3 perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $)
7 This section is surprisingly small because the rest of the FAQ is
8 littered with answers involving regular expressions. For example,
9 decoding a URL and checking whether something is a number are handled
10 with regular expressions, but those answers are found elsewhere in
11 this document (in the section on Data and the Networking one on
12 networking, to be precise).
14 =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
16 Three techniques can make regular expressions maintainable and
21 =item Comments Outside the Regex
23 Describe what you're doing and how you're doing it, using normal Perl
26 # turn the line into the first word, a colon, and the
27 # number of characters on the rest of the line
28 s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;
30 =item Comments Inside the Regex
32 The C</x> modifier causes whitespace to be ignored in a regex pattern
33 (except in a character class), and also allows you to use normal
34 comments there, too. As you can imagine, whitespace and comments help
37 C</x> lets you turn this:
39 s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;
43 s{ < # opening angle bracket
44 (?: # Non-backreffing grouping paren
45 [^>'"] * # 0 or more things that are neither > nor ' nor "
47 ".*?" # a section between double quotes (stingy match)
49 '.*?' # a section between single quotes (stingy match)
50 ) + # all occurring one or more times
51 > # closing angle bracket
52 }{}gsx; # replace with nothing, i.e. delete
54 It's still not quite so clear as prose, but it is very useful for
55 describing the meaning of each part of the pattern.
57 =item Different Delimiters
59 While we normally think of patterns as being delimited with C</>
60 characters, they can be delimited by almost any character. L<perlre>
61 describes this. For example, the C<s///> above uses braces as
62 delimiters. Selecting another delimiter can avoid quoting the
63 delimiter within the pattern:
65 s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
66 s#/usr/local#/usr/share#g; # better
70 =head2 I'm having trouble matching over more than one line. What's wrong?
72 Either you don't have more than one line in the string you're looking at
73 (probably), or else you aren't using the correct modifier(s) on your
76 There are many ways to get multiline data into a string. If you want
77 it to happen automatically while reading input, you'll want to set $/
78 (probably to '' for paragraphs or C<undef> for the whole file) to
79 allow you to read more than one line at a time.
81 Read L<perlre> to help you decide which of C</s> and C</m> (or both)
82 you might want to use: C</s> allows dot to include newline, and C</m>
83 allows caret and dollar to match next to a newline, not just at the
84 end of the string. You do need to make sure that you've actually
85 got a multiline string in there.
87 For example, this program detects duplicate words, even when they span
88 line breaks (but not paragraph ones). For this example, we don't need
89 C</s> because we aren't using dot in a regular expression that we want
90 to cross line boundaries. Neither do we need C</m> because we aren't
91 wanting caret or dollar to match at any point inside the record next
92 to newlines. But it's imperative that $/ be set to something other
93 than the default, or else we won't actually ever have a multiline
96 $/ = ''; # read in more whole paragraph, not just one line
98 while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha
99 print "Duplicate $1 at paragraph $.\n";
103 Here's code that finds sentences that begin with "From " (which would
104 be mangled by many mailers):
106 $/ = ''; # read in more whole paragraph, not just one line
108 while ( /^From /gm ) { # /m makes ^ match next to \n
109 print "leading from in paragraph $.\n";
113 Here's code that finds everything between START and END in a paragraph:
115 undef $/; # read in whole file, not just one line or paragraph
117 while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries
122 =head2 How can I pull out lines between two patterns that are themselves on different lines?
124 You can use Perl's somewhat exotic C<..> operator (documented in
127 perl -ne 'print if /START/ .. /END/' file1 file2 ...
129 If you wanted text and not lines, you would use
131 perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
133 But if you want nested occurrences of C<START> through C<END>, you'll
134 run up against the problem described in the question in this section
135 on matching balanced text.
137 Here's another example of using C<..>:
140 $in_header = 1 .. /^$/;
141 $in_body = /^$/ .. eof();
142 # now choose between them
144 reset if eof(); # fix $.
147 =head2 I put a regular expression into $/ but it didn't work. What's wrong?
149 $/ must be a string, not a regular expression. Awk has to be better
152 Actually, you could do this if you don't mind reading the whole file
156 @records = split /your_pattern/, <FH>;
158 The Net::Telnet module (available from CPAN) has the capability to
159 wait for a pattern in the input stream, or timeout if it doesn't
160 appear within a certain time.
162 ## Create a file with three lines.
164 print FH "The first line\nThe second line\nThe third line\n";
167 ## Get a read/write filehandle to it.
168 $fh = new FileHandle "+<file";
170 ## Attach it to a "stream" object.
172 $file = new Net::Telnet (-fhopen => $fh);
174 ## Search for the second line and print out the third.
175 $file->waitfor('/second line\n/');
176 print $file->getline;
178 =head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS?
180 Here's a lovely Perlish solution by Larry Rosler. It exploits
181 properties of bitwise xor on ASCII strings.
183 $_= "this is a TEsT case";
189 { uc $new | (uc $1 ^ $1) .
190 (uc(substr $1, -1) ^ substr $1, -1) x
191 (length($new) - length $1)
196 And here it is as a subroutine, modelled after the above:
198 sub preserve_case($$) {
199 my ($old, $new) = @_;
200 my $mask = uc $old ^ $old;
203 substr($mask, -1) x (length($new) - length($old))
206 $a = "this is a TEsT case";
207 $a =~ s/(test)/preserve_case($1, "success")/egi;
212 this is a SUcCESS case
214 Just to show that C programmers can write C in any programming language,
215 if you prefer a more C-like solution, the following script makes the
216 substitution have the same case, letter by letter, as the original.
217 (It also happens to run about 240% slower than the Perlish solution runs.)
218 If the substitution has more characters than the string being substituted,
219 the case of the last character is used for the rest of the substitution.
221 # Original by Nathan Torkington, massaged by Jeffrey Friedl
223 sub preserve_case($$)
225 my ($old, $new) = @_;
226 my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
227 my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
228 my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
230 for ($i = 0; $i < $len; $i++) {
231 if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
233 } elsif (lc $c eq $c) {
234 substr($new, $i, 1) = lc(substr($new, $i, 1));
237 substr($new, $i, 1) = uc(substr($new, $i, 1));
241 # finish up with any remaining new (for when new is longer than old)
242 if ($newlen > $oldlen) {
244 substr($new, $oldlen) = lc(substr($new, $oldlen));
245 } elsif ($state == 2) {
246 substr($new, $oldlen) = uc(substr($new, $oldlen));
252 =head2 How can I make C<\w> match national character sets?
256 =head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
258 One alphabetic character would be C</[^\W\d_]/>, no matter what locale
259 you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't
260 consider an underscore a letter).
262 =head2 How can I quote a variable to use in a regex?
264 The Perl parser will expand $variable and @variable references in
265 regular expressions unless the delimiter is a single quote. Remember,
266 too, that the right-hand side of a C<s///> substitution is considered
267 a double-quoted string (see L<perlop> for more details). Remember
268 also that any regex special characters will be acted on unless you
269 precede the substitution with \Q. Here's an example:
273 $rhs = "sleep, no more";
275 $string =~ s/\Q$lhs/$rhs/;
276 # $string is now "to sleep no more"
278 Without the \Q, the regex would also spuriously match "di".
280 =head2 What is C</o> really for?
282 Using a variable in a regular expression match forces a re-evaluation
283 (and perhaps recompilation) each time through. The C</o> modifier
284 locks in the regex the first time it's used. This always happens in a
285 constant regular expression, and in fact, the pattern was compiled
286 into the internal format at the same time your entire program was.
288 Use of C</o> is irrelevant unless variable interpolation is used in
289 the pattern, and if so, the regex engine will neither know nor care
290 whether the variables change after the pattern is evaluated the I<very
293 C</o> is often used to gain an extra measure of efficiency by not
294 performing subsequent evaluations when you know it won't matter
295 (because you know the variables won't change), or more rarely, when
296 you don't want the regex to notice if they do.
298 For example, here's a "paragrep" program:
300 $/ = ''; # paragraph mode
306 =head2 How do I use a regular expression to strip C style comments from a file?
308 While this actually can be done, it's much harder than you'd think.
309 For example, this one-liner
311 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
313 will work in many but not all cases. You see, it's too simple-minded for
314 certain kinds of C programs, in particular, those with what appear to be
315 comments in quoted strings. For that, you'd need something like this,
316 created by Jeffrey Friedl and later modified by Fred Curtis.
320 s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs
323 This could, of course, be more legibly written with the C</x> modifier, adding
324 whitespace and comments. Here it is expanded, courtesy of Fred Curtis.
327 /\* ## Start of /* ... */ comment
328 [^*]*\*+ ## Non-* followed by 1-or-more *'s
331 )* ## 0-or-more things which don't start with /
332 ## but do end with '*'
333 / ## End of /* ... */ comment
335 | ## OR various things which aren't comments:
338 " ## Start of " ... " string
344 " ## End of " ... " string
348 ' ## Start of ' ... ' string
354 ' ## End of ' ... ' string
358 . ## Anything other char
359 [^/"'\\]* ## Chars which doesn't start a comment, string or escape
363 A slight modification also removes C++ comments:
365 s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs;
367 =head2 Can I use Perl regular expressions to match balanced text?
369 Although Perl regular expressions are more powerful than "mathematical"
370 regular expressions, because they feature conveniences like backreferences
371 (C<\1> and its ilk), they still aren't powerful enough -- with
372 the possible exception of bizarre and experimental features in the
373 development-track releases of Perl. You still need to use non-regex
374 techniques to parse balanced text, such as the text enclosed between
375 matching parentheses or braces, for example.
377 An elaborate subroutine (for 7-bit ASCII only) to pull out balanced
378 and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>,
379 or C<(> and C<)> can be found in
380 http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz .
382 The C::Scan module from CPAN contains such subs for internal usage,
383 but they are undocumented.
385 =head2 What does it mean that regexes are greedy? How can I get around it?
387 Most people mean that greedy regexes match as much as they can.
388 Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>,
389 C<{}>) that are greedy rather than the whole pattern; Perl prefers local
390 greed and immediate gratification to overall greed. To get non-greedy
391 versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>).
395 $s1 = $s2 = "I am very very cold";
396 $s1 =~ s/ve.*y //; # I am cold
397 $s2 =~ s/ve.*?y //; # I am very cold
399 Notice how the second substitution stopped matching as soon as it
400 encountered "y ". The C<*?> quantifier effectively tells the regular
401 expression engine to find a match as quickly as possible and pass
402 control on to whatever is next in line, like you would if you were
405 =head2 How do I process each word on each line?
407 Use the split function:
410 foreach $word ( split ) {
411 # do something with $word here
415 Note that this isn't really a word in the English sense; it's just
416 chunks of consecutive non-whitespace characters.
418 To work with only alphanumeric sequences, you might consider
421 foreach $word (m/(\w+)/g) {
422 # do something with $word here
426 =head2 How can I print out a word-frequency or line-frequency summary?
428 To do this, you have to parse out each word in the input stream. We'll
429 pretend that by word you mean chunk of alphabetics, hyphens, or
430 apostrophes, rather than the non-whitespace chunk idea of a word given
431 in the previous question:
434 while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
438 while ( ($word, $count) = each %seen ) {
439 print "$count $word\n";
442 If you wanted to do the same thing for lines, you wouldn't need a
448 while ( ($line, $count) = each %seen ) {
449 print "$count $line";
452 If you want these output in a sorted order, see the section on Hashes.
454 =head2 How can I do approximate matching?
456 See the module String::Approx available from CPAN.
458 =head2 How do I efficiently match many regular expressions at once?
460 The following is extremely inefficient:
462 # slow but obvious way
463 @popstates = qw(CO ON MI WI MN);
464 while (defined($line = <>)) {
465 for $state (@popstates) {
466 if ($line =~ /\b$state\b/i) {
473 That's because Perl has to recompile all those patterns for each of
474 the lines of the file. As of the 5.005 release, there's a much better
475 approach, one which makes use of the new C<qr//> operator:
477 # use spiffy new qr// operator, with /i flag even
479 @popstates = qw(CO ON MI WI MN);
480 @poppats = map { qr/\b$_\b/i } @popstates;
481 while (defined($line = <>)) {
482 for $patobj (@poppats) {
483 print $line if $line =~ /$patobj/;
487 =head2 Why don't word-boundary searches with C<\b> work for me?
489 Two common misconceptions are that C<\b> is a synonym for C<\s+>, and
490 that it's the edge between whitespace characters and non-whitespace
491 characters. Neither is correct. C<\b> is the place between a C<\w>
492 character and a C<\W> character (that is, C<\b> is the edge of a
493 "word"). It's a zero-width assertion, just like C<^>, C<$>, and all
494 the other anchors, so it doesn't consume any characters. L<perlre>
495 describes the behavior of all the regex metacharacters.
497 Here are examples of the incorrect application of C<\b>, with fixes:
499 "two words" =~ /(\w+)\b(\w+)/; # WRONG
500 "two words" =~ /(\w+)\s+(\w+)/; # right
502 " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
503 " =matchless= text" =~ /=(\w+)=/; # right
505 Although they may not do what you thought they did, C<\b> and C<\B>
506 can still be quite useful. For an example of the correct use of
507 C<\b>, see the example of matching duplicate words over multiple
510 An example of using C<\B> is the pattern C<\Bis\B>. This will find
511 occurrences of "is" on the insides of words only, as in "thistle", but
512 not "this" or "island".
514 =head2 Why does using $&, $`, or $' slow my program down?
516 Because once Perl sees that you need one of these variables anywhere in
517 the program, it has to provide them on each and every pattern match.
518 The same mechanism that handles these provides for the use of $1, $2,
519 etc., so you pay the same price for each regex that contains capturing
520 parentheses. But if you never use $&, etc., in your script, then regexes
521 I<without> capturing parentheses won't be penalized. So avoid $&, $',
522 and $` if you can, but if you can't, once you've used them at all, use
523 them at will because you've already paid the price. Remember that some
524 algorithms really appreciate them. As of the 5.005 release. the $&
525 variable is no longer "expensive" the way the other two are.
527 =head2 What good is C<\G> in a regular expression?
529 The notation C<\G> is used in a match or substitution in conjunction the
530 C</g> modifier (and ignored if there's no C</g>) to anchor the regular
531 expression to the point just past where the last match occurred, i.e. the
532 pos() point. A failed match resets the position of C<\G> unless the
533 C</c> modifier is in effect.
535 For example, suppose you had a line of text quoted in standard mail
536 and Usenet notation, (that is, with leading C<< > >> characters), and
537 you want change each leading C<< > >> into a corresponding C<:>. You
538 could do so in this way:
540 s/^(>+)/':' x length($1)/gem;
542 Or, using C<\G>, the much simpler (and faster):
546 A more sophisticated use might involve a tokenizer. The following
547 lex-like example is courtesy of Jeffrey Friedl. It did not work in
548 5.003 due to bugs in that release, but does work in 5.004 or better.
549 (Note the use of C</c>, which prevents a failed match with C</g> from
550 resetting the search position back to the beginning of the string.)
555 m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; };
556 m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; };
557 m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; };
558 m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; };
562 Of course, that could have been written as
567 if ( /\G( \d+\b )/gcx {
568 print "number: $1\n";
571 if ( /\G( \w+ )/gcx {
575 if ( /\G( \s+ )/gcx {
579 if ( /\G( [^\w\d]+ )/gcx {
586 But then you lose the vertical alignment of the regular expressions.
588 =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
590 While it's true that Perl's regular expressions resemble the DFAs
591 (deterministic finite automata) of the egrep(1) program, they are in
592 fact implemented as NFAs (non-deterministic finite automata) to allow
593 backtracking and backreferencing. And they aren't POSIX-style either,
594 because those guarantee worst-case behavior for all cases. (It seems
595 that some people prefer guarantees of consistency, even when what's
596 guaranteed is slowness.) See the book "Mastering Regular Expressions"
597 (from O'Reilly) by Jeffrey Friedl for all the details you could ever
598 hope to know on these matters (a full citation appears in
601 =head2 What's wrong with using grep or map in a void context?
603 Both grep and map build a return list, regardless of their context.
604 This means you're making Perl go to the trouble of building up a
605 return list that you then just ignore. That's no way to treat a
606 programming language, you insensitive scoundrel!
608 =head2 How can I match strings with multibyte characters?
610 This is hard, and there's no good way. Perl does not directly support
611 wide characters. It pretends that a byte and a character are
612 synonymous. The following set of approaches was offered by Jeffrey
613 Friedl, whose article in issue #5 of The Perl Journal talks about this
616 Let's suppose you have some weird Martian encoding where pairs of
617 ASCII uppercase letters encode single Martian letters (i.e. the two
618 bytes "CV" make a single Martian letter, as do the two bytes "SG",
619 "VS", "XX", etc.). Other bytes represent single characters, just like
622 So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the
623 nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
625 Now, say you want to search for the single character C</GX/>. Perl
626 doesn't know about Martian, so it'll find the two bytes "GX" in the "I
627 am CVSGXX!" string, even though that character isn't there: it just
628 looks like it is because "SG" is next to "XX", but there's no real
629 "GX". This is a big problem.
631 Here are a few ways, all painful, to deal with it:
633 $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes
634 # are no longer adjacent.
635 print "found GX!\n" if $martian =~ /GX/;
639 @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
640 # above is conceptually similar to: @chars = $text =~ m/(.)/g;
642 foreach $char (@chars) {
643 print "found GX!\n", last if $char eq 'GX';
648 while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
649 print "found GX!\n", last if $1 eq 'GX';
654 die "sorry, Perl doesn't (yet) have Martian support )-:\n";
656 There are many double- (and multi-) byte encodings commonly used these
657 days. Some versions of these have 1-, 2-, 3-, and 4-byte characters,
660 =head2 How do I match a pattern that is supplied by the user?
662 Well, if it's really a pattern, then just use
664 chomp($pattern = <STDIN>);
665 if ($line =~ /$pattern/) { }
667 Or, since you have no guarantee that your user entered
668 a valid regular expression, trap the exception this way:
670 if (eval { $line =~ /$pattern/ }) { }
672 But if all you really want to search for a string, not a pattern,
673 then you should either use the index() function, which is made for
674 string searching, or if you can't be disabused of using a pattern
675 match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented
680 open (FILE, $input) or die "Couldn't open input $input: $!; aborting";
682 print if /\Q$pattern\E/;
686 =head1 AUTHOR AND COPYRIGHT
688 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
691 When included as part of the Standard Version of Perl, or as part of
692 its complete documentation whether printed or otherwise, this work
693 may be distributed only under the terms of Perl's Artistic License.
694 Any distribution of this file or derivatives thereof I<outside>
695 of that package require that special arrangements be made with
698 Irrespective of its distribution, all code examples in this file
699 are hereby placed into the public domain. You are permitted and
700 encouraged to use this code in your own programs for fun
701 or for profit as you see fit. A simple comment in the code giving
702 credit would be courteous but is not required.