From 68dc074516a6859e3424b48d1647bcb08b1a1a7d Mon Sep 17 00:00:00 2001 From: Perl 5 Porters Date: Sun, 9 Mar 1997 11:57:19 +1200 Subject: [inseparable changes from match from perl-5.003_93 to perl-5.003_94] BUILD PROCESS Subject: Don't use db 2.x, we're not yet ready for it From: Paul Marquess Files: Configure Subject: Warn if #! command is longer than 32 chars From: Chip Salzenberg Files: Configure Subject: patches re perl -wc install{perl,man} Date: Tue, 11 Mar 97 13:13:16 GMT From: Robin Barker Files: installman installperl I got the new installhtml from CPAN (TOMC/scripts/pod2html-v2.0beta.shar.gz) I had problems getting the system call to splitpod at line 376 to work. 1. splitroot was not being found 2. splitroot was not finding its library 3. I changed htmlroot to podroot at line 175 to match the documentation. p5p-msgid: 3180.9703270906@tempest.cise.npl.co.uk private-msgid: 21544.9703111313@tempest.cise.npl.co.uk Subject: 3_93 doesn't install pods Date: Sun, 16 Mar 1997 02:21:35 -0500 From: Spider Boardman Files: installperl Msg-ID: 199703160721.CAA08339@Orb.Nashua.NH.US (applied based on p5p patch as commit 43506a616735d616e03d277d64fbae1e864024bf) Subject: When installing, use File::Copy instead of `cp` From: Chip Salzenberg Files: installperl Subject: Make hint files' warnings more visible Date: Thu, 20 Mar 1997 23:18:03 +0100 (MET) From: Hallvard B Furuseth Files: hints/3b1.sh hints/apollo.sh hints/cxux.sh hints/dcosx.sh hints/dgux.sh hints/esix4.sh hints/freebsd.sh hints/hpux.sh hints/irix_4.sh hints/mips.sh hints/next_3_0.sh hints/os2.sh hints/qnx.sh hints/sco_2_3_3.sh hints/sco_2_3_4.sh hints/solaris_2.sh hints/ultrix_4.sh hints/utekv.sh private-msgid: 199703202218.XAA09041@bombur2.uio.no CORE LANGUAGE CHANGES Subject: Defer creation of array and hash elements as parameters From: Chip Salzenberg Files: dump.c global.sym mg.c op.c op.h perl.h pp.c pp_hot.c proto.h sv.c Subject: New special literal: __PACKAGE__ From: Chip Salzenberg Files: keywords.pl pod/perldata.pod toke.c Subject: Abort compilation at C or C after errors From: Chip Salzenberg Files: op.c pod/perldiag.pod t/pragma/subs.t Subject: allow C Date: Mon, 10 Mar 1997 15:55:44 -0800 From: David Dyck Files: pp.c Msg-ID: 97Mar10.155517pst.35716-2@gateway.fluke.com (applied based on p5p patch as commit 77f720bf92f3d0100352416caeedd57936807ff2) Subject: Regularize C, esp. when y is negative From: Chip Salzenberg Files: pp.c Subject: Flush before C From: Chip Salzenberg Files: pod/perldelta.pod pod/perlfunc.pod pp_sys.c Subject: Close loopholes in prototype mismatch warning From: Chip Salzenberg Files: op.c sv.c toke.c Subject: Warn on C From: Chip Salzenberg Files: op.c pod/perldiag.pod Subject: Don't warn on C From: Chip Salzenberg Files: toke.c CORE PORTABILITY Subject: Don't say 'static var = 1' Date: Sun, 9 Mar 1997 15:19:57 +0200 (EET) From: Jarkko Hietaniemi Files: malloc.c private-msgid: 199703091319.PAA24714@alpha.hut.fi Subject: HP/UX hint comments Date: Fri, 21 Mar 1997 15:43:07 -0500 (EST) From: Andy Dougherty Files: hints/hpux.sh private-msgid: Pine.SOL.3.95q.970321153918.28770B-100000@fractal.lafayette. Subject: VMS update Date: Tue, 11 Mar 1997 22:00:55 -0500 (EST) From: Charles Bailey Files: lib/ExtUtils/MM_VMS.pm lib/Test/Harness.pm t/op/taint.t utils/perlbug.PL vms/descrip.mms Msg-ID: 1997Mar11.220056.1873182@hmivax.humgen.upenn.edu (applied based on p5p patch as commit 2b5725676da60b49978f38b85bb7f8ee20b4cb55) Subject: vmsish.t and related patches Date: Fri, 21 Mar 1997 01:32:47 -0500 (EST) From: Charles Bailey Files: MANIFEST perl.h vms/descrip.mms vms/ext/vmsish.t vms/vms.c private-msgid: 01IGQW3IP1KK005VFB@hmivax.humgen.upenn.edu Subject: Win32 update (four patches) From: Gurusamy Sarathy Files: MANIFEST README.win32 lib/AutoSplit.pm lib/Cwd.pm lib/ExtUtils/Command.pm lib/ExtUtils/Install.pm lib/ExtUtils/MM_OS2.pm lib/ExtUtils/MM_Unix.pm lib/ExtUtils/MM_Win32.pm lib/ExtUtils/MakeMaker.pm lib/ExtUtils/Mksymlists.pm lib/File/Basename.pm lib/File/Path.pm mg.c t/comp/cpp.t t/comp/script.t t/harness t/io/argv.t t/io/dup.t t/io/fs.t t/io/inplace.t t/lib/filehand.t t/lib/io_dup.t t/lib/io_sel.t t/lib/io_taint.t t/op/closure.t t/op/exec.t t/op/glob.t t/op/goto.t t/op/magic.t t/op/misc.t t/op/rand.t t/op/split.t t/op/stat.t t/op/sysio.t t/op/taint.t t/pragma/strict.t t/pragma/subs.t t/pragma/warning.t util.c win32/* DOCUMENTATION Subject: perlfaq.pod Date: Mon, 17 Mar 1997 16:01:40 -0700 From: Tom Christiansen Files: MANIFEST pod/Makefile pod/buildtoc pod/perl.pod pod/perlfaq*.pod pod/roffitall private-msgid: 199703172301.QAA12566@jhereg.perl.com Subject: *.pod changes based on the FAQ Date: Mon, 17 Mar 1997 09:50:14 -0700 (MST) From: Nat Torkington Files: pod/perldata.pod pod/perlfunc.pod pod/perlipc.pod pod/perlop.pod pod/perlre.pod pod/perlrun.pod pod/perlsec.pod pod/perlvar.pod Msg-ID: 199703171650.JAA02655@elara.frii.com (applied based on p5p patch as commit 3c10ad8e31f7d77e71c048b1746912f41cb540f0) Subject: Document that $. is not reset on implicit open From: Chip Salzenberg Files: pod/perldelta.pod Subject: Re: Embedding success with _93 Date: Tue, 11 Mar 1997 17:55:05 -0500 From: Doug MacEachern Files: pod/perldelta.pod Msg-ID: 199703112255.RAA22775@postman.osf.org (applied based on p5p patch as commit 63a6ff3a1dc8d86edb4d8a7ec1548205e32a7114) Subject: Patch to document illegal characters Date: Fri, 14 Mar 1997 09:08:10 -0800 (PST) From: Tom Phoenix Files: pod/perldiag.pod pod/perltrap.pod private-msgid: Pine.GSO.3.96.970314090558.15346J-100000@kelly.teleport.com Subject: Document trap with //o and closures Date: Mon, 10 Mar 1997 18:08:08 -0500 (EST) From: Charles Bailey Files: pod/perltrap.pod Msg-ID: 01IGCHWRNSEU00661G@hmivax.humgen.upenn.edu (applied based on p5p patch as commit a54cb1465fdb400848f23705a6f130bb5c34ab70) Subject: Illegal character in input Date: Mon, 10 Mar 1997 15:21:21 -0800 (PST) From: Tom Phoenix Files: pod/perldiag.pod private-msgid: Pine.GSO.3.95q.970310151512.22489a-100000@kelly.teleport.com Subject: Patch for docs Re: Lost backslash Date: Wed, 19 Mar 1997 07:28:57 -0800 (PST) From: Tom Phoenix Files: pod/perlop.pod private-msgid: Pine.GSO.3.96.970319071438.24834G-100000@kelly.teleport.com Subject: XSUB's doc fix Date: Mon, 10 Mar 1997 11:42:06 -0500 From: Roderick Schertler Files: pod/perlcall.pod pod/perlguts.pod pod/perlxstut.pod Msg-ID: 28804.858012126@eeyore.ibcinc.com (applied based on p5p patch as commit 5f43237038ea7a4151d3bf65aeeecd56ceb78a6a) Subject: Document return from do FILE Date: Tue, 18 Mar 1997 14:50:10 +0000 From: "M.J.T. Guy" Files: pod/perlfunc.pod Msg-ID: E0w70DK-0001yJ-00@ursa.cus.cam.ac.uk (applied based on p5p patch as commit ba8d5fb439878113de8abc9b52d2af237d30fb3c) Subject: Document $^M in perlvar Date: Thu, 20 Mar 97 21:08:33 GMT From: Robin Barker Files: pod/perlvar.pod private-msgid: 6153.9703202108@tempest.cise.npl.co.uk Subject: typos in pods of 5.003_93 Date: 19 Mar 1997 10:39:38 -0600 From: Jim Meyering Files: pod/perlfunc.pod pod/perlguts.pod pod/perlre.pod pod/perltoot.pod pod/perlxs.pod Msg-ID: wpgendbzvhx.fsf@asic.sc.ti.com (applied based on p5p patch as commit 76a9873e006cf8f48f57062b2a0dd40b5ed45a95) Subject: Re: Updates to pod punctuations Date: Fri, 14 Mar 1997 17:00:12 -0500 From: Larry W. Virden Files: pod/*.pod private-msgid: 9703141700.AA22911@cas.org Subject: clarify example in perlfunc Date: Thu, 20 Mar 1997 19:46:01 +0200 (EET) From: Jarkko Hietaniemi Files: pod/perlfunc.pod private-msgid: 199703201746.TAA25195@alpha.hut.fi Subject: Regularize headings in DB_File documentation From: Chip Salzenberg Files: ext/DB_File/DB_File.pm LIBRARY AND EXTENSIONS Subject: New module: autouse.pm Date: Thu, 20 Mar 1997 19:34:30 -0500 (EST) From: Ilya Zakharevich Files: MANIFEST lib/autouse.pm Msg-ID: 199703210034.TAA13469@monk.mps.ohio-state.edu (applied based on p5p patch as commit 6757905eccb6dd0440ef65e8128a277a20f7d943) Subject: Refresh DB_File to 1.12 Date: Wed, 12 Mar 97 15:51:14 GMT From: Paul Marquess Files: ext/DB_File/DB_File.pm ext/DB_File/DB_File.xs Msg-ID: 9703121551.AA07435@claudius.bfsec.bt.co.uk (applied based on p5p patch as commit b3deed9189f963e9994815307931f9084f60d1d9) Subject: In File::Path, some systems can't remove read-only files From: Chip Salzenberg Files: lib/File/Path.pm Subject: Fix bugs revealed by prototype warnings From: Chip Salzenberg Files: ext/Opcode/Opcode.pm lib/ExtUtils/MakeMaker.pm lib/Getopt/Long.pm Subject: Problems with SKIP in makemaker Date: Thu, 20 Mar 1997 23:13:31 -0500 (EST) From: Ilya Zakharevich Files: lib/ExtUtils/MM_Unix.pm Msg-ID: 199703210413.XAA21601@monk.mps.ohio-state.edu (applied based on p5p patch as commit 970322a2e8024294ada6e8d1a027cb98f1f48ee3) Subject: In Exporter, don't C at file scope From: Chip Salzenberg Files: lib/Exporter.pm Subject: fix for Exporter's $SIG{__WARN__} handler Date: Thu, 13 Mar 1997 18:40:51 -0500 From: Roderick Schertler Files: lib/Exporter.pm Msg-ID: 2282.858296451@eeyore.ibcinc.com (applied based on p5p patch as commit 2768ea1aeef34f42d096f198fbe629c8374ca429) Subject: Don't try to substr() refs in Carp From: Chip Salzenberg Files: lib/Carp.pm Subject: Re: NUL in die and other messages Date: Fri, 21 Mar 1997 09:58:17 +0000 From: "M.J.T. Guy" Files: lib/Carp.pm Msg-ID: E0w815V-0005xs-00@ursa.cus.cam.ac.uk (applied based on p5p patch as commit 52a267c574cb66c4bc35601dcf148a1d7a3bc557) OTHER CORE CHANGES Subject: Guard against buffer overflow in yyerror() and related funcs From: Chip Salzenberg Files: toke.c Subject: For bin compat, rename calllist() and he_{,delay}free From: Chip Salzenberg Files: global.sym hv.c op.c perl.c pod/perlguts.pod proto.h Subject: Fix C on tied default handle From: Chip Salzenberg Files: pp_hot.c Subject: Fix C From: Chip Salzenberg Files: op.c Subject: Improve diagnostic on C<@a++>, C<--%a>, @a =~ s/a/b/ From: Chip Salzenberg Files: pp.c pp_hot.c Subject: Don't warn on C<$x{y} .= "z"> when %x is tied From: Chip Salzenberg Files: pp_hot.c Subject: Eliminate 'unreachable code' warnings From: Chip Salzenberg Files: ext/POSIX/POSIX.xs mg.c pp_ctl.c toke.c Subject: printf format corrections for -DDEBUGGING Date: Wed, 19 Mar 1997 12:42:50 -0500 From: Roderick Schertler Files: doop.c malloc.c op.c pp_ctl.c regexec.c sv.c x2p/str.c x2p/util.c Msg-ID: 26592.858793370@eeyore.ibcinc.com (applied based on p5p patch as commit e125f273e351a19a92b69d6244af55abbbf0a26d) Subject: Warn about missing -DMULTIPLICITY if likely a problem Date: Wed, 19 Mar 1997 18:45:53 -0500 From: Doug MacEachern Files: perl.c Msg-ID: 199703192345.SAA15070@postman.osf.org (applied based on p5p patch as commit 71aeea1753924e6e19c2461e241e3f7d8a570e90) --- pod/perlfaq6.pod | 580 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 580 insertions(+) create mode 100644 pod/perlfaq6.pod (limited to 'pod/perlfaq6.pod') diff --git a/pod/perlfaq6.pod b/pod/perlfaq6.pod new file mode 100644 index 0000000000..589d89e495 --- /dev/null +++ b/pod/perlfaq6.pod @@ -0,0 +1,580 @@ +=head1 NAME + +perlfaq6 - Regexps ($Revision: 1.14 $) + +=head1 DESCRIPTION + +This section is surprisingly small because the rest of the FAQ is +littered with answers involving regular expressions. For example, +decoding a URL and checking whether something is a number are handled +with regular expressions, but those answers are found elsewhere in +this document (in the section on Data and the Networking one on +networking, to be precise). + +=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? + +Three techniques can make regular expressions maintainable and +understandable. + +=over 4 + +=item Comments Outside the Regexp + +Describe what you're doing and how you're doing it, using normal Perl +comments. + + # turn the line into the first word, a colon, and the + # number of characters on the rest of the line + s/^(\w+)(.*)/ lc($1) . ":" . length($2) /ge; + +=item Comments Inside the Regexp + +The C modifier causes whitespace to be ignored in a regexp pattern +(except in a character class), and also allows you to use normal +comments there, too. As you can imagine, whitespace and comments help +a lot. + +C lets you turn this: + + s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; + +into this: + + s{ < # opening angle bracket + (?: # Non-backreffing grouping paren + [^>'"] * # 0 or more things that are neither > nor ' nor " + | # or else + ".*?" # a section between double quotes (stingy match) + | # or else + '.*?' # a section between single quotes (stingy match) + ) + # all occurring one or more times + > # closing angle bracket + }{}gsx; # replace with nothing, i.e. delete + +It's still not quite so clear as prose, but it is very useful for +describing the meaning of each part of the pattern. + +=item Different Delimiters + +While we normally think of patterns as being delimited with C +characters, they can be delimited by almost any character. L +describes this. For example, the C above uses braces as +delimiters. Selecting another delimiter can avoid quoting the +delimiter within the pattern: + + s/\/usr\/local/\/usr\/share/g; # bad delimiter choice + s#/usr/local#/usr/share#g; # better + +=back + +=head2 I'm having trouble matching over more than one line. What's wrong? + +Either you don't have newlines in your string, or you aren't using the +correct modifier(s) on your pattern. + +There are many ways to get multiline data into a string. If you want +it to happen automatically while reading input, you'll want to set $/ +(probably to '' for paragraphs or C for the whole file) to +allow you to read more than one line at a time. + +Read L to help you decide which of C and C (or both) +you might want to use: C allows dot to include newline, and C +allows caret and dollar to match next to a newline, not just at the +end of the string. You do need to make sure that you've actually +got a multiline string in there. + +For example, this program detects duplicate words, even when they span +line breaks (but not paragraph ones). For this example, we don't need +C because we aren't using dot in a regular expression that we want +to cross line boundaries. Neither do we need C because we aren't +wanting caret or dollar to match at any point inside the record next +to newlines. But it's imperative that $/ be set to something other +than the default, or else we won't actually ever have a multiline +record read in. + + $/ = ''; # read in more whole paragraph, not just one line + while ( <> ) { + while ( /\b(\w\S+)(\s+\1)+\b/gi ) { + print "Duplicate $1 at paragraph $.\n"; + } + } + +Here's code that finds sentences that begin with "From " (which would +be mangled by many mailers): + + $/ = ''; # read in more whole paragraph, not just one line + while ( <> ) { + while ( /^From /gm ) { # /m makes ^ match next to \n + print "leading from in paragraph $.\n"; + } + } + +Here's code that finds everything between START and END in a paragraph: + + undef $/; # read in whole file, not just one line or paragraph + while ( <> ) { + while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries + print "$1\n"; + } + } + +=head2 How can I pull out lines between two patterns that are themselves on different lines? + +You can use Perl's somewhat exotic C<..> operator (documented in +L): + + perl -ne 'print if /START/ .. /END/' file1 file2 ... + +If you wanted text and not lines, you would use + + perl -0777 -pe 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... + +But if you want nested occurrences of C through C, you'll +run up against the problem described in the question in this section +on matching balanced text. + +=head2 I put a regular expression into $/ but it didn't work. What's wrong? + +$/ must be a string, not a regular expression. Awk has to be better +for something. :-) + +Actually, you could do this if you don't mind reading the whole file into + + undef $/; + @records = split /your_pattern/, ; + +=head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS? + +It depends on what you mean by "preserving case". The following +script makes the substitution have the same case, letter by letter, as +the original. If the substitution has more characters than the string +being substituted, the case of the last character is used for the rest +of the substitution. + + # Original by Nathan Torkington, massaged by Jeffrey Friedl + # + sub preserve_case($$) + { + my ($old, $new) = @_; + my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc + my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); + my ($len) = $oldlen < $newlen ? $oldlen : $newlen; + + for ($i = 0; $i < $len; $i++) { + if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { + $state = 0; + } elsif (lc $c eq $c) { + substr($new, $i, 1) = lc(substr($new, $i, 1)); + $state = 1; + } else { + substr($new, $i, 1) = uc(substr($new, $i, 1)); + $state = 2; + } + } + # finish up with any remaining new (for when new is longer than old) + if ($newlen > $oldlen) { + if ($state == 1) { + substr($new, $oldlen) = lc(substr($new, $oldlen)); + } elsif ($state == 2) { + substr($new, $oldlen) = uc(substr($new, $oldlen)); + } + } + return $new; + } + + $a = "this is a TEsT case"; + $a =~ s/(test)/preserve_case($1, "success")/gie; + print "$a\n"; + +This prints: + + this is a SUcCESS case + +=head2 How can I make C<\w> match accented characters? + +See L. + +=head2 How can I match a locale-smart version of C? + +One alphabetic character would be C, no matter what locale +you're in. Non-alphabetics would be C (assuming you don't +consider an underscore a letter). + +=head2 How can I quote a variable to use in a regexp? + +The Perl parser will expand $variable and @variable references in +regular expressions unless the delimiter is a single quote. Remember, +too, that the right-hand side of a C substitution is considered +a double-quoted string (see L for more details). Remember +also that any regexp special characters will be acted on unless you +precede the substitution with \Q. Here's an example: + + $string = "to die?"; + $lhs = "die?"; + $rhs = "sleep no more"; + + $string =~ s/\Q$lhs/$rhs/; + # $string is now "to sleep no more" + +Without the \Q, the regexp would also spuriously match "di". + +=head2 What is C really for? + +Using a variable in a regular expression match forces a re-evaluation +(and perhaps recompilation) each time through. The C modifier +locks in the regexp the first time it's used. This always happens in a +constant regular expression, and in fact, the pattern was compiled +into the internal format at the same time your entire program was. + +Use of C is irrelevant unless variable interpolation is used in +the pattern, and if so, the regexp engine will neither know nor care +whether the variables change after the pattern is evaluated the I time. + +C is often used to gain an extra measure of efficiency by not +performing subsequent evaluations when you know it won't matter +(because you know the variables won't change), or more rarely, when +you don't want the regexp to notice if they do. + +For example, here's a "paragrep" program: + + $/ = ''; # paragraph mode + $pat = shift; + while (<>) { + print if /$pat/o; + } + +=head2 How do I use a regular expression to strip C style comments from a file? + +While this actually can be done, it's much harder than you'd think. +For example, this one-liner + + perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c + +will work in many but not all cases. You see, it's too simple-minded for +certain kinds of C programs, in particular, those with what appear to be +comments in quoted strings. For that, you'd need something like this, +created by Jeffrey Friedl: + + $/ = undef; + $_ = <>; + s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g; + print; + +This could, of course, be more legibly written with the C modifier, adding +whitespace and comments. + +=head2 Can I use Perl regular expressions to match balanced text? + +Although Perl regular expressions are more powerful than "mathematical" +regular expressions, because they feature conveniences like backreferences +(C<\1> and its ilk), they still aren't powerful enough. You still need +to use non-regexp techniques to parse balanced text, such as the text +enclosed between matching parentheses or braces, for example. + +An elaborate subroutine (for 7-bit ASCII only) to pull out balanced +and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, +or C<(> and C<)> can be found in +http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz . + +The C::Scan module from CPAN contains such subs for internal usage, +but they are undocumented. + +=head2 What does it mean that regexps are greedy? How can I get around it? + +Most people mean that greedy regexps match as much as they can. +Technically speaking, it's actually the quantifiers (C, C<*>, C<+>, +C<{}>) that are greedy rather than the whole pattern; Perl prefers local +greed and immediate gratification to overall greed. To get non-greedy +versions of the same quantifiers, use (C, C<*?>, C<+?>, C<{}?>). + +An example: + + $s1 = $s2 = "I am very very cold"; + $s1 =~ s/ve.*y //; # I am cold + $s2 =~ s/ve.*?y //; # I am very cold + +Notice how the second substitution stopped matching as soon as it +encountered "y ". The C<*?> quantifier effectively tells the regular +expression engine to find a match as quickly as possible and pass +control on to whatever is next in line, like you would if you were +playing hot potato. + +=head2 How do I process each word on each line? + +Use the split function: + + while (<>) { + foreach $word ( split ) { + # do something with $word here + } + } + +Note that this isn't really a word in the English sense; it's just +chunks of consecutive non-whitespace characters. + +To work with only alphanumeric sequences, you might consider + + while (<>) { + foreach $word (m/(\w+)/g) { + # do something with $word here + } + } + +=head2 How can I print out a word-frequency or line-frequency summary? + +To do this, you have to parse out each word in the input stream. We'll +pretend that by word you mean chunk of alphabetics, hyphens, or +apostrophes, rather than the non-whitespace chunk idea of a word given +in the previous question: + + while (<>) { + while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" + $seen{$1}++; + } + } + while ( ($word, $count) = each %seen ) { + print "$count $word\n"; + } + +If you wanted to do the same thing for lines, you wouldn't need a +regular expression: + + while (<>) { + $seen{$_}++; + } + while ( ($line, $count) = each %seen ) { + print "$count $line"; + } + +If you want these output in a sorted order, see the section on Hashes. + +=head2 How can I do approximate matching? + +See the module String::Approx available from CPAN. + +=head2 How do I efficiently match many regular expressions at once? + +The following is super-inefficient: + + while () { + foreach $pat (@patterns) { + if ( /$pat/ ) { + # do something + } + } + } + +Instead, you either need to use one of the experimental Regexp extension +modules from CPAN (which might well be overkill for your purposes), +or else put together something like this, inspired from a routine +in Jeffrey Friedl's book: + + sub _bm_build { + my $condition = shift; + my @regexp = @_; # this MUST not be local(); need my() + my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp); + my $match_func = eval "sub { $expr }"; + die if $@; # propagate $@; this shouldn't happen! + return $match_func; + } + + sub bm_and { _bm_build('&&', @_) } + sub bm_or { _bm_build('||', @_) } + + $f1 = bm_and qw{ + xterm + (?i)window + }; + + $f2 = bm_or qw{ + \b[Ff]ree\b + \bBSD\B + (?i)sys(tem)?\s*[V5]\b + }; + + # feed me /etc/termcap, prolly + while ( <> ) { + print "1: $_" if &$f1; + print "2: $_" if &$f2; + } + +=head2 Why don't word-boundary searches with C<\b> work for me? + +Two common misconceptions are that C<\b> is a synonym for C<\s+>, and +that it's the edge between whitespace characters and non-whitespace +characters. Neither is correct. C<\b> is the place between a C<\w> +character and a C<\W> character (that is, C<\b> is the edge of a +"word"). It's a zero-width assertion, just like C<^>, C<$>, and all +the other anchors, so it doesn't consume any characters. L +describes the behaviour of all the regexp metacharacters. + +Here are examples of the incorrect application of C<\b>, with fixes: + + "two words" =~ /(\w+)\b(\w+)/; # WRONG + "two words" =~ /(\w+)\s+(\w+)/; # right + + " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG + " =matchless= text" =~ /=(\w+)=/; # right + +Although they may not do what you thought they did, C<\b> and C<\B> +can still be quite useful. For an example of the correct use of +C<\b>, see the example of matching duplicate words over multiple +lines. + +An example of using C<\B> is the pattern C<\Bis\B>. This will find +occurrences of "is" on the insides of words only, as in "thistle", but +not "this" or "island". + +=head2 Why does using $&, $`, or $' slow my program down? + +Because once Perl sees that you need one of these variables anywhere +in the program, it has to provide them on each and every pattern +match. The same mechanism that handles these provides for the use of +$1, $2, etc., so you pay the same price for each regexp that contains +capturing parentheses. But if you never use $&, etc., in your script, +then regexps I capturing parentheses won't be penalized. So +avoid $&, $', and $` if you can, but if you can't (and some algorithms +really appreciate them), once you've used them once, use them at will, +because you've already paid the price. + +=head2 What good is C<\G> in a regular expression? + +The notation C<\G> is used in a match or substitution in conjunction the +C modifier (and ignored if there's no C) to anchor the regular +expression to the point just past where the last match occurred, i.e. the +pos() point. + +For example, suppose you had a line of text quoted in standard mail +and Usenet notation, (that is, with leading C> characters), and +you want change each leading C> into a corresponding C<:>. You +could do so in this way: + + s/^(>+)/':' x length($1)/gem; + +Or, using C<\G>, the much simpler (and faster): + + s/\G>/:/g; + +A more sophisticated use might involve a tokenizer. The following +lex-like example is courtesy of Jeffrey Friedl. It did not work in +5.003 due to bugs in that release, but does work in 5.004 or better: + + while (<>) { + chomp; + PARSER: { + m/ \G( \d+\b )/gx && do { print "number: $1\n"; redo; }; + m/ \G( \w+ )/gx && do { print "word: $1\n"; redo; }; + m/ \G( \s+ )/gx && do { print "space: $1\n"; redo; }; + m/ \G( [^\w\d]+ )/gx && do { print "other: $1\n"; redo; }; + } + } + +Of course, that could have been written as + + while (<>) { + chomp; + PARSER: { + if ( /\G( \d+\b )/gx { + print "number: $1\n"; + redo PARSER; + } + if ( /\G( \w+ )/gx { + print "word: $1\n"; + redo PARSER; + } + if ( /\G( \s+ )/gx { + print "space: $1\n"; + redo PARSER; + } + if ( /\G( [^\w\d]+ )/gx { + print "other: $1\n"; + redo PARSER; + } + } + } + +But then you lose the vertical alignment of the regular expressions. + +=head2 Are Perl regexps DFAs or NFAs? Are they POSIX compliant? + +While it's true that Perl's regular expressions resemble the DFAs +(deterministic finite automata) of the egrep(1) program, they are in +fact implemented as NFAs (non-deterministic finite automata) to allow +backtracking and backreferencing. And they aren't POSIX-style either, +because those guarantee worst-case behavior for all cases. (It seems +that some people prefer guarantees of consistency, even when what's +guaranteed is slowness.) See the book "Mastering Regular Expressions" +(from O'Reilly) by Jeffrey Friedl for all the details you could ever +hope to know on these matters (a full citation appears in +L). + +=head2 What's wrong with using grep or map in a void context? + +Strictly speaking, nothing. Stylistically speaking, it's not a good +way to write maintainable code. That's because you're using these +constructs not for their return values but rather for their +side-effects, and side-effects can be mystifying. There's no void +grep() that's not better written as a C (well, C, +technically) loop. + +=head2 How can I match strings with multi-byte characters? + +This is hard, and there's no good way. Perl does not directly support +wide characters. It pretends that a byte and a character are +synonymous. The following set of approaches was offered by Jeffrey +Friedl, whose article in issue #5 of The Perl Journal talks about this +very matter. + +Let's suppose you have some weird Martian encoding where pairs of ASCII +uppercase letters encode single Martian letters (i.e. the two bytes +"CV" make a single Martian letter, as do the two bytes "SG", "VS", +"XX", etc.). Other bytes represent single characters, just like ASCII. + +So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine +characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. + +Now, say you want to search for the single character C. Perl +doesn't know about Martian, so it'll find the two bytes "GX" in the +"I am CVSGXX!" string, even though that character isn't there: it just +looks like it is because "SG" is next to "XX", but there's no real "GX". +This is a big problem. + +Here are a few ways, all painful, to deal with it: + + $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``maritan'' bytes + # are no longer adjacent. + print "found GX!\n" if $martian =~ /GX/; + +Or like this: + + @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; + # above is conceptually similar to: @chars = $text =~ m/(.)/g; + # + foreach $char (@chars) { + print "found GX!\n", last if $char eq 'GX'; + } + +Or like this: + + while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded + print "found GX!\n", last if $1 eq 'GX'; + } + +Or like this: + + die "sorry, Perl doesn't (yet) have Martian support )-:\n"; + +In addition, a sample program which converts half-width to full-width +katakana (in Shift-JIS or EUC encoding) is available from CPAN as + +=for Tom make it so + +There are many double- (and multi-) byte encodings commonly used these +days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, +all mixed. + +=head1 AUTHOR AND COPYRIGHT + +Copyright (c) 1997 Tom Christiansen and Nathan Torkington. +All rights reserved. See L for distribution information. -- cgit v1.2.1