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68dc0745 | 1 | =head1 NAME |
2 | ||
c195e131 | 3 | perlfaq6 - Regular Expressions ($Revision: 10126 $) |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
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 | |
b432a672 AL |
11 | this document (in L<perlfaq9>: "How do I decode or create those %-encodings |
12 | on the web" and L<perlfaq4>: "How do I determine whether a scalar is | |
13 | a number/whole/integer/float", to be precise). | |
68dc0745 | 14 | |
54310121 | 15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? |
d74e8afc ITB |
16 | X<regex, legibility> X<regexp, legibility> |
17 | X<regular expression, legibility> X</x> | |
68dc0745 | 18 | |
19 | Three techniques can make regular expressions maintainable and | |
20 | understandable. | |
21 | ||
22 | =over 4 | |
23 | ||
d92eb7b0 | 24 | =item Comments Outside the Regex |
68dc0745 | 25 | |
26 | Describe what you're doing and how you're doing it, using normal Perl | |
27 | comments. | |
28 | ||
ac9dac7f RGS |
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; | |
68dc0745 | 32 | |
d92eb7b0 | 33 | =item Comments Inside the Regex |
68dc0745 | 34 | |
d92eb7b0 | 35 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
68dc0745 | 36 | (except in a character class), and also allows you to use normal |
37 | comments there, too. As you can imagine, whitespace and comments help | |
38 | a lot. | |
39 | ||
40 | C</x> lets you turn this: | |
41 | ||
ac9dac7f | 42 | s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; |
68dc0745 | 43 | |
44 | into this: | |
45 | ||
ac9dac7f RGS |
46 | s{ < # opening angle bracket |
47 | (?: # Non-backreffing grouping paren | |
48 | [^>'"] * # 0 or more things that are neither > nor ' nor " | |
49 | | # or else | |
50 | ".*?" # a section between double quotes (stingy match) | |
51 | | # or else | |
52 | '.*?' # a section between single quotes (stingy match) | |
53 | ) + # all occurring one or more times | |
54 | > # closing angle bracket | |
55 | }{}gsx; # replace with nothing, i.e. delete | |
68dc0745 | 56 | |
57 | It's still not quite so clear as prose, but it is very useful for | |
58 | describing the meaning of each part of the pattern. | |
59 | ||
60 | =item Different Delimiters | |
61 | ||
62 | While we normally think of patterns as being delimited with C</> | |
63 | characters, they can be delimited by almost any character. L<perlre> | |
64 | describes this. For example, the C<s///> above uses braces as | |
65 | delimiters. Selecting another delimiter can avoid quoting the | |
66 | delimiter within the pattern: | |
67 | ||
ac9dac7f RGS |
68 | s/\/usr\/local/\/usr\/share/g; # bad delimiter choice |
69 | s#/usr/local#/usr/share#g; # better | |
68dc0745 | 70 | |
71 | =back | |
72 | ||
73 | =head2 I'm having trouble matching over more than one line. What's wrong? | |
d74e8afc | 74 | X<regex, multiline> X<regexp, multiline> X<regular expression, multiline> |
68dc0745 | 75 | |
3392b9ec JH |
76 | Either you don't have more than one line in the string you're looking |
77 | at (probably), or else you aren't using the correct modifier(s) on | |
78 | your pattern (possibly). | |
68dc0745 | 79 | |
80 | There are many ways to get multiline data into a string. If you want | |
81 | it to happen automatically while reading input, you'll want to set $/ | |
82 | (probably to '' for paragraphs or C<undef> for the whole file) to | |
83 | allow you to read more than one line at a time. | |
84 | ||
85 | Read L<perlre> to help you decide which of C</s> and C</m> (or both) | |
86 | you might want to use: C</s> allows dot to include newline, and C</m> | |
87 | allows caret and dollar to match next to a newline, not just at the | |
88 | end of the string. You do need to make sure that you've actually | |
89 | got a multiline string in there. | |
90 | ||
91 | For example, this program detects duplicate words, even when they span | |
92 | line breaks (but not paragraph ones). For this example, we don't need | |
93 | C</s> because we aren't using dot in a regular expression that we want | |
94 | to cross line boundaries. Neither do we need C</m> because we aren't | |
95 | wanting caret or dollar to match at any point inside the record next | |
96 | to newlines. But it's imperative that $/ be set to something other | |
97 | than the default, or else we won't actually ever have a multiline | |
98 | record read in. | |
99 | ||
ac9dac7f RGS |
100 | $/ = ''; # read in more whole paragraph, not just one line |
101 | while ( <> ) { | |
102 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha | |
103 | print "Duplicate $1 at paragraph $.\n"; | |
104 | } | |
54310121 | 105 | } |
68dc0745 | 106 | |
107 | Here's code that finds sentences that begin with "From " (which would | |
108 | be mangled by many mailers): | |
109 | ||
ac9dac7f RGS |
110 | $/ = ''; # read in more whole paragraph, not just one line |
111 | while ( <> ) { | |
112 | while ( /^From /gm ) { # /m makes ^ match next to \n | |
113 | print "leading from in paragraph $.\n"; | |
114 | } | |
68dc0745 | 115 | } |
68dc0745 | 116 | |
117 | Here's code that finds everything between START and END in a paragraph: | |
118 | ||
ac9dac7f RGS |
119 | undef $/; # read in whole file, not just one line or paragraph |
120 | while ( <> ) { | |
121 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries | |
122 | print "$1\n"; | |
123 | } | |
68dc0745 | 124 | } |
68dc0745 | 125 | |
126 | =head2 How can I pull out lines between two patterns that are themselves on different lines? | |
d74e8afc | 127 | X<..> |
68dc0745 | 128 | |
129 | You can use Perl's somewhat exotic C<..> operator (documented in | |
130 | L<perlop>): | |
131 | ||
ac9dac7f | 132 | perl -ne 'print if /START/ .. /END/' file1 file2 ... |
68dc0745 | 133 | |
134 | If you wanted text and not lines, you would use | |
135 | ||
ac9dac7f | 136 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
68dc0745 | 137 | |
138 | But if you want nested occurrences of C<START> through C<END>, you'll | |
139 | run up against the problem described in the question in this section | |
140 | on matching balanced text. | |
141 | ||
5a964f20 TC |
142 | Here's another example of using C<..>: |
143 | ||
ac9dac7f RGS |
144 | while (<>) { |
145 | $in_header = 1 .. /^$/; | |
e573f903 | 146 | $in_body = /^$/ .. eof; |
5a964f20 | 147 | # now choose between them |
ac9dac7f | 148 | } continue { |
e573f903 | 149 | $. = 0 if eof; # fix $. |
ac9dac7f | 150 | } |
5a964f20 | 151 | |
68dc0745 | 152 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
d74e8afc ITB |
153 | X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in> |
154 | X<$RS, regexes in> | |
68dc0745 | 155 | |
c195e131 RGS |
156 | $/ has to be a string. You can use these examples if you really need to |
157 | do this. | |
49d635f9 | 158 | |
28b41a80 RGS |
159 | If you have File::Stream, this is easy. |
160 | ||
ac9dac7f RGS |
161 | use File::Stream; |
162 | ||
163 | my $stream = File::Stream->new( | |
164 | $filehandle, | |
165 | separator => qr/\s*,\s*/, | |
166 | ); | |
28b41a80 | 167 | |
ac9dac7f | 168 | print "$_\n" while <$stream>; |
28b41a80 RGS |
169 | |
170 | If you don't have File::Stream, you have to do a little more work. | |
171 | ||
172 | You can use the four argument form of sysread to continually add to | |
197aec24 | 173 | a buffer. After you add to the buffer, you check if you have a |
49d635f9 RGS |
174 | complete line (using your regular expression). |
175 | ||
ac9dac7f RGS |
176 | local $_ = ""; |
177 | while( sysread FH, $_, 8192, length ) { | |
178 | while( s/^((?s).*?)your_pattern/ ) { | |
179 | my $record = $1; | |
180 | # do stuff here. | |
181 | } | |
182 | } | |
197aec24 | 183 | |
49d635f9 RGS |
184 | You can do the same thing with foreach and a match using the |
185 | c flag and the \G anchor, if you do not mind your entire file | |
186 | being in memory at the end. | |
197aec24 | 187 | |
ac9dac7f RGS |
188 | local $_ = ""; |
189 | while( sysread FH, $_, 8192, length ) { | |
190 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { | |
191 | # do stuff here. | |
192 | } | |
193 | substr( $_, 0, pos ) = "" if pos; | |
194 | } | |
68dc0745 | 195 | |
3fe9a6f1 | 196 | |
a6dd486b | 197 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
d74e8afc ITB |
198 | X<replace, case preserving> X<substitute, case preserving> |
199 | X<substitution, case preserving> X<s, case preserving> | |
68dc0745 | 200 | |
d92eb7b0 GS |
201 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
202 | properties of bitwise xor on ASCII strings. | |
203 | ||
ac9dac7f | 204 | $_= "this is a TEsT case"; |
d92eb7b0 | 205 | |
ac9dac7f RGS |
206 | $old = 'test'; |
207 | $new = 'success'; | |
d92eb7b0 | 208 | |
ac9dac7f RGS |
209 | s{(\Q$old\E)} |
210 | { uc $new | (uc $1 ^ $1) . | |
211 | (uc(substr $1, -1) ^ substr $1, -1) x | |
212 | (length($new) - length $1) | |
213 | }egi; | |
d92eb7b0 | 214 | |
ac9dac7f | 215 | print; |
d92eb7b0 | 216 | |
8305e449 | 217 | And here it is as a subroutine, modeled after the above: |
d92eb7b0 | 218 | |
ac9dac7f RGS |
219 | sub preserve_case($$) { |
220 | my ($old, $new) = @_; | |
221 | my $mask = uc $old ^ $old; | |
d92eb7b0 | 222 | |
ac9dac7f RGS |
223 | uc $new | $mask . |
224 | substr($mask, -1) x (length($new) - length($old)) | |
d92eb7b0 GS |
225 | } |
226 | ||
ac9dac7f RGS |
227 | $a = "this is a TEsT case"; |
228 | $a =~ s/(test)/preserve_case($1, "success")/egi; | |
229 | print "$a\n"; | |
d92eb7b0 GS |
230 | |
231 | This prints: | |
232 | ||
ac9dac7f | 233 | this is a SUcCESS case |
d92eb7b0 | 234 | |
74b9445a JP |
235 | As an alternative, to keep the case of the replacement word if it is |
236 | longer than the original, you can use this code, by Jeff Pinyan: | |
237 | ||
ac9dac7f RGS |
238 | sub preserve_case { |
239 | my ($from, $to) = @_; | |
240 | my ($lf, $lt) = map length, @_; | |
7207e29d | 241 | |
ac9dac7f RGS |
242 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
243 | else { $from .= substr $to, $lf } | |
7207e29d | 244 | |
ac9dac7f RGS |
245 | return uc $to | ($from ^ uc $from); |
246 | } | |
74b9445a JP |
247 | |
248 | This changes the sentence to "this is a SUcCess case." | |
249 | ||
d92eb7b0 GS |
250 | Just to show that C programmers can write C in any programming language, |
251 | if you prefer a more C-like solution, the following script makes the | |
252 | substitution have the same case, letter by letter, as the original. | |
253 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
254 | If the substitution has more characters than the string being substituted, | |
255 | the case of the last character is used for the rest of the substitution. | |
68dc0745 | 256 | |
ac9dac7f RGS |
257 | # Original by Nathan Torkington, massaged by Jeffrey Friedl |
258 | # | |
259 | sub preserve_case($$) | |
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 | } | |
68dc0745 | 287 | |
5a964f20 | 288 | =head2 How can I make C<\w> match national character sets? |
d74e8afc | 289 | X<\w> |
68dc0745 | 290 | |
49d635f9 RGS |
291 | Put C<use locale;> in your script. The \w character class is taken |
292 | from the current locale. | |
293 | ||
294 | See L<perllocale> for details. | |
68dc0745 | 295 | |
296 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? | |
d74e8afc | 297 | X<alpha> |
68dc0745 | 298 | |
49d635f9 RGS |
299 | You can use the POSIX character class syntax C</[[:alpha:]]/> |
300 | documented in L<perlre>. | |
301 | ||
302 | No matter which locale you are in, the alphabetic characters are | |
303 | the characters in \w without the digits and the underscore. | |
304 | As a regex, that looks like C</[^\W\d_]/>. Its complement, | |
197aec24 RGS |
305 | the non-alphabetics, is then everything in \W along with |
306 | the digits and the underscore, or C</[\W\d_]/>. | |
68dc0745 | 307 | |
d92eb7b0 | 308 | =head2 How can I quote a variable to use in a regex? |
d74e8afc | 309 | X<regex, escaping> X<regexp, escaping> X<regular expression, escaping> |
68dc0745 | 310 | |
311 | The Perl parser will expand $variable and @variable references in | |
312 | regular expressions unless the delimiter is a single quote. Remember, | |
79a522f5 | 313 | too, that the right-hand side of a C<s///> substitution is considered |
68dc0745 | 314 | a double-quoted string (see L<perlop> for more details). Remember |
d92eb7b0 | 315 | also that any regex special characters will be acted on unless you |
68dc0745 | 316 | precede the substitution with \Q. Here's an example: |
317 | ||
ac9dac7f RGS |
318 | $string = "Placido P. Octopus"; |
319 | $regex = "P."; | |
68dc0745 | 320 | |
ac9dac7f RGS |
321 | $string =~ s/$regex/Polyp/; |
322 | # $string is now "Polypacido P. Octopus" | |
68dc0745 | 323 | |
c83084d1 MJD |
324 | Because C<.> is special in regular expressions, and can match any |
325 | single character, the regex C<P.> here has matched the <Pl> in the | |
326 | original string. | |
327 | ||
328 | To escape the special meaning of C<.>, we use C<\Q>: | |
329 | ||
ac9dac7f RGS |
330 | $string = "Placido P. Octopus"; |
331 | $regex = "P."; | |
c83084d1 | 332 | |
ac9dac7f RGS |
333 | $string =~ s/\Q$regex/Polyp/; |
334 | # $string is now "Placido Polyp Octopus" | |
c83084d1 MJD |
335 | |
336 | The use of C<\Q> causes the <.> in the regex to be treated as a | |
337 | regular character, so that C<P.> matches a C<P> followed by a dot. | |
68dc0745 | 338 | |
339 | =head2 What is C</o> really for? | |
ee891a00 | 340 | X</o, regular expressions> X<compile, regular expressions> |
68dc0745 | 341 | |
ee891a00 | 342 | (contributed by brian d foy) |
68dc0745 | 343 | |
ee891a00 RGS |
344 | The C</o> option for regular expressions (documented in L<perlop> and |
345 | L<perlreref>) tells Perl to compile the regular expression only once. | |
346 | This is only useful when the pattern contains a variable. Perls 5.6 | |
347 | and later handle this automatically if the pattern does not change. | |
68dc0745 | 348 | |
ee891a00 RGS |
349 | Since the match operator C<m//>, the substitution operator C<s///>, |
350 | and the regular expression quoting operator C<qr//> are double-quotish | |
351 | constructs, you can interpolate variables into the pattern. See the | |
352 | answer to "How can I quote a variable to use in a regex?" for more | |
353 | details. | |
68dc0745 | 354 | |
ee891a00 RGS |
355 | This example takes a regular expression from the argument list and |
356 | prints the lines of input that match it: | |
68dc0745 | 357 | |
ee891a00 RGS |
358 | my $pattern = shift @ARGV; |
359 | ||
360 | while( <> ) { | |
361 | print if m/$pattern/; | |
362 | } | |
363 | ||
364 | Versions of Perl prior to 5.6 would recompile the regular expression | |
365 | for each iteration, even if C<$pattern> had not changed. The C</o> | |
366 | would prevent this by telling Perl to compile the pattern the first | |
367 | time, then reuse that for subsequent iterations: | |
368 | ||
369 | my $pattern = shift @ARGV; | |
370 | ||
371 | while( <> ) { | |
372 | print if m/$pattern/o; # useful for Perl < 5.6 | |
373 | } | |
374 | ||
375 | In versions 5.6 and later, Perl won't recompile the regular expression | |
376 | if the variable hasn't changed, so you probably don't need the C</o> | |
377 | option. It doesn't hurt, but it doesn't help either. If you want any | |
378 | version of Perl to compile the regular expression only once even if | |
379 | the variable changes (thus, only using its initial value), you still | |
380 | need the C</o>. | |
381 | ||
382 | You can watch Perl's regular expression engine at work to verify for | |
383 | yourself if Perl is recompiling a regular expression. The C<use re | |
384 | 'debug'> pragma (comes with Perl 5.005 and later) shows the details. | |
385 | With Perls before 5.6, you should see C<re> reporting that its | |
386 | compiling the regular expression on each iteration. With Perl 5.6 or | |
387 | later, you should only see C<re> report that for the first iteration. | |
388 | ||
389 | use re 'debug'; | |
390 | ||
391 | $regex = 'Perl'; | |
392 | foreach ( qw(Perl Java Ruby Python) ) { | |
393 | print STDERR "-" x 73, "\n"; | |
394 | print STDERR "Trying $_...\n"; | |
395 | print STDERR "\t$_ is good!\n" if m/$regex/; | |
396 | } | |
68dc0745 | 397 | |
398 | =head2 How do I use a regular expression to strip C style comments from a file? | |
399 | ||
400 | While this actually can be done, it's much harder than you'd think. | |
401 | For example, this one-liner | |
402 | ||
ac9dac7f | 403 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
68dc0745 | 404 | |
405 | will work in many but not all cases. You see, it's too simple-minded for | |
406 | certain kinds of C programs, in particular, those with what appear to be | |
407 | comments in quoted strings. For that, you'd need something like this, | |
d92eb7b0 | 408 | created by Jeffrey Friedl and later modified by Fred Curtis. |
68dc0745 | 409 | |
ac9dac7f RGS |
410 | $/ = undef; |
411 | $_ = <>; | |
412 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; | |
413 | print; | |
68dc0745 | 414 | |
415 | This could, of course, be more legibly written with the C</x> modifier, adding | |
d92eb7b0 GS |
416 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
417 | ||
418 | s{ | |
419 | /\* ## Start of /* ... */ comment | |
420 | [^*]*\*+ ## Non-* followed by 1-or-more *'s | |
421 | ( | |
422 | [^/*][^*]*\*+ | |
423 | )* ## 0-or-more things which don't start with / | |
424 | ## but do end with '*' | |
425 | / ## End of /* ... */ comment | |
426 | ||
427 | | ## OR various things which aren't comments: | |
428 | ||
429 | ( | |
430 | " ## Start of " ... " string | |
431 | ( | |
432 | \\. ## Escaped char | |
433 | | ## OR | |
434 | [^"\\] ## Non "\ | |
435 | )* | |
436 | " ## End of " ... " string | |
437 | ||
438 | | ## OR | |
439 | ||
440 | ' ## Start of ' ... ' string | |
441 | ( | |
442 | \\. ## Escaped char | |
443 | | ## OR | |
444 | [^'\\] ## Non '\ | |
445 | )* | |
446 | ' ## End of ' ... ' string | |
447 | ||
448 | | ## OR | |
449 | ||
450 | . ## Anything other char | |
451 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape | |
452 | ) | |
c98c5709 | 453 | }{defined $2 ? $2 : ""}gxse; |
d92eb7b0 | 454 | |
e573f903 RGS |
455 | A slight modification also removes C++ comments, as long as they are not |
456 | spread over multiple lines using a continuation character): | |
d92eb7b0 | 457 | |
ac9dac7f | 458 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
68dc0745 | 459 | |
460 | =head2 Can I use Perl regular expressions to match balanced text? | |
d74e8afc ITB |
461 | X<regex, matching balanced test> X<regexp, matching balanced test> |
462 | X<regular expression, matching balanced test> | |
68dc0745 | 463 | |
8305e449 JH |
464 | Historically, Perl regular expressions were not capable of matching |
465 | balanced text. As of more recent versions of perl including 5.6.1 | |
466 | experimental features have been added that make it possible to do this. | |
467 | Look at the documentation for the (??{ }) construct in recent perlre manual | |
468 | pages to see an example of matching balanced parentheses. Be sure to take | |
469 | special notice of the warnings present in the manual before making use | |
470 | of this feature. | |
471 | ||
472 | CPAN contains many modules that can be useful for matching text | |
473 | depending on the context. Damian Conway provides some useful | |
474 | patterns in Regexp::Common. The module Text::Balanced provides a | |
475 | general solution to this problem. | |
476 | ||
477 | One of the common applications of balanced text matching is working | |
478 | with XML and HTML. There are many modules available that support | |
479 | these needs. Two examples are HTML::Parser and XML::Parser. There | |
480 | are many others. | |
68dc0745 | 481 | |
482 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced | |
483 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, | |
484 | or C<(> and C<)> can be found in | |
a93751fa | 485 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
68dc0745 | 486 | |
8305e449 | 487 | The C::Scan module from CPAN also contains such subs for internal use, |
68dc0745 | 488 | but they are undocumented. |
489 | ||
d92eb7b0 | 490 | =head2 What does it mean that regexes are greedy? How can I get around it? |
d74e8afc | 491 | X<greedy> X<greediness> |
68dc0745 | 492 | |
d92eb7b0 | 493 | Most people mean that greedy regexes match as much as they can. |
68dc0745 | 494 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
495 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local | |
496 | greed and immediate gratification to overall greed. To get non-greedy | |
497 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). | |
498 | ||
499 | An example: | |
500 | ||
ac9dac7f RGS |
501 | $s1 = $s2 = "I am very very cold"; |
502 | $s1 =~ s/ve.*y //; # I am cold | |
503 | $s2 =~ s/ve.*?y //; # I am very cold | |
68dc0745 | 504 | |
505 | Notice how the second substitution stopped matching as soon as it | |
506 | encountered "y ". The C<*?> quantifier effectively tells the regular | |
507 | expression engine to find a match as quickly as possible and pass | |
508 | control on to whatever is next in line, like you would if you were | |
509 | playing hot potato. | |
510 | ||
f9ac83b8 | 511 | =head2 How do I process each word on each line? |
d74e8afc | 512 | X<word> |
68dc0745 | 513 | |
514 | Use the split function: | |
515 | ||
ac9dac7f RGS |
516 | while (<>) { |
517 | foreach $word ( split ) { | |
518 | # do something with $word here | |
519 | } | |
197aec24 | 520 | } |
68dc0745 | 521 | |
54310121 | 522 | Note that this isn't really a word in the English sense; it's just |
523 | chunks of consecutive non-whitespace characters. | |
68dc0745 | 524 | |
f1cbbd6e GS |
525 | To work with only alphanumeric sequences (including underscores), you |
526 | might consider | |
68dc0745 | 527 | |
ac9dac7f RGS |
528 | while (<>) { |
529 | foreach $word (m/(\w+)/g) { | |
530 | # do something with $word here | |
531 | } | |
68dc0745 | 532 | } |
68dc0745 | 533 | |
534 | =head2 How can I print out a word-frequency or line-frequency summary? | |
535 | ||
536 | To do this, you have to parse out each word in the input stream. We'll | |
54310121 | 537 | pretend that by word you mean chunk of alphabetics, hyphens, or |
538 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
68dc0745 | 539 | in the previous question: |
540 | ||
ac9dac7f RGS |
541 | while (<>) { |
542 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" | |
543 | $seen{$1}++; | |
544 | } | |
54310121 | 545 | } |
ac9dac7f RGS |
546 | |
547 | while ( ($word, $count) = each %seen ) { | |
548 | print "$count $word\n"; | |
549 | } | |
68dc0745 | 550 | |
551 | If you wanted to do the same thing for lines, you wouldn't need a | |
552 | regular expression: | |
553 | ||
ac9dac7f RGS |
554 | while (<>) { |
555 | $seen{$_}++; | |
556 | } | |
557 | ||
558 | while ( ($line, $count) = each %seen ) { | |
559 | print "$count $line"; | |
560 | } | |
68dc0745 | 561 | |
b432a672 AL |
562 | If you want these output in a sorted order, see L<perlfaq4>: "How do I |
563 | sort a hash (optionally by value instead of key)?". | |
68dc0745 | 564 | |
565 | =head2 How can I do approximate matching? | |
d74e8afc | 566 | X<match, approximate> X<matching, approximate> |
68dc0745 | 567 | |
568 | See the module String::Approx available from CPAN. | |
569 | ||
570 | =head2 How do I efficiently match many regular expressions at once? | |
d74e8afc ITB |
571 | X<regex, efficiency> X<regexp, efficiency> |
572 | X<regular expression, efficiency> | |
68dc0745 | 573 | |
7678cced RGS |
574 | ( contributed by brian d foy ) |
575 | ||
6670e5e7 | 576 | Avoid asking Perl to compile a regular expression every time |
7678cced RGS |
577 | you want to match it. In this example, perl must recompile |
578 | the regular expression for every iteration of the foreach() | |
579 | loop since it has no way to know what $pattern will be. | |
580 | ||
ac9dac7f | 581 | @patterns = qw( foo bar baz ); |
6670e5e7 | 582 | |
ac9dac7f RGS |
583 | LINE: while( <DATA> ) |
584 | { | |
6670e5e7 | 585 | foreach $pattern ( @patterns ) |
7678cced | 586 | { |
ac9dac7f RGS |
587 | if( /\b$pattern\b/i ) |
588 | { | |
589 | print; | |
590 | next LINE; | |
591 | } | |
592 | } | |
7678cced | 593 | } |
68dc0745 | 594 | |
7678cced RGS |
595 | The qr// operator showed up in perl 5.005. It compiles a |
596 | regular expression, but doesn't apply it. When you use the | |
597 | pre-compiled version of the regex, perl does less work. In | |
598 | this example, I inserted a map() to turn each pattern into | |
599 | its pre-compiled form. The rest of the script is the same, | |
600 | but faster. | |
601 | ||
ac9dac7f | 602 | @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); |
7678cced | 603 | |
ac9dac7f RGS |
604 | LINE: while( <> ) |
605 | { | |
6670e5e7 | 606 | foreach $pattern ( @patterns ) |
7678cced | 607 | { |
c195e131 | 608 | print if /$pattern/i; |
ac9dac7f RGS |
609 | next LINE; |
610 | } | |
7678cced | 611 | } |
6670e5e7 | 612 | |
7678cced RGS |
613 | In some cases, you may be able to make several patterns into |
614 | a single regular expression. Beware of situations that require | |
615 | backtracking though. | |
65acb1b1 | 616 | |
7678cced RGS |
617 | $regex = join '|', qw( foo bar baz ); |
618 | ||
ac9dac7f RGS |
619 | LINE: while( <> ) |
620 | { | |
7678cced RGS |
621 | print if /\b(?:$regex)\b/i; |
622 | } | |
623 | ||
624 | For more details on regular expression efficiency, see Mastering | |
625 | Regular Expressions by Jeffrey Freidl. He explains how regular | |
626 | expressions engine work and why some patterns are surprisingly | |
6670e5e7 | 627 | inefficient. Once you understand how perl applies regular |
7678cced | 628 | expressions, you can tune them for individual situations. |
68dc0745 | 629 | |
630 | =head2 Why don't word-boundary searches with C<\b> work for me? | |
d74e8afc | 631 | X<\b> |
68dc0745 | 632 | |
7678cced RGS |
633 | (contributed by brian d foy) |
634 | ||
635 | Ensure that you know what \b really does: it's the boundary between a | |
636 | word character, \w, and something that isn't a word character. That | |
637 | thing that isn't a word character might be \W, but it can also be the | |
638 | start or end of the string. | |
639 | ||
640 | It's not (not!) the boundary between whitespace and non-whitespace, | |
641 | and it's not the stuff between words we use to create sentences. | |
642 | ||
643 | In regex speak, a word boundary (\b) is a "zero width assertion", | |
644 | meaning that it doesn't represent a character in the string, but a | |
645 | condition at a certain position. | |
646 | ||
647 | For the regular expression, /\bPerl\b/, there has to be a word | |
648 | boundary before the "P" and after the "l". As long as something other | |
649 | than a word character precedes the "P" and succeeds the "l", the | |
650 | pattern will match. These strings match /\bPerl\b/. | |
651 | ||
652 | "Perl" # no word char before P or after l | |
653 | "Perl " # same as previous (space is not a word char) | |
654 | "'Perl'" # the ' char is not a word char | |
655 | "Perl's" # no word char before P, non-word char after "l" | |
656 | ||
657 | These strings do not match /\bPerl\b/. | |
658 | ||
659 | "Perl_" # _ is a word char! | |
660 | "Perler" # no word char before P, but one after l | |
6670e5e7 | 661 | |
7678cced | 662 | You don't have to use \b to match words though. You can look for |
d7f8936a | 663 | non-word characters surrounded by word characters. These strings |
7678cced RGS |
664 | match the pattern /\b'\b/. |
665 | ||
666 | "don't" # the ' char is surrounded by "n" and "t" | |
667 | "qep'a'" # the ' char is surrounded by "p" and "a" | |
6670e5e7 | 668 | |
7678cced | 669 | These strings do not match /\b'\b/. |
68dc0745 | 670 | |
7678cced | 671 | "foo'" # there is no word char after non-word ' |
6670e5e7 | 672 | |
7678cced RGS |
673 | You can also use the complement of \b, \B, to specify that there |
674 | should not be a word boundary. | |
68dc0745 | 675 | |
7678cced RGS |
676 | In the pattern /\Bam\B/, there must be a word character before the "a" |
677 | and after the "m". These patterns match /\Bam\B/: | |
68dc0745 | 678 | |
7678cced RGS |
679 | "llama" # "am" surrounded by word chars |
680 | "Samuel" # same | |
6670e5e7 | 681 | |
7678cced | 682 | These strings do not match /\Bam\B/ |
68dc0745 | 683 | |
7678cced RGS |
684 | "Sam" # no word boundary before "a", but one after "m" |
685 | "I am Sam" # "am" surrounded by non-word chars | |
68dc0745 | 686 | |
68dc0745 | 687 | |
688 | =head2 Why does using $&, $`, or $' slow my program down? | |
d74e8afc | 689 | X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`> |
68dc0745 | 690 | |
571e049f | 691 | (contributed by Anno Siegel) |
68dc0745 | 692 | |
571e049f | 693 | Once Perl sees that you need one of these variables anywhere in the |
b68463f7 RGS |
694 | program, it provides them on each and every pattern match. That means |
695 | that on every pattern match the entire string will be copied, part of it | |
696 | to $`, part to $&, and part to $'. Thus the penalty is most severe with | |
697 | long strings and patterns that match often. Avoid $&, $', and $` if you | |
698 | can, but if you can't, once you've used them at all, use them at will | |
699 | because you've already paid the price. Remember that some algorithms | |
700 | really appreciate them. As of the 5.005 release, the $& variable is no | |
701 | longer "expensive" the way the other two are. | |
702 | ||
703 | Since Perl 5.6.1 the special variables @- and @+ can functionally replace | |
704 | $`, $& and $'. These arrays contain pointers to the beginning and end | |
705 | of each match (see perlvar for the full story), so they give you | |
706 | essentially the same information, but without the risk of excessive | |
707 | string copying. | |
6670e5e7 | 708 | |
68dc0745 | 709 | =head2 What good is C<\G> in a regular expression? |
d74e8afc | 710 | X<\G> |
68dc0745 | 711 | |
49d635f9 RGS |
712 | You use the C<\G> anchor to start the next match on the same |
713 | string where the last match left off. The regular | |
714 | expression engine cannot skip over any characters to find | |
715 | the next match with this anchor, so C<\G> is similar to the | |
716 | beginning of string anchor, C<^>. The C<\G> anchor is typically | |
ee891a00 | 717 | used with the C<g> flag. It uses the value of C<pos()> |
49d635f9 | 718 | as the position to start the next match. As the match |
ee891a00 | 719 | operator makes successive matches, it updates C<pos()> with the |
49d635f9 RGS |
720 | position of the next character past the last match (or the |
721 | first character of the next match, depending on how you like | |
ee891a00 | 722 | to look at it). Each string has its own C<pos()> value. |
49d635f9 | 723 | |
ee891a00 | 724 | Suppose you want to match all of consecutive pairs of digits |
49d635f9 RGS |
725 | in a string like "1122a44" and stop matching when you |
726 | encounter non-digits. You want to match C<11> and C<22> but | |
727 | the letter <a> shows up between C<22> and C<44> and you want | |
728 | to stop at C<a>. Simply matching pairs of digits skips over | |
729 | the C<a> and still matches C<44>. | |
730 | ||
731 | $_ = "1122a44"; | |
732 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) | |
733 | ||
ee891a00 | 734 | If you use the C<\G> anchor, you force the match after C<22> to |
49d635f9 RGS |
735 | start with the C<a>. The regular expression cannot match |
736 | there since it does not find a digit, so the next match | |
737 | fails and the match operator returns the pairs it already | |
738 | found. | |
739 | ||
740 | $_ = "1122a44"; | |
741 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) | |
742 | ||
743 | You can also use the C<\G> anchor in scalar context. You | |
744 | still need the C<g> flag. | |
745 | ||
746 | $_ = "1122a44"; | |
747 | while( m/\G(\d\d)/g ) | |
748 | { | |
749 | print "Found $1\n"; | |
750 | } | |
197aec24 | 751 | |
ee891a00 | 752 | After the match fails at the letter C<a>, perl resets C<pos()> |
49d635f9 RGS |
753 | and the next match on the same string starts at the beginning. |
754 | ||
755 | $_ = "1122a44"; | |
756 | while( m/\G(\d\d)/g ) | |
757 | { | |
758 | print "Found $1\n"; | |
759 | } | |
760 | ||
761 | print "Found $1 after while" if m/(\d\d)/g; # finds "11" | |
762 | ||
ee891a00 RGS |
763 | You can disable C<pos()> resets on fail with the C<c> flag, documented |
764 | in L<perlop> and L<perlreref>. Subsequent matches start where the last | |
765 | successful match ended (the value of C<pos()>) even if a match on the | |
766 | same string has failed in the meantime. In this case, the match after | |
767 | the C<while()> loop starts at the C<a> (where the last match stopped), | |
768 | and since it does not use any anchor it can skip over the C<a> to find | |
769 | C<44>. | |
49d635f9 RGS |
770 | |
771 | $_ = "1122a44"; | |
772 | while( m/\G(\d\d)/gc ) | |
773 | { | |
774 | print "Found $1\n"; | |
775 | } | |
776 | ||
777 | print "Found $1 after while" if m/(\d\d)/g; # finds "44" | |
778 | ||
779 | Typically you use the C<\G> anchor with the C<c> flag | |
780 | when you want to try a different match if one fails, | |
781 | such as in a tokenizer. Jeffrey Friedl offers this example | |
782 | which works in 5.004 or later. | |
68dc0745 | 783 | |
ac9dac7f RGS |
784 | while (<>) { |
785 | chomp; | |
786 | PARSER: { | |
787 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; | |
788 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; | |
789 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; | |
790 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; | |
791 | } | |
792 | } | |
68dc0745 | 793 | |
ee891a00 | 794 | For each line, the C<PARSER> loop first tries to match a series |
49d635f9 RGS |
795 | of digits followed by a word boundary. This match has to |
796 | start at the place the last match left off (or the beginning | |
197aec24 | 797 | of the string on the first match). Since C<m/ \G( \d+\b |
49d635f9 RGS |
798 | )/gcx> uses the C<c> flag, if the string does not match that |
799 | regular expression, perl does not reset pos() and the next | |
800 | match starts at the same position to try a different | |
801 | pattern. | |
68dc0745 | 802 | |
d92eb7b0 | 803 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
d74e8afc | 804 | X<DFA> X<NFA> X<POSIX> |
68dc0745 | 805 | |
806 | While it's true that Perl's regular expressions resemble the DFAs | |
807 | (deterministic finite automata) of the egrep(1) program, they are in | |
46fc3d4c | 808 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 | 809 | backtracking and backreferencing. And they aren't POSIX-style either, |
810 | because those guarantee worst-case behavior for all cases. (It seems | |
811 | that some people prefer guarantees of consistency, even when what's | |
812 | guaranteed is slowness.) See the book "Mastering Regular Expressions" | |
813 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
814 | hope to know on these matters (a full citation appears in | |
815 | L<perlfaq2>). | |
816 | ||
788611b6 | 817 | =head2 What's wrong with using grep in a void context? |
d74e8afc | 818 | X<grep> |
68dc0745 | 819 | |
788611b6 A |
820 | The problem is that grep builds a return list, regardless of the context. |
821 | This means you're making Perl go to the trouble of building a list that | |
822 | you then just throw away. If the list is large, you waste both time and space. | |
823 | If your intent is to iterate over the list, then use a for loop for this | |
f05bbc40 | 824 | purpose. |
68dc0745 | 825 | |
788611b6 A |
826 | In perls older than 5.8.1, map suffers from this problem as well. |
827 | But since 5.8.1, this has been fixed, and map is context aware - in void | |
828 | context, no lists are constructed. | |
829 | ||
54310121 | 830 | =head2 How can I match strings with multibyte characters? |
d74e8afc | 831 | X<regex, and multibyte characters> X<regexp, and multibyte characters> |
ac9dac7f | 832 | X<regular expression, and multibyte characters> X<martian> X<encoding, Martian> |
68dc0745 | 833 | |
d9d154f2 JH |
834 | Starting from Perl 5.6 Perl has had some level of multibyte character |
835 | support. Perl 5.8 or later is recommended. Supported multibyte | |
fe854a6f | 836 | character repertoires include Unicode, and legacy encodings |
d9d154f2 JH |
837 | through the Encode module. See L<perluniintro>, L<perlunicode>, |
838 | and L<Encode>. | |
839 | ||
840 | If you are stuck with older Perls, you can do Unicode with the | |
841 | C<Unicode::String> module, and character conversions using the | |
842 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using | |
843 | Japanese encodings, you might try using the jperl 5.005_03. | |
844 | ||
845 | Finally, the following set of approaches was offered by Jeffrey | |
846 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
847 | this very matter. | |
68dc0745 | 848 | |
fc36a67e | 849 | Let's suppose you have some weird Martian encoding where pairs of |
850 | ASCII uppercase letters encode single Martian letters (i.e. the two | |
851 | bytes "CV" make a single Martian letter, as do the two bytes "SG", | |
852 | "VS", "XX", etc.). Other bytes represent single characters, just like | |
853 | ASCII. | |
68dc0745 | 854 | |
fc36a67e | 855 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
856 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. | |
68dc0745 | 857 | |
858 | Now, say you want to search for the single character C</GX/>. Perl | |
fc36a67e | 859 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
860 | am CVSGXX!" string, even though that character isn't there: it just | |
861 | looks like it is because "SG" is next to "XX", but there's no real | |
862 | "GX". This is a big problem. | |
68dc0745 | 863 | |
864 | Here are a few ways, all painful, to deal with it: | |
865 | ||
ac9dac7f RGS |
866 | # Make sure adjacent "martian" bytes are no longer adjacent. |
867 | $martian =~ s/([A-Z][A-Z])/ $1 /g; | |
868 | ||
869 | print "found GX!\n" if $martian =~ /GX/; | |
68dc0745 | 870 | |
871 | Or like this: | |
872 | ||
ac9dac7f RGS |
873 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
874 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
875 | # | |
876 | foreach $char (@chars) { | |
877 | print "found GX!\n", last if $char eq 'GX'; | |
878 | } | |
68dc0745 | 879 | |
880 | Or like this: | |
881 | ||
ac9dac7f RGS |
882 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded |
883 | print "found GX!\n", last if $1 eq 'GX'; | |
884 | } | |
68dc0745 | 885 | |
49d635f9 | 886 | Here's another, slightly less painful, way to do it from Benjamin |
c98c5709 | 887 | Goldberg, who uses a zero-width negative look-behind assertion. |
49d635f9 | 888 | |
c98c5709 | 889 | print "found GX!\n" if $martian =~ m/ |
ac9dac7f RGS |
890 | (?<![A-Z]) |
891 | (?:[A-Z][A-Z])*? | |
892 | GX | |
c98c5709 | 893 | /x; |
197aec24 | 894 | |
49d635f9 | 895 | This succeeds if the "martian" character GX is in the string, and fails |
c98c5709 RGS |
896 | otherwise. If you don't like using (?<!), a zero-width negative |
897 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). | |
49d635f9 RGS |
898 | |
899 | It does have the drawback of putting the wrong thing in $-[0] and $+[0], | |
900 | but this usually can be worked around. | |
68dc0745 | 901 | |
ac9dac7f RGS |
902 | =head2 How do I match a regular expression that's in a variable? |
903 | X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex> | |
904 | X<\E, regex>, X<qr//> | |
65acb1b1 | 905 | |
ac9dac7f | 906 | (contributed by brian d foy) |
65acb1b1 | 907 | |
ac9dac7f RGS |
908 | We don't have to hard-code patterns into the match operator (or |
909 | anything else that works with regular expressions). We can put the | |
910 | pattern in a variable for later use. | |
65acb1b1 | 911 | |
ac9dac7f RGS |
912 | The match operator is a double quote context, so you can interpolate |
913 | your variable just like a double quoted string. In this case, you | |
914 | read the regular expression as user input and store it in C<$regex>. | |
915 | Once you have the pattern in C<$regex>, you use that variable in the | |
916 | match operator. | |
65acb1b1 | 917 | |
ac9dac7f | 918 | chomp( my $regex = <STDIN> ); |
65acb1b1 | 919 | |
ac9dac7f | 920 | if( $string =~ m/$regex/ ) { ... } |
65acb1b1 | 921 | |
ac9dac7f RGS |
922 | Any regular expression special characters in C<$regex> are still |
923 | special, and the pattern still has to be valid or Perl will complain. | |
924 | For instance, in this pattern there is an unpaired parenthesis. | |
65acb1b1 | 925 | |
ac9dac7f RGS |
926 | my $regex = "Unmatched ( paren"; |
927 | ||
928 | "Two parens to bind them all" =~ m/$regex/; | |
929 | ||
930 | When Perl compiles the regular expression, it treats the parenthesis | |
931 | as the start of a memory match. When it doesn't find the closing | |
932 | parenthesis, it complains: | |
933 | ||
934 | Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. | |
935 | ||
936 | You can get around this in several ways depending on our situation. | |
937 | First, if you don't want any of the characters in the string to be | |
938 | special, you can escape them with C<quotemeta> before you use the string. | |
939 | ||
940 | chomp( my $regex = <STDIN> ); | |
941 | $regex = quotemeta( $regex ); | |
942 | ||
943 | if( $string =~ m/$regex/ ) { ... } | |
944 | ||
945 | You can also do this directly in the match operator using the C<\Q> | |
946 | and C<\E> sequences. The C<\Q> tells Perl where to start escaping | |
947 | special characters, and the C<\E> tells it where to stop (see L<perlop> | |
948 | for more details). | |
949 | ||
950 | chomp( my $regex = <STDIN> ); | |
951 | ||
952 | if( $string =~ m/\Q$regex\E/ ) { ... } | |
953 | ||
954 | Alternately, you can use C<qr//>, the regular expression quote operator (see | |
955 | L<perlop> for more details). It quotes and perhaps compiles the pattern, | |
956 | and you can apply regular expression flags to the pattern. | |
957 | ||
958 | chomp( my $input = <STDIN> ); | |
959 | ||
960 | my $regex = qr/$input/is; | |
961 | ||
962 | $string =~ m/$regex/ # same as m/$input/is; | |
963 | ||
964 | You might also want to trap any errors by wrapping an C<eval> block | |
965 | around the whole thing. | |
966 | ||
967 | chomp( my $input = <STDIN> ); | |
968 | ||
969 | eval { | |
970 | if( $string =~ m/\Q$input\E/ ) { ... } | |
971 | }; | |
972 | warn $@ if $@; | |
973 | ||
974 | Or... | |
975 | ||
976 | my $regex = eval { qr/$input/is }; | |
977 | if( defined $regex ) { | |
978 | $string =~ m/$regex/; | |
979 | } | |
980 | else { | |
981 | warn $@; | |
982 | } | |
65acb1b1 | 983 | |
500071f4 RGS |
984 | =head1 REVISION |
985 | ||
c195e131 | 986 | Revision: $Revision: 10126 $ |
500071f4 | 987 | |
c195e131 | 988 | Date: $Date: 2007-10-27 21:29:20 +0200 (Sat, 27 Oct 2007) $ |
500071f4 RGS |
989 | |
990 | See L<perlfaq> for source control details and availability. | |
991 | ||
68dc0745 | 992 | =head1 AUTHOR AND COPYRIGHT |
993 | ||
ee891a00 | 994 | Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and |
7678cced | 995 | other authors as noted. All rights reserved. |
5a964f20 | 996 | |
5a7beb56 JH |
997 | This documentation is free; you can redistribute it and/or modify it |
998 | under the same terms as Perl itself. | |
5a964f20 TC |
999 | |
1000 | Irrespective of its distribution, all code examples in this file | |
1001 | are hereby placed into the public domain. You are permitted and | |
1002 | encouraged to use this code in your own programs for fun | |
1003 | or for profit as you see fit. A simple comment in the code giving | |
1004 | credit would be courteous but is not required. |