3 perlfaq6 - Regexps ($Revision: 1.16 $, $Date: 1997/03/25 18:16:56 $)
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 Regexp
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) /ge;
30 =item Comments Inside the Regexp
32 The C</x> modifier causes whitespace to be ignored in a regexp 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 newlines in your string, or you aren't using the
73 correct modifier(s) on your pattern.
75 There are many ways to get multiline data into a string. If you want
76 it to happen automatically while reading input, you'll want to set $/
77 (probably to '' for paragraphs or C<undef> for the whole file) to
78 allow you to read more than one line at a time.
80 Read L<perlre> to help you decide which of C</s> and C</m> (or both)
81 you might want to use: C</s> allows dot to include newline, and C</m>
82 allows caret and dollar to match next to a newline, not just at the
83 end of the string. You do need to make sure that you've actually
84 got a multiline string in there.
86 For example, this program detects duplicate words, even when they span
87 line breaks (but not paragraph ones). For this example, we don't need
88 C</s> because we aren't using dot in a regular expression that we want
89 to cross line boundaries. Neither do we need C</m> because we aren't
90 wanting caret or dollar to match at any point inside the record next
91 to newlines. But it's imperative that $/ be set to something other
92 than the default, or else we won't actually ever have a multiline
95 $/ = ''; # read in more whole paragraph, not just one line
97 while ( /\b(\w\S+)(\s+\1)+\b/gi ) {
98 print "Duplicate $1 at paragraph $.\n";
102 Here's code that finds sentences that begin with "From " (which would
103 be mangled by many mailers):
105 $/ = ''; # read in more whole paragraph, not just one line
107 while ( /^From /gm ) { # /m makes ^ match next to \n
108 print "leading from in paragraph $.\n";
112 Here's code that finds everything between START and END in a paragraph:
114 undef $/; # read in whole file, not just one line or paragraph
116 while ( /START(.*?)END/sm ) { # /s makes . cross line boundaries
121 =head2 How can I pull out lines between two patterns that are themselves on different lines?
123 You can use Perl's somewhat exotic C<..> operator (documented in
126 perl -ne 'print if /START/ .. /END/' file1 file2 ...
128 If you wanted text and not lines, you would use
130 perl -0777 -pe 'print "$1\n" while /START(.*?)END/gs' file1 file2 ...
132 But if you want nested occurrences of C<START> through C<END>, you'll
133 run up against the problem described in the question in this section
134 on matching balanced text.
136 =head2 I put a regular expression into $/ but it didn't work. What's wrong?
138 $/ must be a string, not a regular expression. Awk has to be better
141 Actually, you could do this if you don't mind reading the whole file into
144 @records = split /your_pattern/, <FH>;
146 The Net::Telnet module (available from CPAN) has the capability to
147 wait for a pattern in the input stream, or timeout if it doesn't
148 appear within a certain time.
150 ## Create a file with three lines.
152 print FH "The first line\nThe second line\nThe third line\n";
155 ## Get a read/write filehandle to it.
156 $fh = new FileHandle "+<file";
158 ## Attach it to a "stream" object.
160 $file = new Net::Telnet (-fhopen => $fh);
162 ## Search for the second line and print out the third.
163 $file->waitfor('/second line\n/');
164 print $file->getline;
166 =head2 How do I substitute case insensitively on the LHS, but preserving case on the RHS?
168 It depends on what you mean by "preserving case". The following
169 script makes the substitution have the same case, letter by letter, as
170 the original. If the substitution has more characters than the string
171 being substituted, the case of the last character is used for the rest
174 # Original by Nathan Torkington, massaged by Jeffrey Friedl
176 sub preserve_case($$)
178 my ($old, $new) = @_;
179 my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
180 my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
181 my ($len) = $oldlen < $newlen ? $oldlen : $newlen;
183 for ($i = 0; $i < $len; $i++) {
184 if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
186 } elsif (lc $c eq $c) {
187 substr($new, $i, 1) = lc(substr($new, $i, 1));
190 substr($new, $i, 1) = uc(substr($new, $i, 1));
194 # finish up with any remaining new (for when new is longer than old)
195 if ($newlen > $oldlen) {
197 substr($new, $oldlen) = lc(substr($new, $oldlen));
198 } elsif ($state == 2) {
199 substr($new, $oldlen) = uc(substr($new, $oldlen));
205 $a = "this is a TEsT case";
206 $a =~ s/(test)/preserve_case($1, "success")/gie;
211 this is a SUcCESS case
213 =head2 How can I make C<\w> match accented characters?
217 =head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
219 One alphabetic character would be C</[^\W\d_]/>, no matter what locale
220 you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't
221 consider an underscore a letter).
223 =head2 How can I quote a variable to use in a regexp?
225 The Perl parser will expand $variable and @variable references in
226 regular expressions unless the delimiter is a single quote. Remember,
227 too, that the right-hand side of a C<s///> substitution is considered
228 a double-quoted string (see L<perlop> for more details). Remember
229 also that any regexp special characters will be acted on unless you
230 precede the substitution with \Q. Here's an example:
234 $rhs = "sleep no more";
236 $string =~ s/\Q$lhs/$rhs/;
237 # $string is now "to sleep no more"
239 Without the \Q, the regexp would also spuriously match "di".
241 =head2 What is C</o> really for?
243 Using a variable in a regular expression match forces a reevaluation
244 (and perhaps recompilation) each time through. The C</o> modifier
245 locks in the regexp the first time it's used. This always happens in a
246 constant regular expression, and in fact, the pattern was compiled
247 into the internal format at the same time your entire program was.
249 Use of C</o> is irrelevant unless variable interpolation is used in
250 the pattern, and if so, the regexp engine will neither know nor care
251 whether the variables change after the pattern is evaluated the I<very
254 C</o> is often used to gain an extra measure of efficiency by not
255 performing subsequent evaluations when you know it won't matter
256 (because you know the variables won't change), or more rarely, when
257 you don't want the regexp to notice if they do.
259 For example, here's a "paragrep" program:
261 $/ = ''; # paragraph mode
267 =head2 How do I use a regular expression to strip C style comments from a file?
269 While this actually can be done, it's much harder than you'd think.
270 For example, this one-liner
272 perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c
274 will work in many but not all cases. You see, it's too simple-minded for
275 certain kinds of C programs, in particular, those with what appear to be
276 comments in quoted strings. For that, you'd need something like this,
277 created by Jeffrey Friedl:
281 s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|\n+|.[^/"'\\]*)#$2#g;
284 This could, of course, be more legibly written with the C</x> modifier, adding
285 whitespace and comments.
287 =head2 Can I use Perl regular expressions to match balanced text?
289 Although Perl regular expressions are more powerful than "mathematical"
290 regular expressions, because they feature conveniences like backreferences
291 (C<\1> and its ilk), they still aren't powerful enough. You still need
292 to use non-regexp techniques to parse balanced text, such as the text
293 enclosed between matching parentheses or braces, for example.
295 An elaborate subroutine (for 7-bit ASCII only) to pull out balanced
296 and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>,
297 or C<(> and C<)> can be found in
298 http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz .
300 The C::Scan module from CPAN contains such subs for internal usage,
301 but they are undocumented.
303 =head2 What does it mean that regexps are greedy? How can I get around it?
305 Most people mean that greedy regexps match as much as they can.
306 Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>,
307 C<{}>) that are greedy rather than the whole pattern; Perl prefers local
308 greed and immediate gratification to overall greed. To get non-greedy
309 versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>).
313 $s1 = $s2 = "I am very very cold";
314 $s1 =~ s/ve.*y //; # I am cold
315 $s2 =~ s/ve.*?y //; # I am very cold
317 Notice how the second substitution stopped matching as soon as it
318 encountered "y ". The C<*?> quantifier effectively tells the regular
319 expression engine to find a match as quickly as possible and pass
320 control on to whatever is next in line, like you would if you were
323 =head2 How do I process each word on each line?
325 Use the split function:
328 foreach $word ( split ) {
329 # do something with $word here
333 Note that this isn't really a word in the English sense; it's just
334 chunks of consecutive non-whitespace characters.
336 To work with only alphanumeric sequences, you might consider
339 foreach $word (m/(\w+)/g) {
340 # do something with $word here
344 =head2 How can I print out a word-frequency or line-frequency summary?
346 To do this, you have to parse out each word in the input stream. We'll
347 pretend that by word you mean chunk of alphabetics, hyphens, or
348 apostrophes, rather than the non-whitespace chunk idea of a word given
349 in the previous question:
352 while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
356 while ( ($word, $count) = each %seen ) {
357 print "$count $word\n";
360 If you wanted to do the same thing for lines, you wouldn't need a
366 while ( ($line, $count) = each %seen ) {
367 print "$count $line";
370 If you want these output in a sorted order, see the section on Hashes.
372 =head2 How can I do approximate matching?
374 See the module String::Approx available from CPAN.
376 =head2 How do I efficiently match many regular expressions at once?
378 The following is super-inefficient:
381 foreach $pat (@patterns) {
388 Instead, you either need to use one of the experimental Regexp extension
389 modules from CPAN (which might well be overkill for your purposes),
390 or else put together something like this, inspired from a routine
391 in Jeffrey Friedl's book:
394 my $condition = shift;
395 my @regexp = @_; # this MUST not be local(); need my()
396 my $expr = join $condition => map { "m/\$regexp[$_]/o" } (0..$#regexp);
397 my $match_func = eval "sub { $expr }";
398 die if $@; # propagate $@; this shouldn't happen!
402 sub bm_and { _bm_build('&&', @_) }
403 sub bm_or { _bm_build('||', @_) }
413 (?i)sys(tem)?\s*[V5]\b
416 # feed me /etc/termcap, prolly
418 print "1: $_" if &$f1;
419 print "2: $_" if &$f2;
422 =head2 Why don't word-boundary searches with C<\b> work for me?
424 Two common misconceptions are that C<\b> is a synonym for C<\s+>, and
425 that it's the edge between whitespace characters and non-whitespace
426 characters. Neither is correct. C<\b> is the place between a C<\w>
427 character and a C<\W> character (that is, C<\b> is the edge of a
428 "word"). It's a zero-width assertion, just like C<^>, C<$>, and all
429 the other anchors, so it doesn't consume any characters. L<perlre>
430 describes the behaviour of all the regexp metacharacters.
432 Here are examples of the incorrect application of C<\b>, with fixes:
434 "two words" =~ /(\w+)\b(\w+)/; # WRONG
435 "two words" =~ /(\w+)\s+(\w+)/; # right
437 " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG
438 " =matchless= text" =~ /=(\w+)=/; # right
440 Although they may not do what you thought they did, C<\b> and C<\B>
441 can still be quite useful. For an example of the correct use of
442 C<\b>, see the example of matching duplicate words over multiple
445 An example of using C<\B> is the pattern C<\Bis\B>. This will find
446 occurrences of "is" on the insides of words only, as in "thistle", but
447 not "this" or "island".
449 =head2 Why does using $&, $`, or $' slow my program down?
451 Because once Perl sees that you need one of these variables anywhere
452 in the program, it has to provide them on each and every pattern
453 match. The same mechanism that handles these provides for the use of
454 $1, $2, etc., so you pay the same price for each regexp that contains
455 capturing parentheses. But if you never use $&, etc., in your script,
456 then regexps I<without> capturing parentheses won't be penalized. So
457 avoid $&, $', and $` if you can, but if you can't (and some algorithms
458 really appreciate them), once you've used them once, use them at will,
459 because you've already paid the price.
461 =head2 What good is C<\G> in a regular expression?
463 The notation C<\G> is used in a match or substitution in conjunction the
464 C</g> modifier (and ignored if there's no C</g>) to anchor the regular
465 expression to the point just past where the last match occurred, i.e. the
468 For example, suppose you had a line of text quoted in standard mail
469 and Usenet notation, (that is, with leading C<E<gt>> characters), and
470 you want change each leading C<E<gt>> into a corresponding C<:>. You
471 could do so in this way:
473 s/^(>+)/':' x length($1)/gem;
475 Or, using C<\G>, the much simpler (and faster):
479 A more sophisticated use might involve a tokenizer. The following
480 lex-like example is courtesy of Jeffrey Friedl. It did not work in
481 5.003 due to bugs in that release, but does work in 5.004 or better:
486 m/ \G( \d+\b )/gx && do { print "number: $1\n"; redo; };
487 m/ \G( \w+ )/gx && do { print "word: $1\n"; redo; };
488 m/ \G( \s+ )/gx && do { print "space: $1\n"; redo; };
489 m/ \G( [^\w\d]+ )/gx && do { print "other: $1\n"; redo; };
493 Of course, that could have been written as
498 if ( /\G( \d+\b )/gx {
499 print "number: $1\n";
510 if ( /\G( [^\w\d]+ )/gx {
517 But then you lose the vertical alignment of the regular expressions.
519 =head2 Are Perl regexps DFAs or NFAs? Are they POSIX compliant?
521 While it's true that Perl's regular expressions resemble the DFAs
522 (deterministic finite automata) of the egrep(1) program, they are in
523 fact implemented as NFAs (nondeterministic finite automata) to allow
524 backtracking and backreferencing. And they aren't POSIX-style either,
525 because those guarantee worst-case behavior for all cases. (It seems
526 that some people prefer guarantees of consistency, even when what's
527 guaranteed is slowness.) See the book "Mastering Regular Expressions"
528 (from O'Reilly) by Jeffrey Friedl for all the details you could ever
529 hope to know on these matters (a full citation appears in
532 =head2 What's wrong with using grep or map in a void context?
534 Strictly speaking, nothing. Stylistically speaking, it's not a good
535 way to write maintainable code. That's because you're using these
536 constructs not for their return values but rather for their
537 side-effects, and side-effects can be mystifying. There's no void
538 grep() that's not better written as a C<for> (well, C<foreach>,
541 =head2 How can I match strings with multibyte characters?
543 This is hard, and there's no good way. Perl does not directly support
544 wide characters. It pretends that a byte and a character are
545 synonymous. The following set of approaches was offered by Jeffrey
546 Friedl, whose article in issue #5 of The Perl Journal talks about this
549 Let's suppose you have some weird Martian encoding where pairs of ASCII
550 uppercase letters encode single Martian letters (i.e. the two bytes
551 "CV" make a single Martian letter, as do the two bytes "SG", "VS",
552 "XX", etc.). Other bytes represent single characters, just like ASCII.
554 So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine
555 characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
557 Now, say you want to search for the single character C</GX/>. Perl
558 doesn't know about Martian, so it'll find the two bytes "GX" in the
559 "I am CVSGXX!" string, even though that character isn't there: it just
560 looks like it is because "SG" is next to "XX", but there's no real "GX".
561 This is a big problem.
563 Here are a few ways, all painful, to deal with it:
565 $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes
566 # are no longer adjacent.
567 print "found GX!\n" if $martian =~ /GX/;
571 @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g;
572 # above is conceptually similar to: @chars = $text =~ m/(.)/g;
574 foreach $char (@chars) {
575 print "found GX!\n", last if $char eq 'GX';
580 while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
581 print "found GX!\n", last if $1 eq 'GX';
586 die "sorry, Perl doesn't (yet) have Martian support )-:\n";
588 In addition, a sample program which converts half-width to full-width
589 katakana (in Shift-JIS or EUC encoding) is available from CPAN as
593 There are many double (and multi) byte encodings commonly used these
594 days. Some versions of these have 1-, 2-, 3-, and 4-byte characters,
597 =head1 AUTHOR AND COPYRIGHT
599 Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
600 All rights reserved. See L<perlfaq> for distribution information.