1perlfaq6(3)           User Contributed Perl Documentation          perlfaq6(3)
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NAME

6       perlfaq6 - Regular Expressions
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VERSION

9       version 5.20191102
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DESCRIPTION

12       This section is surprisingly small because the rest of the FAQ is
13       littered with answers involving regular expressions. For example,
14       decoding a URL and checking whether something is a number can be
15       handled with regular expressions, but those answers are found elsewhere
16       in this document (in perlfaq9: "How do I decode or create those
17       %-encodings on the web" and perlfaq4: "How do I determine whether a
18       scalar is a number/whole/integer/float", to be precise).
19
20   How can I hope to use regular expressions without creating illegible and
21       unmaintainable code?
22       Three techniques can make regular expressions maintainable and
23       understandable.
24
25       Comments Outside the Regex
26           Describe what you're doing and how you're doing it, using normal
27           Perl comments.
28
29               # turn the line into the first word, a colon, and the
30               # number of characters on the rest of the line
31               s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;
32
33       Comments Inside the Regex
34           The "/x" modifier causes whitespace to be ignored in a regex
35           pattern (except in a character class and a few other places), and
36           also allows you to use normal comments there, too. As you can
37           imagine, whitespace and comments help a lot.
38
39           "/x" lets you turn this:
40
41               s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;
42
43           into this:
44
45               s{ <                    # opening angle bracket
46                   (?:                 # Non-backreffing grouping paren
47                       [^>'"] *        # 0 or more things that are neither > nor ' nor "
48                           |           #    or else
49                       ".*?"           # a section between double quotes (stingy match)
50                           |           #    or else
51                       '.*?'           # a section between single quotes (stingy match)
52                   ) +                 #   all occurring one or more times
53                   >                   # closing angle bracket
54               }{}gsx;                 # replace with nothing, i.e. delete
55
56           It's still not quite so clear as prose, but it is very useful for
57           describing the meaning of each part of the pattern.
58
59       Different Delimiters
60           While we normally think of patterns as being delimited with "/"
61           characters, they can be delimited by almost any character. perlre
62           describes this. For example, the "s///" above uses braces as
63           delimiters. Selecting another delimiter can avoid quoting the
64           delimiter within the pattern:
65
66               s/\/usr\/local/\/usr\/share/g;    # bad delimiter choice
67               s#/usr/local#/usr/share#g;        # better
68
69           Using logically paired delimiters can be even more readable:
70
71               s{/usr/local/}{/usr/share}g;      # better still
72
73   I'm having trouble matching over more than one line. What's wrong?
74       Either you don't have more than one line in the string you're looking
75       at (probably), or else you aren't using the correct modifier(s) on your
76       pattern (possibly).
77
78       There are many ways to get multiline data into a string. If you want it
79       to happen automatically while reading input, you'll want to set $/
80       (probably to '' for paragraphs or "undef" for the whole file) to allow
81       you to read more than one line at a time.
82
83       Read perlre to help you decide which of "/s" and "/m" (or both) you
84       might want to use: "/s" allows dot to include newline, and "/m" allows
85       caret and dollar to match next to a newline, not just at the end of the
86       string. You do need to make sure that you've actually got a multiline
87       string in there.
88
89       For example, this program detects duplicate words, even when they span
90       line breaks (but not paragraph ones). For this example, we don't need
91       "/s" because we aren't using dot in a regular expression that we want
92       to cross line boundaries. Neither do we need "/m" because we don't want
93       caret or dollar to match at any point inside the record next to
94       newlines. But it's imperative that $/ be set to something other than
95       the default, or else we won't actually ever have a multiline record
96       read in.
97
98           $/ = '';          # read in whole paragraph, not just one line
99           while ( <> ) {
100               while ( /\b([\w'-]+)(\s+\g1)+\b/gi ) {     # word starts alpha
101                   print "Duplicate $1 at paragraph $.\n";
102               }
103           }
104
105       Here's some code that finds sentences that begin with "From " (which
106       would be mangled by many mailers):
107
108           $/ = '';          # read in whole paragraph, not just one line
109           while ( <> ) {
110               while ( /^From /gm ) { # /m makes ^ match next to \n
111               print "leading From in paragraph $.\n";
112               }
113           }
114
115       Here's code that finds everything between START and END in a paragraph:
116
117           undef $/;          # read in whole file, not just one line or paragraph
118           while ( <> ) {
119               while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
120                   print "$1\n";
121               }
122           }
123
124   How can I pull out lines between two patterns that are themselves on
125       different lines?
126       You can use Perl's somewhat exotic ".." operator (documented in
127       perlop):
128
129           perl -ne 'print if /START/ .. /END/' file1 file2 ...
130
131       If you wanted text and not lines, you would use
132
133           perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
134
135       But if you want nested occurrences of "START" through "END", you'll run
136       up against the problem described in the question in this section on
137       matching balanced text.
138
139       Here's another example of using "..":
140
141           while (<>) {
142               my $in_header =   1  .. /^$/;
143               my $in_body   = /^$/ .. eof;
144           # now choose between them
145           } continue {
146               $. = 0 if eof;    # fix $.
147           }
148
149   How do I match XML, HTML, or other nasty, ugly things with a regex?
150       Do not use regexes. Use a module and forget about the regular
151       expressions. The XML::LibXML, HTML::TokeParser and HTML::TreeBuilder
152       modules are good starts, although each namespace has other parsing
153       modules specialized for certain tasks and different ways of doing it.
154       Start at CPAN Search ( <http://metacpan.org/> ) and wonder at all the
155       work people have done for you already! :)
156
157   I put a regular expression into $/ but it didn't work. What's wrong?
158       $/ has to be a string. You can use these examples if you really need to
159       do this.
160
161       If you have File::Stream, this is easy.
162
163           use File::Stream;
164
165           my $stream = File::Stream->new(
166               $filehandle,
167               separator => qr/\s*,\s*/,
168               );
169
170           print "$_\n" while <$stream>;
171
172       If you don't have File::Stream, you have to do a little more work.
173
174       You can use the four-argument form of sysread to continually add to a
175       buffer. After you add to the buffer, you check if you have a complete
176       line (using your regular expression).
177
178           local $_ = "";
179           while( sysread FH, $_, 8192, length ) {
180               while( s/^((?s).*?)your_pattern// ) {
181                   my $record = $1;
182                   # do stuff here.
183               }
184           }
185
186       You can do the same thing with foreach and a match using the c flag and
187       the \G anchor, if you do not mind your entire file being in memory at
188       the end.
189
190           local $_ = "";
191           while( sysread FH, $_, 8192, length ) {
192               foreach my $record ( m/\G((?s).*?)your_pattern/gc ) {
193                   # do stuff here.
194               }
195               substr( $_, 0, pos ) = "" if pos;
196           }
197
198   How do I substitute case-insensitively on the LHS while preserving case on
199       the RHS?
200       Here's a lovely Perlish solution by Larry Rosler. It exploits
201       properties of bitwise xor on ASCII strings.
202
203           $_= "this is a TEsT case";
204
205           $old = 'test';
206           $new = 'success';
207
208           s{(\Q$old\E)}
209           { uc $new | (uc $1 ^ $1) .
210               (uc(substr $1, -1) ^ substr $1, -1) x
211               (length($new) - length $1)
212           }egi;
213
214           print;
215
216       And here it is as a subroutine, modeled after the above:
217
218           sub preserve_case {
219               my ($old, $new) = @_;
220               my $mask = uc $old ^ $old;
221
222               uc $new | $mask .
223                   substr($mask, -1) x (length($new) - length($old))
224           }
225
226           $string = "this is a TEsT case";
227           $string =~ s/(test)/preserve_case($1, "success")/egi;
228           print "$string\n";
229
230       This prints:
231
232           this is a SUcCESS case
233
234       As an alternative, to keep the case of the replacement word if it is
235       longer than the original, you can use this code, by Jeff Pinyan:
236
237           sub preserve_case {
238               my ($from, $to) = @_;
239               my ($lf, $lt) = map length, @_;
240
241               if ($lt < $lf) { $from = substr $from, 0, $lt }
242               else { $from .= substr $to, $lf }
243
244               return uc $to | ($from ^ uc $from);
245           }
246
247       This changes the sentence to "this is a SUcCess case."
248
249       Just to show that C programmers can write C in any programming
250       language, if you prefer a more C-like solution, the following script
251       makes the substitution have the same case, letter by letter, as the
252       original.  (It also happens to run about 240% slower than the Perlish
253       solution runs.)  If the substitution has more characters than the
254       string being substituted, the case of the last character is used for
255       the rest of the substitution.
256
257           # Original by Nathan Torkington, massaged by Jeffrey Friedl
258           #
259           sub preserve_case
260           {
261               my ($old, $new) = @_;
262               my $state = 0; # 0 = no change; 1 = lc; 2 = uc
263               my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
264               my $len = $oldlen < $newlen ? $oldlen : $newlen;
265
266               for ($i = 0; $i < $len; $i++) {
267                   if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
268                       $state = 0;
269                   } elsif (lc $c eq $c) {
270                       substr($new, $i, 1) = lc(substr($new, $i, 1));
271                       $state = 1;
272                   } else {
273                       substr($new, $i, 1) = uc(substr($new, $i, 1));
274                       $state = 2;
275                   }
276               }
277               # finish up with any remaining new (for when new is longer than old)
278               if ($newlen > $oldlen) {
279                   if ($state == 1) {
280                       substr($new, $oldlen) = lc(substr($new, $oldlen));
281                   } elsif ($state == 2) {
282                       substr($new, $oldlen) = uc(substr($new, $oldlen));
283                   }
284               }
285               return $new;
286           }
287
288   How can I make "\w" match national character sets?
289       Put "use locale;" in your script. The \w character class is taken from
290       the current locale.
291
292       See perllocale for details.
293
294   How can I match a locale-smart version of "/[a-zA-Z]/"?
295       You can use the POSIX character class syntax "/[[:alpha:]]/" documented
296       in perlre.
297
298       No matter which locale you are in, the alphabetic characters are the
299       characters in \w without the digits and the underscore.  As a regex,
300       that looks like "/[^\W\d_]/". Its complement, the non-alphabetics, is
301       then everything in \W along with the digits and the underscore, or
302       "/[\W\d_]/".
303
304   How can I quote a variable to use in a regex?
305       The Perl parser will expand $variable and @variable references in
306       regular expressions unless the delimiter is a single quote. Remember,
307       too, that the right-hand side of a "s///" substitution is considered a
308       double-quoted string (see perlop for more details). Remember also that
309       any regex special characters will be acted on unless you precede the
310       substitution with \Q. Here's an example:
311
312           $string = "Placido P. Octopus";
313           $regex  = "P.";
314
315           $string =~ s/$regex/Polyp/;
316           # $string is now "Polypacido P. Octopus"
317
318       Because "." is special in regular expressions, and can match any single
319       character, the regex "P." here has matched the <Pl> in the original
320       string.
321
322       To escape the special meaning of ".", we use "\Q":
323
324           $string = "Placido P. Octopus";
325           $regex  = "P.";
326
327           $string =~ s/\Q$regex/Polyp/;
328           # $string is now "Placido Polyp Octopus"
329
330       The use of "\Q" causes the "." in the regex to be treated as a regular
331       character, so that "P." matches a "P" followed by a dot.
332
333   What is "/o" really for?
334       (contributed by brian d foy)
335
336       The "/o" option for regular expressions (documented in perlop and
337       perlreref) tells Perl to compile the regular expression only once.
338       This is only useful when the pattern contains a variable. Perls 5.6 and
339       later handle this automatically if the pattern does not change.
340
341       Since the match operator "m//", the substitution operator "s///", and
342       the regular expression quoting operator "qr//" are double-quotish
343       constructs, you can interpolate variables into the pattern. See the
344       answer to "How can I quote a variable to use in a regex?" for more
345       details.
346
347       This example takes a regular expression from the argument list and
348       prints the lines of input that match it:
349
350           my $pattern = shift @ARGV;
351
352           while( <> ) {
353               print if m/$pattern/;
354           }
355
356       Versions of Perl prior to 5.6 would recompile the regular expression
357       for each iteration, even if $pattern had not changed. The "/o" would
358       prevent this by telling Perl to compile the pattern the first time,
359       then reuse that for subsequent iterations:
360
361           my $pattern = shift @ARGV;
362
363           while( <> ) {
364               print if m/$pattern/o; # useful for Perl < 5.6
365           }
366
367       In versions 5.6 and later, Perl won't recompile the regular expression
368       if the variable hasn't changed, so you probably don't need the "/o"
369       option. It doesn't hurt, but it doesn't help either. If you want any
370       version of Perl to compile the regular expression only once even if the
371       variable changes (thus, only using its initial value), you still need
372       the "/o".
373
374       You can watch Perl's regular expression engine at work to verify for
375       yourself if Perl is recompiling a regular expression. The "use re
376       'debug'" pragma (comes with Perl 5.005 and later) shows the details.
377       With Perls before 5.6, you should see "re" reporting that its compiling
378       the regular expression on each iteration. With Perl 5.6 or later, you
379       should only see "re" report that for the first iteration.
380
381           use re 'debug';
382
383           my $regex = 'Perl';
384           foreach ( qw(Perl Java Ruby Python) ) {
385               print STDERR "-" x 73, "\n";
386               print STDERR "Trying $_...\n";
387               print STDERR "\t$_ is good!\n" if m/$regex/;
388           }
389
390   How do I use a regular expression to strip C-style comments from a file?
391       While this actually can be done, it's much harder than you'd think.
392       For example, this one-liner
393
394           perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
395
396       will work in many but not all cases. You see, it's too simple-minded
397       for certain kinds of C programs, in particular, those with what appear
398       to be comments in quoted strings. For that, you'd need something like
399       this, created by Jeffrey Friedl and later modified by Fred Curtis.
400
401           $/ = undef;
402           $_ = <>;
403           s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
404           print;
405
406       This could, of course, be more legibly written with the "/x" modifier,
407       adding whitespace and comments. Here it is expanded, courtesy of Fred
408       Curtis.
409
410           s{
411              /\*         ##  Start of /* ... */ comment
412              [^*]*\*+    ##  Non-* followed by 1-or-more *'s
413              (
414                [^/*][^*]*\*+
415              )*          ##  0-or-more things which don't start with /
416                          ##    but do end with '*'
417              /           ##  End of /* ... */ comment
418
419            |         ##     OR  various things which aren't comments:
420
421              (
422                "           ##  Start of " ... " string
423                (
424                  \\.           ##  Escaped char
425                |               ##    OR
426                  [^"\\]        ##  Non "\
427                )*
428                "           ##  End of " ... " string
429
430              |         ##     OR
431
432                '           ##  Start of ' ... ' string
433                (
434                  \\.           ##  Escaped char
435                |               ##    OR
436                  [^'\\]        ##  Non '\
437                )*
438                '           ##  End of ' ... ' string
439
440              |         ##     OR
441
442                .           ##  Anything other char
443                [^/"'\\]*   ##  Chars which doesn't start a comment, string or escape
444              )
445            }{defined $2 ? $2 : ""}gxse;
446
447       A slight modification also removes C++ comments, possibly spanning
448       multiple lines using a continuation character:
449
450        s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//([^\\]|[^\n][\n]?)*?\n|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $3 ? $3 : ""#gse;
451
452   Can I use Perl regular expressions to match balanced text?
453       (contributed by brian d foy)
454
455       Your first try should probably be the Text::Balanced module, which is
456       in the Perl standard library since Perl 5.8. It has a variety of
457       functions to deal with tricky text. The Regexp::Common module can also
458       help by providing canned patterns you can use.
459
460       As of Perl 5.10, you can match balanced text with regular expressions
461       using recursive patterns. Before Perl 5.10, you had to resort to
462       various tricks such as using Perl code in "(??{})" sequences.
463
464       Here's an example using a recursive regular expression. The goal is to
465       capture all of the text within angle brackets, including the text in
466       nested angle brackets. This sample text has two "major" groups: a group
467       with one level of nesting and a group with two levels of nesting. There
468       are five total groups in angle brackets:
469
470           I have some <brackets in <nested brackets> > and
471           <another group <nested once <nested twice> > >
472           and that's it.
473
474       The regular expression to match the balanced text uses two new (to Perl
475       5.10) regular expression features. These are covered in perlre and this
476       example is a modified version of one in that documentation.
477
478       First, adding the new possessive "+" to any quantifier finds the
479       longest match and does not backtrack. That's important since you want
480       to handle any angle brackets through the recursion, not backtracking.
481       The group "[^<>]++" finds one or more non-angle brackets without
482       backtracking.
483
484       Second, the new "(?PARNO)" refers to the sub-pattern in the particular
485       capture group given by "PARNO". In the following regex, the first
486       capture group finds (and remembers) the balanced text, and you need
487       that same pattern within the first buffer to get past the nested text.
488       That's the recursive part. The "(?1)" uses the pattern in the outer
489       capture group as an independent part of the regex.
490
491       Putting it all together, you have:
492
493           #!/usr/local/bin/perl5.10.0
494
495           my $string =<<"HERE";
496           I have some <brackets in <nested brackets> > and
497           <another group <nested once <nested twice> > >
498           and that's it.
499           HERE
500
501           my @groups = $string =~ m/
502                   (                   # start of capture group 1
503                   <                   # match an opening angle bracket
504                       (?:
505                           [^<>]++     # one or more non angle brackets, non backtracking
506                             |
507                           (?1)        # found < or >, so recurse to capture group 1
508                       )*
509                   >                   # match a closing angle bracket
510                   )                   # end of capture group 1
511                   /xg;
512
513           $" = "\n\t";
514           print "Found:\n\t@groups\n";
515
516       The output shows that Perl found the two major groups:
517
518           Found:
519               <brackets in <nested brackets> >
520               <another group <nested once <nested twice> > >
521
522       With a little extra work, you can get all of the groups in angle
523       brackets even if they are in other angle brackets too. Each time you
524       get a balanced match, remove its outer delimiter (that's the one you
525       just matched so don't match it again) and add it to a queue of strings
526       to process. Keep doing that until you get no matches:
527
528           #!/usr/local/bin/perl5.10.0
529
530           my @queue =<<"HERE";
531           I have some <brackets in <nested brackets> > and
532           <another group <nested once <nested twice> > >
533           and that's it.
534           HERE
535
536           my $regex = qr/
537                   (                   # start of bracket 1
538                   <                   # match an opening angle bracket
539                       (?:
540                           [^<>]++     # one or more non angle brackets, non backtracking
541                             |
542                           (?1)        # recurse to bracket 1
543                       )*
544                   >                   # match a closing angle bracket
545                   )                   # end of bracket 1
546                   /x;
547
548           $" = "\n\t";
549
550           while( @queue ) {
551               my $string = shift @queue;
552
553               my @groups = $string =~ m/$regex/g;
554               print "Found:\n\t@groups\n\n" if @groups;
555
556               unshift @queue, map { s/^<//; s/>$//; $_ } @groups;
557           }
558
559       The output shows all of the groups. The outermost matches show up first
560       and the nested matches show up later:
561
562           Found:
563               <brackets in <nested brackets> >
564               <another group <nested once <nested twice> > >
565
566           Found:
567               <nested brackets>
568
569           Found:
570               <nested once <nested twice> >
571
572           Found:
573               <nested twice>
574
575   What does it mean that regexes are greedy? How can I get around it?
576       Most people mean that greedy regexes match as much as they can.
577       Technically speaking, it's actually the quantifiers ("?", "*", "+",
578       "{}") that are greedy rather than the whole pattern; Perl prefers local
579       greed and immediate gratification to overall greed. To get non-greedy
580       versions of the same quantifiers, use ("??", "*?", "+?", "{}?").
581
582       An example:
583
584           my $s1 = my $s2 = "I am very very cold";
585           $s1 =~ s/ve.*y //;      # I am cold
586           $s2 =~ s/ve.*?y //;     # I am very cold
587
588       Notice how the second substitution stopped matching as soon as it
589       encountered "y ". The "*?" quantifier effectively tells the regular
590       expression engine to find a match as quickly as possible and pass
591       control on to whatever is next in line, as you would if you were
592       playing hot potato.
593
594   How do I process each word on each line?
595       Use the split function:
596
597           while (<>) {
598               foreach my $word ( split ) {
599                   # do something with $word here
600               }
601           }
602
603       Note that this isn't really a word in the English sense; it's just
604       chunks of consecutive non-whitespace characters.
605
606       To work with only alphanumeric sequences (including underscores), you
607       might consider
608
609           while (<>) {
610               foreach $word (m/(\w+)/g) {
611                   # do something with $word here
612               }
613           }
614
615   How can I print out a word-frequency or line-frequency summary?
616       To do this, you have to parse out each word in the input stream. We'll
617       pretend that by word you mean chunk of alphabetics, hyphens, or
618       apostrophes, rather than the non-whitespace chunk idea of a word given
619       in the previous question:
620
621           my (%seen);
622           while (<>) {
623               while ( /(\b[^\W_\d][\w'-]+\b)/g ) {   # misses "`sheep'"
624                   $seen{$1}++;
625               }
626           }
627
628           while ( my ($word, $count) = each %seen ) {
629               print "$count $word\n";
630           }
631
632       If you wanted to do the same thing for lines, you wouldn't need a
633       regular expression:
634
635           my (%seen);
636
637           while (<>) {
638               $seen{$_}++;
639           }
640
641           while ( my ($line, $count) = each %seen ) {
642               print "$count $line";
643           }
644
645       If you want these output in a sorted order, see perlfaq4: "How do I
646       sort a hash (optionally by value instead of key)?".
647
648   How can I do approximate matching?
649       See the module String::Approx available from CPAN.
650
651   How do I efficiently match many regular expressions at once?
652       (contributed by brian d foy)
653
654       If you have Perl 5.10 or later, this is almost trivial. You just smart
655       match against an array of regular expression objects:
656
657           my @patterns = ( qr/Fr.d/, qr/B.rn.y/, qr/W.lm./ );
658
659           if( $string ~~ @patterns ) {
660               ...
661           };
662
663       The smart match stops when it finds a match, so it doesn't have to try
664       every expression.
665
666       Earlier than Perl 5.10, you have a bit of work to do. You want to avoid
667       compiling a regular expression every time you want to match it.  In
668       this example, perl must recompile the regular expression for every
669       iteration of the "foreach" loop since it has no way to know what
670       $pattern will be:
671
672           my @patterns = qw( foo bar baz );
673
674           LINE: while( <DATA> ) {
675               foreach $pattern ( @patterns ) {
676                   if( /\b$pattern\b/i ) {
677                       print;
678                       next LINE;
679                   }
680               }
681           }
682
683       The "qr//" operator showed up in perl 5.005. It compiles a regular
684       expression, but doesn't apply it. When you use the pre-compiled version
685       of the regex, perl does less work. In this example, I inserted a "map"
686       to turn each pattern into its pre-compiled form. The rest of the script
687       is the same, but faster:
688
689           my @patterns = map { qr/\b$_\b/i } qw( foo bar baz );
690
691           LINE: while( <> ) {
692               foreach $pattern ( @patterns ) {
693                   if( /$pattern/ ) {
694                       print;
695                       next LINE;
696                   }
697               }
698           }
699
700       In some cases, you may be able to make several patterns into a single
701       regular expression. Beware of situations that require backtracking
702       though.
703
704           my $regex = join '|', qw( foo bar baz );
705
706           LINE: while( <> ) {
707               print if /\b(?:$regex)\b/i;
708           }
709
710       For more details on regular expression efficiency, see Mastering
711       Regular Expressions by Jeffrey Friedl. He explains how the regular
712       expressions engine works and why some patterns are surprisingly
713       inefficient. Once you understand how perl applies regular expressions,
714       you can tune them for individual situations.
715
716   Why don't word-boundary searches with "\b" work for me?
717       (contributed by brian d foy)
718
719       Ensure that you know what \b really does: it's the boundary between a
720       word character, \w, and something that isn't a word character. That
721       thing that isn't a word character might be \W, but it can also be the
722       start or end of the string.
723
724       It's not (not!) the boundary between whitespace and non-whitespace, and
725       it's not the stuff between words we use to create sentences.
726
727       In regex speak, a word boundary (\b) is a "zero width assertion",
728       meaning that it doesn't represent a character in the string, but a
729       condition at a certain position.
730
731       For the regular expression, /\bPerl\b/, there has to be a word boundary
732       before the "P" and after the "l". As long as something other than a
733       word character precedes the "P" and succeeds the "l", the pattern will
734       match. These strings match /\bPerl\b/.
735
736           "Perl"    # no word char before "P" or after "l"
737           "Perl "   # same as previous (space is not a word char)
738           "'Perl'"  # the "'" char is not a word char
739           "Perl's"  # no word char before "P", non-word char after "l"
740
741       These strings do not match /\bPerl\b/.
742
743           "Perl_"   # "_" is a word char!
744           "Perler"  # no word char before "P", but one after "l"
745
746       You don't have to use \b to match words though. You can look for non-
747       word characters surrounded by word characters. These strings match the
748       pattern /\b'\b/.
749
750           "don't"   # the "'" char is surrounded by "n" and "t"
751           "qep'a'"  # the "'" char is surrounded by "p" and "a"
752
753       These strings do not match /\b'\b/.
754
755           "foo'"    # there is no word char after non-word "'"
756
757       You can also use the complement of \b, \B, to specify that there should
758       not be a word boundary.
759
760       In the pattern /\Bam\B/, there must be a word character before the "a"
761       and after the "m". These patterns match /\Bam\B/:
762
763           "llama"   # "am" surrounded by word chars
764           "Samuel"  # same
765
766       These strings do not match /\Bam\B/
767
768           "Sam"      # no word boundary before "a", but one after "m"
769           "I am Sam" # "am" surrounded by non-word chars
770
771   Why does using $&, $`, or $' slow my program down?
772       (contributed by Anno Siegel)
773
774       Once Perl sees that you need one of these variables anywhere in the
775       program, it provides them on each and every pattern match. That means
776       that on every pattern match the entire string will be copied, part of
777       it to $`, part to $&, and part to $'. Thus the penalty is most severe
778       with long strings and patterns that match often. Avoid $&, $', and $`
779       if you can, but if you can't, once you've used them at all, use them at
780       will because you've already paid the price. Remember that some
781       algorithms really appreciate them. As of the 5.005 release, the $&
782       variable is no longer "expensive" the way the other two are.
783
784       Since Perl 5.6.1 the special variables @- and @+ can functionally
785       replace $`, $& and $'. These arrays contain pointers to the beginning
786       and end of each match (see perlvar for the full story), so they give
787       you essentially the same information, but without the risk of excessive
788       string copying.
789
790       Perl 5.10 added three specials, "${^MATCH}", "${^PREMATCH}", and
791       "${^POSTMATCH}" to do the same job but without the global performance
792       penalty. Perl 5.10 only sets these variables if you compile or execute
793       the regular expression with the "/p" modifier.
794
795   What good is "\G" in a regular expression?
796       You use the "\G" anchor to start the next match on the same string
797       where the last match left off. The regular expression engine cannot
798       skip over any characters to find the next match with this anchor, so
799       "\G" is similar to the beginning of string anchor, "^". The "\G" anchor
800       is typically used with the "g" modifier. It uses the value of "pos()"
801       as the position to start the next match. As the match operator makes
802       successive matches, it updates "pos()" with the position of the next
803       character past the last match (or the first character of the next
804       match, depending on how you like to look at it). Each string has its
805       own "pos()" value.
806
807       Suppose you want to match all of consecutive pairs of digits in a
808       string like "1122a44" and stop matching when you encounter non-digits.
809       You want to match 11 and 22 but the letter "a" shows up between 22 and
810       44 and you want to stop at "a". Simply matching pairs of digits skips
811       over the "a" and still matches 44.
812
813           $_ = "1122a44";
814           my @pairs = m/(\d\d)/g;   # qw( 11 22 44 )
815
816       If you use the "\G" anchor, you force the match after 22 to start with
817       the "a". The regular expression cannot match there since it does not
818       find a digit, so the next match fails and the match operator returns
819       the pairs it already found.
820
821           $_ = "1122a44";
822           my @pairs = m/\G(\d\d)/g; # qw( 11 22 )
823
824       You can also use the "\G" anchor in scalar context. You still need the
825       "g" modifier.
826
827           $_ = "1122a44";
828           while( m/\G(\d\d)/g ) {
829               print "Found $1\n";
830           }
831
832       After the match fails at the letter "a", perl resets "pos()" and the
833       next match on the same string starts at the beginning.
834
835           $_ = "1122a44";
836           while( m/\G(\d\d)/g ) {
837               print "Found $1\n";
838           }
839
840           print "Found $1 after while" if m/(\d\d)/g; # finds "11"
841
842       You can disable "pos()" resets on fail with the "c" modifier,
843       documented in perlop and perlreref. Subsequent matches start where the
844       last successful match ended (the value of "pos()") even if a match on
845       the same string has failed in the meantime. In this case, the match
846       after the "while()" loop starts at the "a" (where the last match
847       stopped), and since it does not use any anchor it can skip over the "a"
848       to find 44.
849
850           $_ = "1122a44";
851           while( m/\G(\d\d)/gc ) {
852               print "Found $1\n";
853           }
854
855           print "Found $1 after while" if m/(\d\d)/g; # finds "44"
856
857       Typically you use the "\G" anchor with the "c" modifier when you want
858       to try a different match if one fails, such as in a tokenizer. Jeffrey
859       Friedl offers this example which works in 5.004 or later.
860
861           while (<>) {
862               chomp;
863               PARSER: {
864                   m/ \G( \d+\b    )/gcx   && do { print "number: $1\n";  redo; };
865                   m/ \G( \w+      )/gcx   && do { print "word:   $1\n";  redo; };
866                   m/ \G( \s+      )/gcx   && do { print "space:  $1\n";  redo; };
867                   m/ \G( [^\w\d]+ )/gcx   && do { print "other:  $1\n";  redo; };
868               }
869           }
870
871       For each line, the "PARSER" loop first tries to match a series of
872       digits followed by a word boundary. This match has to start at the
873       place the last match left off (or the beginning of the string on the
874       first match). Since "m/ \G( \d+\b )/gcx" uses the "c" modifier, if the
875       string does not match that regular expression, perl does not reset
876       pos() and the next match starts at the same position to try a different
877       pattern.
878
879   Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
880       While it's true that Perl's regular expressions resemble the DFAs
881       (deterministic finite automata) of the egrep(1) program, they are in
882       fact implemented as NFAs (non-deterministic finite automata) to allow
883       backtracking and backreferencing. And they aren't POSIX-style either,
884       because those guarantee worst-case behavior for all cases. (It seems
885       that some people prefer guarantees of consistency, even when what's
886       guaranteed is slowness.) See the book "Mastering Regular Expressions"
887       (from O'Reilly) by Jeffrey Friedl for all the details you could ever
888       hope to know on these matters (a full citation appears in perlfaq2).
889
890   What's wrong with using grep in a void context?
891       The problem is that grep builds a return list, regardless of the
892       context.  This means you're making Perl go to the trouble of building a
893       list that you then just throw away. If the list is large, you waste
894       both time and space.  If your intent is to iterate over the list, then
895       use a for loop for this purpose.
896
897       In perls older than 5.8.1, map suffers from this problem as well.  But
898       since 5.8.1, this has been fixed, and map is context aware - in void
899       context, no lists are constructed.
900
901   How can I match strings with multibyte characters?
902       Starting from Perl 5.6 Perl has had some level of multibyte character
903       support. Perl 5.8 or later is recommended. Supported multibyte
904       character repertoires include Unicode, and legacy encodings through the
905       Encode module. See perluniintro, perlunicode, and Encode.
906
907       If you are stuck with older Perls, you can do Unicode with the
908       Unicode::String module, and character conversions using the
909       Unicode::Map8 and Unicode::Map modules. If you are using Japanese
910       encodings, you might try using the jperl 5.005_03.
911
912       Finally, the following set of approaches was offered by Jeffrey Friedl,
913       whose article in issue #5 of The Perl Journal talks about this very
914       matter.
915
916       Let's suppose you have some weird Martian encoding where pairs of ASCII
917       uppercase letters encode single Martian letters (i.e. the two bytes
918       "CV" make a single Martian letter, as do the two bytes "SG", "VS",
919       "XX", etc.). Other bytes represent single characters, just like ASCII.
920
921       So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the
922       nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
923
924       Now, say you want to search for the single character "/GX/". Perl
925       doesn't know about Martian, so it'll find the two bytes "GX" in the "I
926       am CVSGXX!" string, even though that character isn't there: it just
927       looks like it is because "SG" is next to "XX", but there's no real
928       "GX". This is a big problem.
929
930       Here are a few ways, all painful, to deal with it:
931
932           # Make sure adjacent "martian" bytes are no longer adjacent.
933           $martian =~ s/([A-Z][A-Z])/ $1 /g;
934
935           print "found GX!\n" if $martian =~ /GX/;
936
937       Or like this:
938
939           my @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
940           # above is conceptually similar to:     my @chars = $text =~ m/(.)/g;
941           #
942           foreach my $char (@chars) {
943               print "found GX!\n", last if $char eq 'GX';
944           }
945
946       Or like this:
947
948           while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) {  # \G probably unneeded
949               if ($1 eq 'GX') {
950                   print "found GX!\n";
951                   last;
952               }
953           }
954
955       Here's another, slightly less painful, way to do it from Benjamin
956       Goldberg, who uses a zero-width negative look-behind assertion.
957
958           print "found GX!\n" if    $martian =~ m/
959               (?<![A-Z])
960               (?:[A-Z][A-Z])*?
961               GX
962               /x;
963
964       This succeeds if the "martian" character GX is in the string, and fails
965       otherwise. If you don't like using (?<!), a zero-width negative look-
966       behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]).
967
968       It does have the drawback of putting the wrong thing in $-[0] and
969       $+[0], but this usually can be worked around.
970
971   How do I match a regular expression that's in a variable?
972       (contributed by brian d foy)
973
974       We don't have to hard-code patterns into the match operator (or
975       anything else that works with regular expressions). We can put the
976       pattern in a variable for later use.
977
978       The match operator is a double quote context, so you can interpolate
979       your variable just like a double quoted string. In this case, you read
980       the regular expression as user input and store it in $regex.  Once you
981       have the pattern in $regex, you use that variable in the match
982       operator.
983
984           chomp( my $regex = <STDIN> );
985
986           if( $string =~ m/$regex/ ) { ... }
987
988       Any regular expression special characters in $regex are still special,
989       and the pattern still has to be valid or Perl will complain.  For
990       instance, in this pattern there is an unpaired parenthesis.
991
992           my $regex = "Unmatched ( paren";
993
994           "Two parens to bind them all" =~ m/$regex/;
995
996       When Perl compiles the regular expression, it treats the parenthesis as
997       the start of a memory match. When it doesn't find the closing
998       parenthesis, it complains:
999
1000           Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE  paren/ at script line 3.
1001
1002       You can get around this in several ways depending on our situation.
1003       First, if you don't want any of the characters in the string to be
1004       special, you can escape them with "quotemeta" before you use the
1005       string.
1006
1007           chomp( my $regex = <STDIN> );
1008           $regex = quotemeta( $regex );
1009
1010           if( $string =~ m/$regex/ ) { ... }
1011
1012       You can also do this directly in the match operator using the "\Q" and
1013       "\E" sequences. The "\Q" tells Perl where to start escaping special
1014       characters, and the "\E" tells it where to stop (see perlop for more
1015       details).
1016
1017           chomp( my $regex = <STDIN> );
1018
1019           if( $string =~ m/\Q$regex\E/ ) { ... }
1020
1021       Alternately, you can use "qr//", the regular expression quote operator
1022       (see perlop for more details). It quotes and perhaps compiles the
1023       pattern, and you can apply regular expression flags to the pattern.
1024
1025           chomp( my $input = <STDIN> );
1026
1027           my $regex = qr/$input/is;
1028
1029           $string =~ m/$regex/  # same as m/$input/is;
1030
1031       You might also want to trap any errors by wrapping an "eval" block
1032       around the whole thing.
1033
1034           chomp( my $input = <STDIN> );
1035
1036           eval {
1037               if( $string =~ m/\Q$input\E/ ) { ... }
1038           };
1039           warn $@ if $@;
1040
1041       Or...
1042
1043           my $regex = eval { qr/$input/is };
1044           if( defined $regex ) {
1045               $string =~ m/$regex/;
1046           }
1047           else {
1048               warn $@;
1049           }
1050
1052       Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
1053       authors as noted. All rights reserved.
1054
1055       This documentation is free; you can redistribute it and/or modify it
1056       under the same terms as Perl itself.
1057
1058       Irrespective of its distribution, all code examples in this file are
1059       hereby placed into the public domain. You are permitted and encouraged
1060       to use this code in your own programs for fun or for profit as you see
1061       fit. A simple comment in the code giving credit would be courteous but
1062       is not required.
1063
1064
1065
1066perl v5.30.0                      2019-11-04                       perlfaq6(3)
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