()\*+?|^$</pre>
<h5>
<a name="boost_regex.syntax.perl_syntax.h2"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.wildcard"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.wildcard">Wildcard</a>
</h5>
<p>
The single character '.' when used outside of a character set will match
any single character except:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The NULL character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
<code class="literal">match_not_dot_null</code></a> is passed to the matching
algorithms.
</li>
<li class="listitem">
The newline character when the <a class="link" href="../ref/match_flag_type.html" title="match_flag_type">flag
<code class="literal">match_not_dot_newline</code></a> is passed to the matching
algorithms.
</li>
</ul></div>
<h5>
<a name="boost_regex.syntax.perl_syntax.h3"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.anchors"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.anchors">Anchors</a>
</h5>
<p>
A '^' character shall match the start of a line.
</p>
<p>
A '$' character shall match the end of a line.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h4"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.marked_sub_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.marked_sub_expressions">Marked sub-expressions</a>
</h5>
<p>
A section beginning <code class="literal">(</code> and ending <code class="literal">)</code>
acts as a marked sub-expression. Whatever matched the sub-expression is split
out in a separate field by the matching algorithms. Marked sub-expressions
can also repeated, or referred to by a back-reference.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h5"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_marking_grouping"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_marking_grouping">Non-marking
grouping</a>
</h5>
<p>
A marked sub-expression is useful to lexically group part of a regular expression,
but has the side-effect of spitting out an extra field in the result. As
an alternative you can lexically group part of a regular expression, without
generating a marked sub-expression by using <code class="literal">(?:</code> and <code class="literal">)</code>
, for example <code class="literal">(?:ab)+</code> will repeat <code class="literal">ab</code>
without splitting out any separate sub-expressions.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h6"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.repeats">Repeats</a>
</h5>
<p>
Any atom (a single character, a marked sub-expression, or a character class)
can be repeated with the <code class="literal">*</code>, <code class="literal">+</code>, <code class="literal">?</code>,
and <code class="literal">{}</code> operators.
</p>
<p>
The <code class="literal">*</code> operator will match the preceding atom zero or more
times, for example the expression <code class="literal">a*b</code> will match any of
the following:
</p>
<pre class="programlisting"><span class="identifier">b</span>
<span class="identifier">ab</span>
<span class="identifier">aaaaaaaab</span>
</pre>
<p>
The <code class="literal">+</code> operator will match the preceding atom one or more
times, for example the expression <code class="literal">a+b</code> will match any of
the following:
</p>
<pre class="programlisting"><span class="identifier">ab</span>
<span class="identifier">aaaaaaaab</span>
</pre>
<p>
But will not match:
</p>
<pre class="programlisting"><span class="identifier">b</span>
</pre>
<p>
The <code class="literal">?</code> operator will match the preceding atom zero or one
times, for example the expression ca?b will match any of the following:
</p>
<pre class="programlisting"><span class="identifier">cb</span>
<span class="identifier">cab</span>
</pre>
<p>
But will not match:
</p>
<pre class="programlisting"><span class="identifier">caab</span>
</pre>
<p>
An atom can also be repeated with a bounded repeat:
</p>
<p>
<code class="literal">a{n}</code> Matches 'a' repeated exactly n times.
</p>
<p>
<code class="literal">a{n,}</code> Matches 'a' repeated n or more times.
</p>
<p>
<code class="literal">a{n, m}</code> Matches 'a' repeated between n and m times inclusive.
</p>
<p>
For example:
</p>
<pre class="programlisting">^a{2,3}$</pre>
<p>
Will match either of:
</p>
<pre class="programlisting"><span class="identifier">aa</span>
<span class="identifier">aaa</span>
</pre>
<p>
But neither of:
</p>
<pre class="programlisting"><span class="identifier">a</span>
<span class="identifier">aaaa</span>
</pre>
<p>
Note that the "{" and "}" characters will treated as
ordinary literals when used in a context that is not a repeat: this matches
Perl 5.x behavior. For example in the expressions "ab{1", "ab1}"
and "a{b}c" the curly brackets are all treated as literals and
<span class="emphasis"><em>no error will be raised</em></span>.
</p>
<p>
It is an error to use a repeat operator, if the preceding construct can not
be repeated, for example:
</p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">(*)</span>
</pre>
<p>
Will raise an error, as there is nothing for the <code class="literal">*</code> operator
to be applied to.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h7"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_greedy_repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_greedy_repeats">Non
greedy repeats</a>
</h5>
<p>
The normal repeat operators are "greedy", that is to say they will
consume as much input as possible. There are non-greedy versions available
that will consume as little input as possible while still producing a match.
</p>
<p>
<code class="literal">*?</code> Matches the previous atom zero or more times, while
consuming as little input as possible.
</p>
<p>
<code class="literal">+?</code> Matches the previous atom one or more times, while
consuming as little input as possible.
</p>
<p>
<code class="literal">??</code> Matches the previous atom zero or one times, while
consuming as little input as possible.
</p>
<p>
<code class="literal">{n,}?</code> Matches the previous atom n or more times, while
consuming as little input as possible.
</p>
<p>
<code class="literal">{n,m}?</code> Matches the previous atom between n and m times,
while consuming as little input as possible.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h8"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.possessive_repeats"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.possessive_repeats">Possessive
repeats</a>
</h5>
<p>
By default when a repeated pattern does not match then the engine will backtrack
until a match is found. However, this behaviour can sometime be undesireble
so there are also "possessive" repeats: these match as much as
possible and do not then allow backtracking if the rest of the expression
fails to match.
</p>
<p>
<code class="literal">*+</code> Matches the previous atom zero or more times, while
giving nothing back.
</p>
<p>
<code class="literal">++</code> Matches the previous atom one or more times, while
giving nothing back.
</p>
<p>
<code class="literal">?+</code> Matches the previous atom zero or one times, while
giving nothing back.
</p>
<p>
<code class="literal">{n,}+</code> Matches the previous atom n or more times, while
giving nothing back.
</p>
<p>
<code class="literal">{n,m}+</code> Matches the previous atom between n and m times,
while giving nothing back.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h9"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.back_references"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.back_references">Back
references</a>
</h5>
<p>
An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
is in the range 1-9, matches the same string that was matched by sub-expression
<span class="emphasis"><em>n</em></span>. For example the expression:
</p>
<pre class="programlisting">^(a*).*\1$</pre>
<p>
Will match the string:
</p>
<pre class="programlisting"><span class="identifier">aaabbaaa</span>
</pre>
<p>
But not the string:
</p>
<pre class="programlisting"><span class="identifier">aaabba</span>
</pre>
<p>
You can also use the \g escape for the same function, for example:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Escape
</p>
</th>
<th>
<p>
Meaning
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="literal">\g1</code>
</p>
</td>
<td>
<p>
Match whatever matched sub-expression 1
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\g{1}</code>
</p>
</td>
<td>
<p>
Match whatever matched sub-expression 1: this form allows for safer
parsing of the expression in cases like <code class="literal">\g{1}2</code>
or for indexes higher than 9 as in <code class="literal">\g{1234}</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\g-1</code>
</p>
</td>
<td>
<p>
Match whatever matched the last opened sub-expression
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\g{-2}</code>
</p>
</td>
<td>
<p>
Match whatever matched the last but one opened sub-expression
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\g{one}</code>
</p>
</td>
<td>
<p>
Match whatever matched the sub-expression named "one"
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
Finally the \k escape can be used to refer to named subexpressions, for example
<code class="literal">\k<two></code> will match whatever matched the subexpression
named "two".
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h10"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.alternation"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.alternation">Alternation</a>
</h5>
<p>
The <code class="literal">|</code> operator will match either of its arguments, so
for example: <code class="literal">abc|def</code> will match either "abc"
or "def".
</p>
<p>
Parenthesis can be used to group alternations, for example: <code class="literal">ab(d|ef)</code>
will match either of "abd" or "abef".
</p>
<p>
Empty alternatives are not allowed (these are almost always a mistake), but
if you really want an empty alternative use <code class="literal">(?:)</code> as a
placeholder, for example:
</p>
<p>
<code class="literal">|abc</code> is not a valid expression, but
</p>
<p>
<code class="literal">(?:)|abc</code> is and is equivalent, also the expression:
</p>
<p>
<code class="literal">(?:abc)??</code> has exactly the same effect.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h11"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_sets"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_sets">Character
sets</a>
</h5>
<p>
A character set is a bracket-expression starting with <code class="literal">[</code>
and ending with <code class="literal"></code>], it defines a set of characters, and
matches any single character that is a member of that set.
</p>
<p>
A bracket expression may contain any combination of the following:
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h12"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.single_characters"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.single_characters">Single
characters</a>
</h6>
<p>
For example <code class="literal">[abc]</code>, will match any of the characters 'a',
'b', or 'c'.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h13"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_ranges"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_ranges">Character
ranges</a>
</h6>
<p>
For example <code class="literal">[a-c]</code> will match any single character in the
range 'a' to 'c'. By default, for Perl regular expressions, a character x
is within the range y to z, if the code point of the character lies within
the codepoints of the endpoints of the range. Alternatively, if you set the
<a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions"><code class="literal">collate</code>
flag</a> when constructing the regular expression, then ranges are locale
sensitive.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h14"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.negation"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.negation">Negation</a>
</h6>
<p>
If the bracket-expression begins with the ^ character, then it matches the
complement of the characters it contains, for example <code class="literal">[^a-c]</code>
matches any character that is not in the range <code class="literal">a-c</code>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h15"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_classes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_classes">Character
classes</a>
</h6>
<p>
An expression of the form <code class="literal">[[:name:]]</code> matches the named
character class "name", for example <code class="literal">[[:lower:]]</code>
matches any lower case character. See <a class="link" href="character_classes.html" title="Character Class Names">character
class names</a>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h16"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.collating_elements"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.collating_elements">Collating
Elements</a>
</h6>
<p>
An expression of the form <code class="literal">[[.col.]]</code> matches the collating
element <span class="emphasis"><em>col</em></span>. A collating element is any single character,
or any sequence of characters that collates as a single unit. Collating elements
may also be used as the end point of a range, for example: <code class="literal">[[.ae.]-c]</code>
matches the character sequence "ae", plus any single character
in the range "ae"-c, assuming that "ae" is treated as
a single collating element in the current locale.
</p>
<p>
As an extension, a collating element may also be specified via it's <a class="link" href="collating_names.html" title="Collating Names">symbolic name</a>, for example:
</p>
<pre class="programlisting"><span class="special">[[.</span><span class="identifier">NUL</span><span class="special">.]]</span>
</pre>
<p>
matches a <code class="literal">\0</code> character.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h17"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.equivalence_classes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.equivalence_classes">Equivalence
classes</a>
</h6>
<p>
An expression of the form <code class="literal">[[=col=]]</code>, matches any character
or collating element whose primary sort key is the same as that for collating
element <span class="emphasis"><em>col</em></span>, as with collating elements the name <span class="emphasis"><em>col</em></span>
may be a <a class="link" href="collating_names.html" title="Collating Names">symbolic name</a>.
A primary sort key is one that ignores case, accentation, or locale-specific
tailorings; so for example <code class="computeroutput"><span class="special">[[=</span><span class="identifier">a</span><span class="special">=]]</span></code> matches
any of the characters: a, À, Á, Â, Ã, Ä, Å, A, à, á, â, ã, ä and å. Unfortunately implementation
of this is reliant on the platform's collation and localisation support;
this feature can not be relied upon to work portably across all platforms,
or even all locales on one platform.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h18"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.escaped_characters"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.escaped_characters">Escaped
Characters</a>
</h6>
<p>
All the escape sequences that match a single character, or a single character
class are permitted within a character class definition. For example <code class="computeroutput"><span class="special">[\[\]]</span></code> would match either of <code class="computeroutput"><span class="special">[</span></code> or <code class="computeroutput"><span class="special">]</span></code>
while <code class="computeroutput"><span class="special">[\</span><span class="identifier">W</span><span class="special">\</span><span class="identifier">d</span><span class="special">]</span></code>
would match any character that is either a "digit", <span class="emphasis"><em>or</em></span>
is <span class="emphasis"><em>not</em></span> a "word" character.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h19"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.combinations"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.combinations">Combinations</a>
</h6>
<p>
All of the above can be combined in one character set declaration, for example:
<code class="literal">[[:digit:]a-c[.NUL.]]</code>.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h20"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.escapes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.escapes">Escapes</a>
</h5>
<p>
Any special character preceded by an escape shall match itself.
</p>
<p>
The following escape sequences are all synonyms for single characters:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Escape
</p>
</th>
<th>
<p>
Character
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="literal">\a</code>
</p>
</td>
<td>
<p>
<code class="literal">\a</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\e</code>
</p>
</td>
<td>
<p>
<code class="literal">0x1B</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\f</code>
</p>
</td>
<td>
<p>
<code class="literal">\f</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal"><br> </code>
</p>
</td>
<td>
<p>
<code class="literal"><br> </code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\r</code>
</p>
</td>
<td>
<p>
<code class="literal">\r</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\t</code>
</p>
</td>
<td>
<p>
<code class="literal">\t</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\v</code>
</p>
</td>
<td>
<p>
<code class="literal">\v</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\b</code>
</p>
</td>
<td>
<p>
<code class="literal">\b</code> (but only inside a character class declaration).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\cX</code>
</p>
</td>
<td>
<p>
An ASCII escape sequence - the character whose code point is X
% 32
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\xdd</code>
</p>
</td>
<td>
<p>
A hexadecimal escape sequence - matches the single character whose
code point is 0xdd.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\x{dddd}</code>
</p>
</td>
<td>
<p>
A hexadecimal escape sequence - matches the single character whose
code point is 0xdddd.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\0ddd</code>
</p>
</td>
<td>
<p>
An octal escape sequence - matches the single character whose code
point is 0ddd.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">\N{name}</code>
</p>
</td>
<td>
<p>
Matches the single character which has the <a class="link" href="collating_names.html" title="Collating Names">symbolic
name</a> <span class="emphasis"><em>name</em></span>. For example <code class="literal">\N{newline}</code>
matches the single character \n.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.perl_syntax.h21"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax._quot_single_character_quot__character_classes_"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax._quot_single_character_quot__character_classes_">"Single
character" character classes:</a>
</h6>
<p>
Any escaped character <span class="emphasis"><em>x</em></span>, if <span class="emphasis"><em>x</em></span> is
the name of a character class shall match any character that is a member
of that class, and any escaped character <span class="emphasis"><em>X</em></span>, if <span class="emphasis"><em>x</em></span>
is the name of a character class, shall match any character not in that class.
</p>
<p>
The following are supported by default:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Escape sequence
</p>
</th>
<th>
<p>
Equivalent to
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">d</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">l</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">s</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">u</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">w</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">h</span></code>
</p>
</td>
<td>
<p>
Horizontal whitespace
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">v</span></code>
</p>
</td>
<td>
<p>
Vertical whitespace
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">D</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">L</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">S</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">U</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">W</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">H</span></code>
</p>
</td>
<td>
<p>
Not Horizontal whitespace
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">V</span></code>
</p>
</td>
<td>
<p>
Not Vertical whitespace
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.perl_syntax.h22"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.character_properties"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.character_properties">Character
Properties</a>
</h6>
<p>
The character property names in the following table are all equivalent to
the <a class="link" href="character_classes.html" title="Character Class Names">names used in character
classes</a>.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Form
</p>
</th>
<th>
<p>
Description
</p>
</th>
<th>
<p>
Equivalent character set form
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">pX</span></code>
</p>
</td>
<td>
<p>
Matches any character that has the property X.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">p</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
</p>
</td>
<td>
<p>
Matches any character that has the property Name.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">PX</span></code>
</p>
</td>
<td>
<p>
Matches any character that does not have the property X.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">\</span><span class="identifier">P</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
</p>
</td>
<td>
<p>
Matches any character that does not have the property Name.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="special">[^[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
For example <code class="literal">\pd</code> matches any "digit" character,
as does <code class="literal">\p{digit}</code>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h23"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.word_boundaries"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.word_boundaries">Word
Boundaries</a>
</h6>
<p>
The following escape sequences match the boundaries of words:
</p>
<p>
<code class="literal"><</code> Matches the start of a word.
</p>
<p>
<code class="literal">></code> Matches the end of a word.
</p>
<p>
<code class="literal">\b</code> Matches a word boundary (the start or end of a word).
</p>
<p>
<code class="literal">\B</code> Matches only when not at a word boundary.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h24"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.buffer_boundaries"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.buffer_boundaries">Buffer
boundaries</a>
</h6>
<p>
The following match only at buffer boundaries: a "buffer" in this
context is the whole of the input text that is being matched against (note
that ^ and $ may match embedded newlines within the text).
</p>
<p>
\` Matches at the start of a buffer only.
</p>
<p>
\' Matches at the end of a buffer only.
</p>
<p>
\A Matches at the start of a buffer only (the same as <code class="literal">\`</code>).
</p>
<p>
\z Matches at the end of a buffer only (the same as <code class="literal">\'</code>).
</p>
<p>
\Z Matches a zero-width assertion consisting of an optional sequence of newlines
at the end of a buffer: equivalent to the regular expression <code class="literal">(?=\v*\z)</code>.
Note that this is subtly different from Perl which behaves as if matching
<code class="literal">(?=\n?\z)</code>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h25"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.continuation_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.continuation_escape">Continuation
Escape</a>
</h6>
<p>
The sequence <code class="literal">\G</code> matches only at the end of the last match
found, or at the start of the text being matched if no previous match was
found. This escape useful if you're iterating over the matches contained
within a text, and you want each subsequence match to start where the last
one ended.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h26"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.quoting_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.quoting_escape">Quoting
escape</a>
</h6>
<p>
The escape sequence <code class="literal">\Q</code> begins a "quoted sequence":
all the subsequent characters are treated as literals, until either the end
of the regular expression or \E is found. For example the expression: <code class="literal">\Q*+\Ea+</code>
would match either of:
</p>
<pre class="programlisting"><span class="special">\*+</span><span class="identifier">a</span>
<span class="special">\*+</span><span class="identifier">aaa</span>
</pre>
<h6>
<a name="boost_regex.syntax.perl_syntax.h27"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.unicode_escapes"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.unicode_escapes">Unicode
escapes</a>
</h6>
<p>
<code class="literal">\C</code> Matches a single code point: in Boost regex this has
exactly the same effect as a "." operator. <code class="literal">\X</code>
Matches a combining character sequence: that is any non-combining character
followed by a sequence of zero or more combining characters.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h28"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.matching_line_endings"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.matching_line_endings">Matching Line
Endings</a>
</h6>
<p>
The escape sequence <code class="literal">\R</code> matches any line ending character
sequence, specifically it is identical to the expression <code class="literal">(?>\x0D\x0A?|[\x0A-\x0C\x85\x{2028}\x{2029}])</code>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h29"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.keeping_back_some_text"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.keeping_back_some_text">Keeping back
some text</a>
</h6>
<p>
<code class="literal">\K</code> Resets the start location of $0 to the current text
position: in other words everything to the left of \K is "kept back"
and does not form part of the regular expression match. $` is updated accordingly.
</p>
<p>
For example <code class="literal">foo\Kbar</code> matched against the text "foobar"
would return the match "bar" for $0 and "foo" for $`.
This can be used to simulate variable width lookbehind assertions.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h30"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.any_other_escape"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.any_other_escape">Any
other escape</a>
</h6>
<p>
Any other escape sequence matches the character that is escaped, for example
\@ matches a literal '@'.
</p>
<h5>
<a name="boost_regex.syntax.perl_syntax.h31"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.perl_extended_patterns"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.perl_extended_patterns">Perl Extended
Patterns</a>
</h5>
<p>
Perl-specific extensions to the regular expression syntax all start with
<code class="literal">(?</code>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h32"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.named_subexpressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.named_subexpressions">Named
Subexpressions</a>
</h6>
<p>
You can create a named subexpression using:
</p>
<pre class="programlisting"><span class="special">(?<</span><span class="identifier">NAME</span><span class="special">></span><span class="identifier">expression</span><span class="special">)</span>
</pre>
<p>
Which can be then be referred to by the name <span class="emphasis"><em>NAME</em></span>. Alternatively
you can delimit the name using 'NAME' as in:
</p>
<pre class="programlisting"><span class="special">(?</span><span class="char">'NAME'</span><span class="identifier">expression</span><span class="special">)</span>
</pre>
<p>
These named subexpressions can be referred to in a backreference using either
<code class="literal">\g{NAME}</code> or <code class="literal">\k<NAME></code> and can
also be referred to by name in a <a class="link" href="../format/perl_format.html" title="Perl Format String Syntax">Perl</a>
format string for search and replace operations, or in the <a class="link" href="../ref/match_results.html" title="match_results"><code class="computeroutput"><span class="identifier">match_results</span></code></a> member functions.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h33"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.comments"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.comments">Comments</a>
</h6>
<p>
<code class="literal">(?# ... )</code> is treated as a comment, it's contents are ignored.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h34"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.modifiers"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.modifiers">Modifiers</a>
</h6>
<p>
<code class="literal">(?imsx-imsx ... )</code> alters which of the perl modifiers are
in effect within the pattern, changes take effect from the point that the
block is first seen and extend to any enclosing <code class="literal">)</code>. Letters
before a '-' turn that perl modifier on, letters afterward, turn it off.
</p>
<p>
<code class="literal">(?imsx-imsx:pattern)</code> applies the specified modifiers to
pattern only.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h35"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.non_marking_groups"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.non_marking_groups">Non-marking
groups</a>
</h6>
<p>
<code class="literal">(?:pattern)</code> lexically groups pattern, without generating
an additional sub-expression.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h36"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.branch_reset"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.branch_reset">Branch
reset</a>
</h6>
<p>
<code class="literal">(?|pattern)</code> resets the subexpression count at the start
of each "|" alternative within <span class="emphasis"><em>pattern</em></span>.
</p>
<p>
The sub-expression count following this construct is that of whichever branch
had the largest number of sub-expressions. This construct is useful when
you want to capture one of a number of alternative matches in a single sub-expression
index.
</p>
<p>
In the following example the index of each sub-expression is shown below
the expression:
</p>
<pre class="programlisting"># before ---------------branch-reset----------- after
/ ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# 1 2 2 3 2 3 4
</pre>
<h6>
<a name="boost_regex.syntax.perl_syntax.h37"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.lookahead"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.lookahead">Lookahead</a>
</h6>
<p>
<code class="literal">(?=pattern)</code> consumes zero characters, only if pattern
matches.
</p>
<p>
<code class="literal">(?!pattern)</code> consumes zero characters, only if pattern
does not match.
</p>
<p>
Lookahead is typically used to create the logical AND of two regular expressions,
for example if a password must contain a lower case letter, an upper case
letter, a punctuation symbol, and be at least 6 characters long, then the
expression:
</p>
<pre class="programlisting"><span class="special">(?=.*[[:</span><span class="identifier">lower</span><span class="special">:]])(?=.*[[:</span><span class="identifier">upper</span><span class="special">:]])(?=.*[[:</span><span class="identifier">punct</span><span class="special">:]]).{</span><span class="number">6</span><span class="special">,}</span>
</pre>
<p>
could be used to validate the password.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h38"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.lookbehind"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.lookbehind">Lookbehind</a>
</h6>
<p>
<code class="literal">(?<=pattern)</code> consumes zero characters, only if pattern
could be matched against the characters preceding the current position (pattern
must be of fixed length).
</p>
<p>
<code class="literal">(?<!pattern)</code> consumes zero characters, only if pattern
could not be matched against the characters preceding the current position
(pattern must be of fixed length).
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h39"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.independent_sub_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.independent_sub_expressions">Independent
sub-expressions</a>
</h6>
<p>
<code class="literal">(?>pattern)</code> <span class="emphasis"><em>pattern</em></span> is matched
independently of the surrounding patterns, the expression will never backtrack
into <span class="emphasis"><em>pattern</em></span>. Independent sub-expressions are typically
used to improve performance; only the best possible match for pattern will
be considered, if this doesn't allow the expression as a whole to match then
no match is found at all.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h40"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.recursive_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.recursive_expressions">Recursive
Expressions</a>
</h6>
<p>
<code class="literal">(?<span class="emphasis"><em>N</em></span>) (?-<span class="emphasis"><em>N</em></span>) (?+<span class="emphasis"><em>N</em></span>)
(?R) (?0) (?&NAME)</code>
</p>
<p>
<code class="literal">(?R)</code> and <code class="literal">(?0)</code> recurse to the start
of the entire pattern.
</p>
<p>
<code class="literal">(?<span class="emphasis"><em>N</em></span>)</code> executes sub-expression <span class="emphasis"><em>N</em></span>
recursively, for example <code class="literal">(?2)</code> will recurse to sub-expression
2.
</p>
<p>
<code class="literal">(?-<span class="emphasis"><em>N</em></span>)</code> and <code class="literal">(?+<span class="emphasis"><em>N</em></span>)</code>
are relative recursions, so for example <code class="literal">(?-1)</code> recurses
to the last sub-expression to be declared, and <code class="literal">(?+1)</code> recurses
to the next sub-expression to be declared.
</p>
<p>
<code class="literal">(?&NAME)</code> recurses to named sub-expression <span class="emphasis"><em>NAME</em></span>.
</p>
<h6>
<a name="boost_regex.syntax.perl_syntax.h41"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.conditional_expressions"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.conditional_expressions">Conditional
Expressions</a>
</h6>
<p>
<code class="literal">(?(condition)yes-pattern|no-pattern)</code> attempts to match
<span class="emphasis"><em>yes-pattern</em></span> if the <span class="emphasis"><em>condition</em></span> is
true, otherwise attempts to match <span class="emphasis"><em>no-pattern</em></span>.
</p>
<p>
<code class="literal">(?(condition)yes-pattern)</code> attempts to match <span class="emphasis"><em>yes-pattern</em></span>
if the <span class="emphasis"><em>condition</em></span> is true, otherwise matches the NULL
string.
</p>
<p>
<span class="emphasis"><em>condition</em></span> may be either: a forward lookahead assert,
the index of a marked sub-expression (the condition becomes true if the sub-expression
has been matched), or an index of a recursion (the condition become true
if we are executing directly inside the specified recursion).
</p>
<p>
Here is a summary of the possible predicates:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<code class="literal">(?(?=assert)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
if the forward look-ahead assert matches, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(?!assert)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
if the forward look-ahead assert does not match, otherwise executes
<span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(<span class="emphasis"><em>N</em></span>)yes-pattern|no-pattern)</code>
Executes <span class="emphasis"><em>yes-pattern</em></span> if subexpression <span class="emphasis"><em>N</em></span>
has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(<<span class="emphasis"><em>name</em></span>>)yes-pattern|no-pattern)</code>
Executes <span class="emphasis"><em>yes-pattern</em></span> if named subexpression <span class="emphasis"><em>name</em></span>
has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?('<span class="emphasis"><em>name</em></span>')yes-pattern|no-pattern)</code>
Executes <span class="emphasis"><em>yes-pattern</em></span> if named subexpression <span class="emphasis"><em>name</em></span>
has been matched, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(R)yes-pattern|no-pattern)</code> Executes <span class="emphasis"><em>yes-pattern</em></span>
if we are executing inside a recursion, otherwise executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(R<span class="emphasis"><em>N</em></span>)yes-pattern|no-pattern)</code>
Executes <span class="emphasis"><em>yes-pattern</em></span> if we are executing inside
a recursion to sub-expression <span class="emphasis"><em>N</em></span>, otherwise executes
<span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(R&<span class="emphasis"><em>name</em></span>)yes-pattern|no-pattern)</code>
Executes <span class="emphasis"><em>yes-pattern</em></span> if we are executing inside
a recursion to named sub-expression <span class="emphasis"><em>name</em></span>, otherwise
executes <span class="emphasis"><em>no-pattern</em></span>.
</li>
<li class="listitem">
<code class="literal">(?(DEFINE)never-exectuted-pattern)</code> Defines a block
of code that is never executed and matches no characters: this is usually
used to define one or more named sub-expressions which are referred to
from elsewhere in the pattern.
</li>
</ul></div>
<h5>
<a name="boost_regex.syntax.perl_syntax.h42"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.operator_precedence"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.operator_precedence">Operator
precedence</a>
</h5>
<p>
The order of precedence for of operators is as follows:
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
Collation-related bracket symbols <code class="computeroutput"><span class="special">[==]</span>
<span class="special">[::]</span> <span class="special">[..]</span></code>
</li>
<li class="listitem">
Escaped characters [^]
</li>
<li class="listitem">
Character set (bracket expression) <code class="computeroutput"><span class="special">[]</span></code>
</li>
<li class="listitem">
Grouping <code class="literal">()</code>
</li>
<li class="listitem">
Single-character-ERE duplication <code class="literal">* + ? {m,n}</code>
</li>
<li class="listitem">
Concatenation
</li>
<li class="listitem">
Anchoring ^$
</li>
<li class="listitem">
Alternation |
</li>
</ol></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h43"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.what_gets_matched"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.what_gets_matched">What
gets matched</a>
</h4>
<p>
If you view the regular expression as a directed (possibly cyclic) graph,
then the best match found is the first match found by a depth-first-search
performed on that graph, while matching the input text.
</p>
<p>
Alternatively:
</p>
<p>
The best match found is the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">leftmost
match</a>, with individual elements matched as follows;
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Construct
</p>
</th>
<th>
<p>
What gets matched
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="literal">AtomA AtomB</code>
</p>
</td>
<td>
<p>
Locates the best match for <span class="emphasis"><em>AtomA</em></span> that has
a following match for <span class="emphasis"><em>AtomB</em></span>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">Expression1 | Expression2</code>
</p>
</td>
<td>
<p>
If <span class="emphasis"><em>Expresion1</em></span> can be matched then returns
that match, otherwise attempts to match <span class="emphasis"><em>Expression2</em></span>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">S{N}</code>
</p>
</td>
<td>
<p>
Matches <span class="emphasis"><em>S</em></span> repeated exactly N times.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">S{N,M}</code>
</p>
</td>
<td>
<p>
Matches S repeated between N and M times, and as many times as
possible.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">S{N,M}?</code>
</p>
</td>
<td>
<p>
Matches S repeated between N and M times, and as few times as possible.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">S?, S*, S+</code>
</p>
</td>
<td>
<p>
The same as <code class="literal">S{0,1}</code>, <code class="literal">S{0,UINT_MAX}</code>,
<code class="literal">S{1,UINT_MAX}</code> respectively.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">S??, S*?, S+?</code>
</p>
</td>
<td>
<p>
The same as <code class="literal">S{0,1}?</code>, <code class="literal">S{0,UINT_MAX}?</code>,
<code class="literal">S{1,UINT_MAX}?</code> respectively.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">(?>S)</code>
</p>
</td>
<td>
<p>
Matches the best match for <span class="emphasis"><em>S</em></span>, and only that.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">(?=S), (?<=S)</code>
</p>
</td>
<td>
<p>
Matches only the best match for <span class="emphasis"><em>S</em></span> (this is
only visible if there are capturing parenthesis within <span class="emphasis"><em>S</em></span>).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">(?!S), (?<!S)</code>
</p>
</td>
<td>
<p>
Considers only whether a match for S exists or not.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="literal">(?(condition)yes-pattern | no-pattern)</code>
</p>
</td>
<td>
<p>
If condition is true, then only yes-pattern is considered, otherwise
only no-pattern is considered.
</p>
</td>
</tr>
</tbody>
</table></div>
<h4>
<a name="boost_regex.syntax.perl_syntax.h44"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.variations"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.variations">Variations</a>
</h4>
<p>
The <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">options
<code class="literal">normal</code>, <code class="literal">ECMAScript</code>, <code class="literal">JavaScript</code>
and <code class="literal">JScript</code></a> are all synonyms for <code class="literal">perl</code>.
</p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h45"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.options"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.options">Options</a>
</h4>
<p>
There are a <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">variety
of flags</a> that may be combined with the <code class="literal">perl</code> option
when constructing the regular expression, in particular note that the <code class="literal">newline_alt</code>
option alters the syntax, while the <code class="literal">collate</code>, <code class="literal">nosubs</code>
and <code class="literal">icase</code> options modify how the case and locale sensitivity
are to be applied.
</p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h46"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.pattern_modifiers"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.pattern_modifiers">Pattern
Modifiers</a>
</h4>
<p>
The perl <code class="literal">smix</code> modifiers can either be applied using a
<code class="literal">(?smix-smix)</code> prefix to the regular expression, or with
one of the <a class="link" href="../ref/syntax_option_type/syntax_option_type_perl.html" title="Options for Perl Regular Expressions">regex-compile
time flags <code class="literal">no_mod_m</code>, <code class="literal">mod_x</code>, <code class="literal">mod_s</code>,
and <code class="literal">no_mod_s</code></a>.
</p>
<h4>
<a name="boost_regex.syntax.perl_syntax.h47"></a>
<span class="phrase"><a name="boost_regex.syntax.perl_syntax.references"></a></span><a class="link" href="perl_syntax.html#boost_regex.syntax.perl_syntax.references">References</a>
</h4>
<p>
<a href="http://perldoc.perl.org/perlre.html" target="_top">Perl 5.8</a>.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../syntax.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../syntax.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="basic_extended.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
