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68dc0745 | 1 | =head1 NAME |
2 | ||
b68463f7 | 3 | perlfaq6 - Regular Expressions ($Revision: 1.35 $, $Date: 2005/08/10 15:55:08 $) |
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? |
68dc0745 | 16 | |
17 | Three techniques can make regular expressions maintainable and | |
18 | understandable. | |
19 | ||
20 | =over 4 | |
21 | ||
d92eb7b0 | 22 | =item Comments Outside the Regex |
68dc0745 | 23 | |
24 | Describe what you're doing and how you're doing it, using normal Perl | |
25 | comments. | |
26 | ||
27 | # turn the line into the first word, a colon, and the | |
28 | # number of characters on the rest of the line | |
5a964f20 | 29 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; |
68dc0745 | 30 | |
d92eb7b0 | 31 | =item Comments Inside the Regex |
68dc0745 | 32 | |
d92eb7b0 | 33 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
68dc0745 | 34 | (except in a character class), and also allows you to use normal |
35 | comments there, too. As you can imagine, whitespace and comments help | |
36 | a lot. | |
37 | ||
38 | C</x> lets you turn this: | |
39 | ||
40 | s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs; | |
41 | ||
42 | into this: | |
43 | ||
44 | s{ < # opening angle bracket | |
45 | (?: # Non-backreffing grouping paren | |
46 | [^>'"] * # 0 or more things that are neither > nor ' nor " | |
47 | | # or else | |
48 | ".*?" # a section between double quotes (stingy match) | |
49 | | # or else | |
50 | '.*?' # a section between single quotes (stingy match) | |
51 | ) + # all occurring one or more times | |
52 | > # closing angle bracket | |
53 | }{}gsx; # replace with nothing, i.e. delete | |
54 | ||
55 | It's still not quite so clear as prose, but it is very useful for | |
56 | describing the meaning of each part of the pattern. | |
57 | ||
58 | =item Different Delimiters | |
59 | ||
60 | While we normally think of patterns as being delimited with C</> | |
61 | characters, they can be delimited by almost any character. L<perlre> | |
62 | describes this. For example, the C<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 | =back | |
70 | ||
71 | =head2 I'm having trouble matching over more than one line. What's wrong? | |
72 | ||
3392b9ec JH |
73 | Either you don't have more than one line in the string you're looking |
74 | at (probably), or else you aren't using the correct modifier(s) on | |
75 | your pattern (possibly). | |
68dc0745 | 76 | |
77 | There are many ways to get multiline data into a string. If you want | |
78 | it to happen automatically while reading input, you'll want to set $/ | |
79 | (probably to '' for paragraphs or C<undef> for the whole file) to | |
80 | allow you to read more than one line at a time. | |
81 | ||
82 | Read L<perlre> to help you decide which of C</s> and C</m> (or both) | |
83 | you might want to use: C</s> allows dot to include newline, and C</m> | |
84 | allows caret and dollar to match next to a newline, not just at the | |
85 | end of the string. You do need to make sure that you've actually | |
86 | got a multiline string in there. | |
87 | ||
88 | For example, this program detects duplicate words, even when they span | |
89 | line breaks (but not paragraph ones). For this example, we don't need | |
90 | C</s> because we aren't using dot in a regular expression that we want | |
91 | to cross line boundaries. Neither do we need C</m> because we aren't | |
92 | wanting caret or dollar to match at any point inside the record next | |
93 | to newlines. But it's imperative that $/ be set to something other | |
94 | than the default, or else we won't actually ever have a multiline | |
95 | record read in. | |
96 | ||
97 | $/ = ''; # read in more whole paragraph, not just one line | |
98 | while ( <> ) { | |
5a964f20 | 99 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha |
68dc0745 | 100 | print "Duplicate $1 at paragraph $.\n"; |
54310121 | 101 | } |
102 | } | |
68dc0745 | 103 | |
104 | Here's code that finds sentences that begin with "From " (which would | |
105 | be mangled by many mailers): | |
106 | ||
107 | $/ = ''; # read in more whole paragraph, not just one line | |
108 | while ( <> ) { | |
109 | while ( /^From /gm ) { # /m makes ^ match next to \n | |
110 | print "leading from in paragraph $.\n"; | |
111 | } | |
112 | } | |
113 | ||
114 | Here's code that finds everything between START and END in a paragraph: | |
115 | ||
116 | undef $/; # read in whole file, not just one line or paragraph | |
117 | while ( <> ) { | |
fd89e497 | 118 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
68dc0745 | 119 | print "$1\n"; |
120 | } | |
121 | } | |
122 | ||
123 | =head2 How can I pull out lines between two patterns that are themselves on different lines? | |
124 | ||
125 | You can use Perl's somewhat exotic C<..> operator (documented in | |
126 | L<perlop>): | |
127 | ||
128 | perl -ne 'print if /START/ .. /END/' file1 file2 ... | |
129 | ||
130 | If you wanted text and not lines, you would use | |
131 | ||
65acb1b1 | 132 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
68dc0745 | 133 | |
134 | But if you want nested occurrences of C<START> through C<END>, you'll | |
135 | run up against the problem described in the question in this section | |
136 | on matching balanced text. | |
137 | ||
5a964f20 TC |
138 | Here's another example of using C<..>: |
139 | ||
140 | while (<>) { | |
141 | $in_header = 1 .. /^$/; | |
142 | $in_body = /^$/ .. eof(); | |
143 | # now choose between them | |
144 | } continue { | |
145 | reset if eof(); # fix $. | |
197aec24 | 146 | } |
5a964f20 | 147 | |
68dc0745 | 148 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
149 | ||
197aec24 | 150 | Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10, |
49d635f9 RGS |
151 | but don't get your hopes up. Until then, you can use these examples |
152 | if you really need to do this. | |
153 | ||
28b41a80 RGS |
154 | If you have File::Stream, this is easy. |
155 | ||
156 | use File::Stream; | |
157 | my $stream = File::Stream->new( | |
158 | $filehandle, | |
159 | separator => qr/\s*,\s*/, | |
160 | ); | |
161 | ||
162 | print "$_\n" while <$stream>; | |
163 | ||
164 | If you don't have File::Stream, you have to do a little more work. | |
165 | ||
166 | You can use the four argument form of sysread to continually add to | |
197aec24 | 167 | a buffer. After you add to the buffer, you check if you have a |
49d635f9 RGS |
168 | complete line (using your regular expression). |
169 | ||
170 | local $_ = ""; | |
171 | while( sysread FH, $_, 8192, length ) { | |
172 | while( s/^((?s).*?)your_pattern/ ) { | |
173 | my $record = $1; | |
174 | # do stuff here. | |
175 | } | |
176 | } | |
197aec24 | 177 | |
49d635f9 RGS |
178 | You can do the same thing with foreach and a match using the |
179 | c flag and the \G anchor, if you do not mind your entire file | |
180 | being in memory at the end. | |
197aec24 | 181 | |
49d635f9 RGS |
182 | local $_ = ""; |
183 | while( sysread FH, $_, 8192, length ) { | |
184 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { | |
185 | # do stuff here. | |
186 | } | |
187 | substr( $_, 0, pos ) = "" if pos; | |
188 | } | |
68dc0745 | 189 | |
3fe9a6f1 | 190 | |
a6dd486b | 191 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
68dc0745 | 192 | |
d92eb7b0 GS |
193 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
194 | properties of bitwise xor on ASCII strings. | |
195 | ||
196 | $_= "this is a TEsT case"; | |
197 | ||
198 | $old = 'test'; | |
199 | $new = 'success'; | |
200 | ||
575cc754 | 201 | s{(\Q$old\E)} |
d92eb7b0 GS |
202 | { uc $new | (uc $1 ^ $1) . |
203 | (uc(substr $1, -1) ^ substr $1, -1) x | |
204 | (length($new) - length $1) | |
205 | }egi; | |
206 | ||
207 | print; | |
208 | ||
8305e449 | 209 | And here it is as a subroutine, modeled after the above: |
d92eb7b0 GS |
210 | |
211 | sub preserve_case($$) { | |
212 | my ($old, $new) = @_; | |
213 | my $mask = uc $old ^ $old; | |
214 | ||
215 | uc $new | $mask . | |
197aec24 | 216 | substr($mask, -1) x (length($new) - length($old)) |
d92eb7b0 GS |
217 | } |
218 | ||
219 | $a = "this is a TEsT case"; | |
220 | $a =~ s/(test)/preserve_case($1, "success")/egi; | |
221 | print "$a\n"; | |
222 | ||
223 | This prints: | |
224 | ||
225 | this is a SUcCESS case | |
226 | ||
74b9445a JP |
227 | As an alternative, to keep the case of the replacement word if it is |
228 | longer than the original, you can use this code, by Jeff Pinyan: | |
229 | ||
230 | sub preserve_case { | |
231 | my ($from, $to) = @_; | |
232 | my ($lf, $lt) = map length, @_; | |
7207e29d | 233 | |
74b9445a JP |
234 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
235 | else { $from .= substr $to, $lf } | |
7207e29d | 236 | |
74b9445a JP |
237 | return uc $to | ($from ^ uc $from); |
238 | } | |
239 | ||
240 | This changes the sentence to "this is a SUcCess case." | |
241 | ||
d92eb7b0 GS |
242 | Just to show that C programmers can write C in any programming language, |
243 | if you prefer a more C-like solution, the following script makes the | |
244 | substitution have the same case, letter by letter, as the original. | |
245 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
246 | If the substitution has more characters than the string being substituted, | |
247 | the case of the last character is used for the rest of the substitution. | |
68dc0745 | 248 | |
249 | # Original by Nathan Torkington, massaged by Jeffrey Friedl | |
250 | # | |
251 | sub preserve_case($$) | |
252 | { | |
253 | my ($old, $new) = @_; | |
254 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
255 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
256 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
257 | ||
258 | for ($i = 0; $i < $len; $i++) { | |
259 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { | |
260 | $state = 0; | |
261 | } elsif (lc $c eq $c) { | |
262 | substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
263 | $state = 1; | |
264 | } else { | |
265 | substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
266 | $state = 2; | |
267 | } | |
268 | } | |
269 | # finish up with any remaining new (for when new is longer than old) | |
270 | if ($newlen > $oldlen) { | |
271 | if ($state == 1) { | |
272 | substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
273 | } elsif ($state == 2) { | |
274 | substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
275 | } | |
276 | } | |
277 | return $new; | |
278 | } | |
279 | ||
5a964f20 | 280 | =head2 How can I make C<\w> match national character sets? |
68dc0745 | 281 | |
49d635f9 RGS |
282 | Put C<use locale;> in your script. The \w character class is taken |
283 | from the current locale. | |
284 | ||
285 | See L<perllocale> for details. | |
68dc0745 | 286 | |
287 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? | |
288 | ||
49d635f9 RGS |
289 | You can use the POSIX character class syntax C</[[:alpha:]]/> |
290 | documented in L<perlre>. | |
291 | ||
292 | No matter which locale you are in, the alphabetic characters are | |
293 | the characters in \w without the digits and the underscore. | |
294 | As a regex, that looks like C</[^\W\d_]/>. Its complement, | |
197aec24 RGS |
295 | the non-alphabetics, is then everything in \W along with |
296 | the digits and the underscore, or C</[\W\d_]/>. | |
68dc0745 | 297 | |
d92eb7b0 | 298 | =head2 How can I quote a variable to use in a regex? |
68dc0745 | 299 | |
300 | The Perl parser will expand $variable and @variable references in | |
301 | regular expressions unless the delimiter is a single quote. Remember, | |
79a522f5 | 302 | too, that the right-hand side of a C<s///> substitution is considered |
68dc0745 | 303 | a double-quoted string (see L<perlop> for more details). Remember |
d92eb7b0 | 304 | also that any regex special characters will be acted on unless you |
68dc0745 | 305 | precede the substitution with \Q. Here's an example: |
306 | ||
c83084d1 MJD |
307 | $string = "Placido P. Octopus"; |
308 | $regex = "P."; | |
68dc0745 | 309 | |
c83084d1 MJD |
310 | $string =~ s/$regex/Polyp/; |
311 | # $string is now "Polypacido P. Octopus" | |
68dc0745 | 312 | |
c83084d1 MJD |
313 | Because C<.> is special in regular expressions, and can match any |
314 | single character, the regex C<P.> here has matched the <Pl> in the | |
315 | original string. | |
316 | ||
317 | To escape the special meaning of C<.>, we use C<\Q>: | |
318 | ||
319 | $string = "Placido P. Octopus"; | |
320 | $regex = "P."; | |
321 | ||
322 | $string =~ s/\Q$regex/Polyp/; | |
323 | # $string is now "Placido Polyp Octopus" | |
324 | ||
325 | The use of C<\Q> causes the <.> in the regex to be treated as a | |
326 | regular character, so that C<P.> matches a C<P> followed by a dot. | |
68dc0745 | 327 | |
328 | =head2 What is C</o> really for? | |
329 | ||
46fc3d4c | 330 | Using a variable in a regular expression match forces a re-evaluation |
a6dd486b JB |
331 | (and perhaps recompilation) each time the regular expression is |
332 | encountered. The C</o> modifier locks in the regex the first time | |
333 | it's used. This always happens in a constant regular expression, and | |
334 | in fact, the pattern was compiled into the internal format at the same | |
335 | time your entire program was. | |
68dc0745 | 336 | |
337 | Use of C</o> is irrelevant unless variable interpolation is used in | |
d92eb7b0 | 338 | the pattern, and if so, the regex engine will neither know nor care |
68dc0745 | 339 | whether the variables change after the pattern is evaluated the I<very |
340 | first> time. | |
341 | ||
342 | C</o> is often used to gain an extra measure of efficiency by not | |
343 | performing subsequent evaluations when you know it won't matter | |
344 | (because you know the variables won't change), or more rarely, when | |
d92eb7b0 | 345 | you don't want the regex to notice if they do. |
68dc0745 | 346 | |
347 | For example, here's a "paragrep" program: | |
348 | ||
349 | $/ = ''; # paragraph mode | |
350 | $pat = shift; | |
351 | while (<>) { | |
352 | print if /$pat/o; | |
353 | } | |
354 | ||
355 | =head2 How do I use a regular expression to strip C style comments from a file? | |
356 | ||
357 | While this actually can be done, it's much harder than you'd think. | |
358 | For example, this one-liner | |
359 | ||
360 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c | |
361 | ||
362 | will work in many but not all cases. You see, it's too simple-minded for | |
363 | certain kinds of C programs, in particular, those with what appear to be | |
364 | comments in quoted strings. For that, you'd need something like this, | |
d92eb7b0 | 365 | created by Jeffrey Friedl and later modified by Fred Curtis. |
68dc0745 | 366 | |
367 | $/ = undef; | |
368 | $_ = <>; | |
c98c5709 | 369 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
68dc0745 | 370 | print; |
371 | ||
372 | This could, of course, be more legibly written with the C</x> modifier, adding | |
d92eb7b0 GS |
373 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
374 | ||
375 | s{ | |
376 | /\* ## Start of /* ... */ comment | |
377 | [^*]*\*+ ## Non-* followed by 1-or-more *'s | |
378 | ( | |
379 | [^/*][^*]*\*+ | |
380 | )* ## 0-or-more things which don't start with / | |
381 | ## but do end with '*' | |
382 | / ## End of /* ... */ comment | |
383 | ||
384 | | ## OR various things which aren't comments: | |
385 | ||
386 | ( | |
387 | " ## Start of " ... " string | |
388 | ( | |
389 | \\. ## Escaped char | |
390 | | ## OR | |
391 | [^"\\] ## Non "\ | |
392 | )* | |
393 | " ## End of " ... " string | |
394 | ||
395 | | ## OR | |
396 | ||
397 | ' ## Start of ' ... ' string | |
398 | ( | |
399 | \\. ## Escaped char | |
400 | | ## OR | |
401 | [^'\\] ## Non '\ | |
402 | )* | |
403 | ' ## End of ' ... ' string | |
404 | ||
405 | | ## OR | |
406 | ||
407 | . ## Anything other char | |
408 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape | |
409 | ) | |
c98c5709 | 410 | }{defined $2 ? $2 : ""}gxse; |
d92eb7b0 GS |
411 | |
412 | A slight modification also removes C++ comments: | |
413 | ||
c98c5709 | 414 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; |
68dc0745 | 415 | |
416 | =head2 Can I use Perl regular expressions to match balanced text? | |
417 | ||
8305e449 JH |
418 | Historically, Perl regular expressions were not capable of matching |
419 | balanced text. As of more recent versions of perl including 5.6.1 | |
420 | experimental features have been added that make it possible to do this. | |
421 | Look at the documentation for the (??{ }) construct in recent perlre manual | |
422 | pages to see an example of matching balanced parentheses. Be sure to take | |
423 | special notice of the warnings present in the manual before making use | |
424 | of this feature. | |
425 | ||
426 | CPAN contains many modules that can be useful for matching text | |
427 | depending on the context. Damian Conway provides some useful | |
428 | patterns in Regexp::Common. The module Text::Balanced provides a | |
429 | general solution to this problem. | |
430 | ||
431 | One of the common applications of balanced text matching is working | |
432 | with XML and HTML. There are many modules available that support | |
433 | these needs. Two examples are HTML::Parser and XML::Parser. There | |
434 | are many others. | |
68dc0745 | 435 | |
436 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced | |
437 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, | |
438 | or C<(> and C<)> can be found in | |
a93751fa | 439 | http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz . |
68dc0745 | 440 | |
8305e449 | 441 | The C::Scan module from CPAN also contains such subs for internal use, |
68dc0745 | 442 | but they are undocumented. |
443 | ||
d92eb7b0 | 444 | =head2 What does it mean that regexes are greedy? How can I get around it? |
68dc0745 | 445 | |
d92eb7b0 | 446 | Most people mean that greedy regexes match as much as they can. |
68dc0745 | 447 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
448 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local | |
449 | greed and immediate gratification to overall greed. To get non-greedy | |
450 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). | |
451 | ||
452 | An example: | |
453 | ||
454 | $s1 = $s2 = "I am very very cold"; | |
455 | $s1 =~ s/ve.*y //; # I am cold | |
456 | $s2 =~ s/ve.*?y //; # I am very cold | |
457 | ||
458 | Notice how the second substitution stopped matching as soon as it | |
459 | encountered "y ". The C<*?> quantifier effectively tells the regular | |
460 | expression engine to find a match as quickly as possible and pass | |
461 | control on to whatever is next in line, like you would if you were | |
462 | playing hot potato. | |
463 | ||
f9ac83b8 | 464 | =head2 How do I process each word on each line? |
68dc0745 | 465 | |
466 | Use the split function: | |
467 | ||
468 | while (<>) { | |
197aec24 | 469 | foreach $word ( split ) { |
68dc0745 | 470 | # do something with $word here |
197aec24 | 471 | } |
54310121 | 472 | } |
68dc0745 | 473 | |
54310121 | 474 | Note that this isn't really a word in the English sense; it's just |
475 | chunks of consecutive non-whitespace characters. | |
68dc0745 | 476 | |
f1cbbd6e GS |
477 | To work with only alphanumeric sequences (including underscores), you |
478 | might consider | |
68dc0745 | 479 | |
480 | while (<>) { | |
481 | foreach $word (m/(\w+)/g) { | |
482 | # do something with $word here | |
483 | } | |
484 | } | |
485 | ||
486 | =head2 How can I print out a word-frequency or line-frequency summary? | |
487 | ||
488 | To do this, you have to parse out each word in the input stream. We'll | |
54310121 | 489 | pretend that by word you mean chunk of alphabetics, hyphens, or |
490 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
68dc0745 | 491 | in the previous question: |
492 | ||
493 | while (<>) { | |
494 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" | |
495 | $seen{$1}++; | |
54310121 | 496 | } |
497 | } | |
68dc0745 | 498 | while ( ($word, $count) = each %seen ) { |
499 | print "$count $word\n"; | |
54310121 | 500 | } |
68dc0745 | 501 | |
502 | If you wanted to do the same thing for lines, you wouldn't need a | |
503 | regular expression: | |
504 | ||
197aec24 | 505 | while (<>) { |
68dc0745 | 506 | $seen{$_}++; |
54310121 | 507 | } |
68dc0745 | 508 | while ( ($line, $count) = each %seen ) { |
509 | print "$count $line"; | |
510 | } | |
511 | ||
b432a672 AL |
512 | If you want these output in a sorted order, see L<perlfaq4>: "How do I |
513 | sort a hash (optionally by value instead of key)?". | |
68dc0745 | 514 | |
515 | =head2 How can I do approximate matching? | |
516 | ||
517 | See the module String::Approx available from CPAN. | |
518 | ||
519 | =head2 How do I efficiently match many regular expressions at once? | |
520 | ||
7678cced RGS |
521 | ( contributed by brian d foy ) |
522 | ||
6670e5e7 | 523 | Avoid asking Perl to compile a regular expression every time |
7678cced RGS |
524 | you want to match it. In this example, perl must recompile |
525 | the regular expression for every iteration of the foreach() | |
526 | loop since it has no way to know what $pattern will be. | |
527 | ||
528 | @patterns = qw( foo bar baz ); | |
6670e5e7 RGS |
529 | |
530 | LINE: while( <> ) | |
7678cced | 531 | { |
6670e5e7 | 532 | foreach $pattern ( @patterns ) |
7678cced RGS |
533 | { |
534 | print if /\b$pattern\b/i; | |
535 | next LINE; | |
536 | } | |
537 | } | |
68dc0745 | 538 | |
7678cced RGS |
539 | The qr// operator showed up in perl 5.005. It compiles a |
540 | regular expression, but doesn't apply it. When you use the | |
541 | pre-compiled version of the regex, perl does less work. In | |
542 | this example, I inserted a map() to turn each pattern into | |
543 | its pre-compiled form. The rest of the script is the same, | |
544 | but faster. | |
545 | ||
546 | @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); | |
547 | ||
6670e5e7 | 548 | LINE: while( <> ) |
7678cced | 549 | { |
6670e5e7 | 550 | foreach $pattern ( @patterns ) |
7678cced RGS |
551 | { |
552 | print if /\b$pattern\b/i; | |
553 | next LINE; | |
554 | } | |
555 | } | |
6670e5e7 | 556 | |
7678cced RGS |
557 | In some cases, you may be able to make several patterns into |
558 | a single regular expression. Beware of situations that require | |
559 | backtracking though. | |
65acb1b1 | 560 | |
7678cced RGS |
561 | $regex = join '|', qw( foo bar baz ); |
562 | ||
6670e5e7 | 563 | LINE: while( <> ) |
7678cced RGS |
564 | { |
565 | print if /\b(?:$regex)\b/i; | |
566 | } | |
567 | ||
568 | For more details on regular expression efficiency, see Mastering | |
569 | Regular Expressions by Jeffrey Freidl. He explains how regular | |
570 | expressions engine work and why some patterns are surprisingly | |
6670e5e7 | 571 | inefficient. Once you understand how perl applies regular |
7678cced | 572 | expressions, you can tune them for individual situations. |
68dc0745 | 573 | |
574 | =head2 Why don't word-boundary searches with C<\b> work for me? | |
575 | ||
7678cced RGS |
576 | (contributed by brian d foy) |
577 | ||
578 | Ensure that you know what \b really does: it's the boundary between a | |
579 | word character, \w, and something that isn't a word character. That | |
580 | thing that isn't a word character might be \W, but it can also be the | |
581 | start or end of the string. | |
582 | ||
583 | It's not (not!) the boundary between whitespace and non-whitespace, | |
584 | and it's not the stuff between words we use to create sentences. | |
585 | ||
586 | In regex speak, a word boundary (\b) is a "zero width assertion", | |
587 | meaning that it doesn't represent a character in the string, but a | |
588 | condition at a certain position. | |
589 | ||
590 | For the regular expression, /\bPerl\b/, there has to be a word | |
591 | boundary before the "P" and after the "l". As long as something other | |
592 | than a word character precedes the "P" and succeeds the "l", the | |
593 | pattern will match. These strings match /\bPerl\b/. | |
594 | ||
595 | "Perl" # no word char before P or after l | |
596 | "Perl " # same as previous (space is not a word char) | |
597 | "'Perl'" # the ' char is not a word char | |
598 | "Perl's" # no word char before P, non-word char after "l" | |
599 | ||
600 | These strings do not match /\bPerl\b/. | |
601 | ||
602 | "Perl_" # _ is a word char! | |
603 | "Perler" # no word char before P, but one after l | |
6670e5e7 | 604 | |
7678cced | 605 | You don't have to use \b to match words though. You can look for |
d7f8936a | 606 | non-word characters surrounded by word characters. These strings |
7678cced RGS |
607 | match the pattern /\b'\b/. |
608 | ||
609 | "don't" # the ' char is surrounded by "n" and "t" | |
610 | "qep'a'" # the ' char is surrounded by "p" and "a" | |
6670e5e7 | 611 | |
7678cced | 612 | These strings do not match /\b'\b/. |
68dc0745 | 613 | |
7678cced | 614 | "foo'" # there is no word char after non-word ' |
6670e5e7 | 615 | |
7678cced RGS |
616 | You can also use the complement of \b, \B, to specify that there |
617 | should not be a word boundary. | |
68dc0745 | 618 | |
7678cced RGS |
619 | In the pattern /\Bam\B/, there must be a word character before the "a" |
620 | and after the "m". These patterns match /\Bam\B/: | |
68dc0745 | 621 | |
7678cced RGS |
622 | "llama" # "am" surrounded by word chars |
623 | "Samuel" # same | |
6670e5e7 | 624 | |
7678cced | 625 | These strings do not match /\Bam\B/ |
68dc0745 | 626 | |
7678cced RGS |
627 | "Sam" # no word boundary before "a", but one after "m" |
628 | "I am Sam" # "am" surrounded by non-word chars | |
68dc0745 | 629 | |
68dc0745 | 630 | |
631 | =head2 Why does using $&, $`, or $' slow my program down? | |
632 | ||
571e049f | 633 | (contributed by Anno Siegel) |
68dc0745 | 634 | |
571e049f | 635 | Once Perl sees that you need one of these variables anywhere in the |
b68463f7 RGS |
636 | program, it provides them on each and every pattern match. That means |
637 | that on every pattern match the entire string will be copied, part of it | |
638 | to $`, part to $&, and part to $'. Thus the penalty is most severe with | |
639 | long strings and patterns that match often. Avoid $&, $', and $` if you | |
640 | can, but if you can't, once you've used them at all, use them at will | |
641 | because you've already paid the price. Remember that some algorithms | |
642 | really appreciate them. As of the 5.005 release, the $& variable is no | |
643 | longer "expensive" the way the other two are. | |
644 | ||
645 | Since Perl 5.6.1 the special variables @- and @+ can functionally replace | |
646 | $`, $& and $'. These arrays contain pointers to the beginning and end | |
647 | of each match (see perlvar for the full story), so they give you | |
648 | essentially the same information, but without the risk of excessive | |
649 | string copying. | |
6670e5e7 | 650 | |
68dc0745 | 651 | =head2 What good is C<\G> in a regular expression? |
652 | ||
49d635f9 RGS |
653 | You use the C<\G> anchor to start the next match on the same |
654 | string where the last match left off. The regular | |
655 | expression engine cannot skip over any characters to find | |
656 | the next match with this anchor, so C<\G> is similar to the | |
657 | beginning of string anchor, C<^>. The C<\G> anchor is typically | |
658 | used with the C<g> flag. It uses the value of pos() | |
659 | as the position to start the next match. As the match | |
660 | operator makes successive matches, it updates pos() with the | |
661 | position of the next character past the last match (or the | |
662 | first character of the next match, depending on how you like | |
663 | to look at it). Each string has its own pos() value. | |
664 | ||
665 | Suppose you want to match all of consective pairs of digits | |
666 | in a string like "1122a44" and stop matching when you | |
667 | encounter non-digits. You want to match C<11> and C<22> but | |
668 | the letter <a> shows up between C<22> and C<44> and you want | |
669 | to stop at C<a>. Simply matching pairs of digits skips over | |
670 | the C<a> and still matches C<44>. | |
671 | ||
672 | $_ = "1122a44"; | |
673 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) | |
674 | ||
675 | If you use the \G anchor, you force the match after C<22> to | |
676 | start with the C<a>. The regular expression cannot match | |
677 | there since it does not find a digit, so the next match | |
678 | fails and the match operator returns the pairs it already | |
679 | found. | |
680 | ||
681 | $_ = "1122a44"; | |
682 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) | |
683 | ||
684 | You can also use the C<\G> anchor in scalar context. You | |
685 | still need the C<g> flag. | |
686 | ||
687 | $_ = "1122a44"; | |
688 | while( m/\G(\d\d)/g ) | |
689 | { | |
690 | print "Found $1\n"; | |
691 | } | |
197aec24 | 692 | |
49d635f9 RGS |
693 | After the match fails at the letter C<a>, perl resets pos() |
694 | and the next match on the same string starts at the beginning. | |
695 | ||
696 | $_ = "1122a44"; | |
697 | while( m/\G(\d\d)/g ) | |
698 | { | |
699 | print "Found $1\n"; | |
700 | } | |
701 | ||
702 | print "Found $1 after while" if m/(\d\d)/g; # finds "11" | |
703 | ||
704 | You can disable pos() resets on fail with the C<c> flag. | |
705 | Subsequent matches start where the last successful match | |
706 | ended (the value of pos()) even if a match on the same | |
707 | string as failed in the meantime. In this case, the match | |
708 | after the while() loop starts at the C<a> (where the last | |
709 | match stopped), and since it does not use any anchor it can | |
710 | skip over the C<a> to find "44". | |
711 | ||
712 | $_ = "1122a44"; | |
713 | while( m/\G(\d\d)/gc ) | |
714 | { | |
715 | print "Found $1\n"; | |
716 | } | |
717 | ||
718 | print "Found $1 after while" if m/(\d\d)/g; # finds "44" | |
719 | ||
720 | Typically you use the C<\G> anchor with the C<c> flag | |
721 | when you want to try a different match if one fails, | |
722 | such as in a tokenizer. Jeffrey Friedl offers this example | |
723 | which works in 5.004 or later. | |
68dc0745 | 724 | |
725 | while (<>) { | |
726 | chomp; | |
727 | PARSER: { | |
49d635f9 RGS |
728 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; |
729 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; | |
730 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; | |
731 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; | |
68dc0745 | 732 | } |
733 | } | |
734 | ||
49d635f9 RGS |
735 | For each line, the PARSER loop first tries to match a series |
736 | of digits followed by a word boundary. This match has to | |
737 | start at the place the last match left off (or the beginning | |
197aec24 | 738 | of the string on the first match). Since C<m/ \G( \d+\b |
49d635f9 RGS |
739 | )/gcx> uses the C<c> flag, if the string does not match that |
740 | regular expression, perl does not reset pos() and the next | |
741 | match starts at the same position to try a different | |
742 | pattern. | |
68dc0745 | 743 | |
d92eb7b0 | 744 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
68dc0745 | 745 | |
746 | While it's true that Perl's regular expressions resemble the DFAs | |
747 | (deterministic finite automata) of the egrep(1) program, they are in | |
46fc3d4c | 748 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 | 749 | backtracking and backreferencing. And they aren't POSIX-style either, |
750 | because those guarantee worst-case behavior for all cases. (It seems | |
751 | that some people prefer guarantees of consistency, even when what's | |
752 | guaranteed is slowness.) See the book "Mastering Regular Expressions" | |
753 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
754 | hope to know on these matters (a full citation appears in | |
755 | L<perlfaq2>). | |
756 | ||
788611b6 | 757 | =head2 What's wrong with using grep in a void context? |
68dc0745 | 758 | |
788611b6 A |
759 | The problem is that grep builds a return list, regardless of the context. |
760 | This means you're making Perl go to the trouble of building a list that | |
761 | you then just throw away. If the list is large, you waste both time and space. | |
762 | If your intent is to iterate over the list, then use a for loop for this | |
f05bbc40 | 763 | purpose. |
68dc0745 | 764 | |
788611b6 A |
765 | In perls older than 5.8.1, map suffers from this problem as well. |
766 | But since 5.8.1, this has been fixed, and map is context aware - in void | |
767 | context, no lists are constructed. | |
768 | ||
54310121 | 769 | =head2 How can I match strings with multibyte characters? |
68dc0745 | 770 | |
d9d154f2 JH |
771 | Starting from Perl 5.6 Perl has had some level of multibyte character |
772 | support. Perl 5.8 or later is recommended. Supported multibyte | |
fe854a6f | 773 | character repertoires include Unicode, and legacy encodings |
d9d154f2 JH |
774 | through the Encode module. See L<perluniintro>, L<perlunicode>, |
775 | and L<Encode>. | |
776 | ||
777 | If you are stuck with older Perls, you can do Unicode with the | |
778 | C<Unicode::String> module, and character conversions using the | |
779 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using | |
780 | Japanese encodings, you might try using the jperl 5.005_03. | |
781 | ||
782 | Finally, the following set of approaches was offered by Jeffrey | |
783 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
784 | this very matter. | |
68dc0745 | 785 | |
fc36a67e | 786 | Let's suppose you have some weird Martian encoding where pairs of |
787 | ASCII uppercase letters encode single Martian letters (i.e. the two | |
788 | bytes "CV" make a single Martian letter, as do the two bytes "SG", | |
789 | "VS", "XX", etc.). Other bytes represent single characters, just like | |
790 | ASCII. | |
68dc0745 | 791 | |
fc36a67e | 792 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
793 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. | |
68dc0745 | 794 | |
795 | Now, say you want to search for the single character C</GX/>. Perl | |
fc36a67e | 796 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
797 | am CVSGXX!" string, even though that character isn't there: it just | |
798 | looks like it is because "SG" is next to "XX", but there's no real | |
799 | "GX". This is a big problem. | |
68dc0745 | 800 | |
801 | Here are a few ways, all painful, to deal with it: | |
802 | ||
b432a672 | 803 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent "martian" |
49d635f9 | 804 | # bytes are no longer adjacent. |
68dc0745 | 805 | print "found GX!\n" if $martian =~ /GX/; |
806 | ||
807 | Or like this: | |
808 | ||
809 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; | |
810 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
811 | # | |
812 | foreach $char (@chars) { | |
813 | print "found GX!\n", last if $char eq 'GX'; | |
814 | } | |
815 | ||
816 | Or like this: | |
817 | ||
818 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded | |
54310121 | 819 | print "found GX!\n", last if $1 eq 'GX'; |
68dc0745 | 820 | } |
821 | ||
49d635f9 | 822 | Here's another, slightly less painful, way to do it from Benjamin |
c98c5709 | 823 | Goldberg, who uses a zero-width negative look-behind assertion. |
49d635f9 | 824 | |
c98c5709 RGS |
825 | print "found GX!\n" if $martian =~ m/ |
826 | (?<![A-Z]) | |
827 | (?:[A-Z][A-Z])*? | |
828 | GX | |
829 | /x; | |
197aec24 | 830 | |
49d635f9 | 831 | This succeeds if the "martian" character GX is in the string, and fails |
c98c5709 RGS |
832 | otherwise. If you don't like using (?<!), a zero-width negative |
833 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). | |
49d635f9 RGS |
834 | |
835 | It does have the drawback of putting the wrong thing in $-[0] and $+[0], | |
836 | but this usually can be worked around. | |
68dc0745 | 837 | |
65acb1b1 TC |
838 | =head2 How do I match a pattern that is supplied by the user? |
839 | ||
840 | Well, if it's really a pattern, then just use | |
841 | ||
842 | chomp($pattern = <STDIN>); | |
843 | if ($line =~ /$pattern/) { } | |
844 | ||
a6dd486b | 845 | Alternatively, since you have no guarantee that your user entered |
65acb1b1 TC |
846 | a valid regular expression, trap the exception this way: |
847 | ||
848 | if (eval { $line =~ /$pattern/ }) { } | |
849 | ||
a6dd486b | 850 | If all you really want to search for a string, not a pattern, |
65acb1b1 TC |
851 | then you should either use the index() function, which is made for |
852 | string searching, or if you can't be disabused of using a pattern | |
853 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented | |
854 | in L<perlre>. | |
855 | ||
856 | $pattern = <STDIN>; | |
857 | ||
858 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; | |
859 | while (<FILE>) { | |
860 | print if /\Q$pattern\E/; | |
861 | } | |
862 | close FILE; | |
863 | ||
68dc0745 | 864 | =head1 AUTHOR AND COPYRIGHT |
865 | ||
7678cced RGS |
866 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
867 | other authors as noted. All rights reserved. | |
5a964f20 | 868 | |
5a7beb56 JH |
869 | This documentation is free; you can redistribute it and/or modify it |
870 | under the same terms as Perl itself. | |
5a964f20 TC |
871 | |
872 | Irrespective of its distribution, all code examples in this file | |
873 | are hereby placed into the public domain. You are permitted and | |
874 | encouraged to use this code in your own programs for fun | |
875 | or for profit as you see fit. A simple comment in the code giving | |
876 | credit would be courteous but is not required. |