Commit | Line | Data |
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68dc0745 | 1 | =head1 NAME |
2 | ||
109f0441 | 3 | perlfaq6 - Regular Expressions |
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 |
7b059540 KW |
36 | (except in a character class and a few other places), and also allows you to |
37 | use normal comments there, too. As you can imagine, whitespace and comments | |
38 | help a lot. | |
68dc0745 | 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 | ||
109f0441 | 100 | $/ = ''; # read in whole paragraph, not just one line |
ac9dac7f RGS |
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 | ||
109f0441 | 110 | $/ = ''; # read in whole paragraph, not just one line |
ac9dac7f RGS |
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 | |
109f0441 S |
152 | =head2 How do I match XML, HTML, or other nasty, ugly things with a regex? |
153 | X<regex, XML> X<regex, HTML> X<XML> X<HTML> X<pain> X<frustration> | |
154 | X<sucking out, will to live> | |
155 | ||
156 | (contributed by brian d foy) | |
157 | ||
158 | If you just want to get work done, use a module and forget about the | |
159 | regular expressions. The C<XML::Parser> and C<HTML::Parser> modules | |
160 | are good starts, although each namespace has other parsing modules | |
161 | specialized for certain tasks and different ways of doing it. Start at | |
162 | CPAN Search ( http://search.cpan.org ) and wonder at all the work people | |
163 | have done for you already! :) | |
164 | ||
165 | The problem with things such as XML is that they have balanced text | |
166 | containing multiple levels of balanced text, but sometimes it isn't | |
167 | balanced text, as in an empty tag (C<< <br/> >>, for instance). Even then, | |
168 | things can occur out-of-order. Just when you think you've got a | |
169 | pattern that matches your input, someone throws you a curveball. | |
170 | ||
171 | If you'd like to do it the hard way, scratching and clawing your way | |
589a5df2 | 172 | toward a right answer but constantly being disappointed, besieged by |
109f0441 S |
173 | bug reports, and weary from the inordinate amount of time you have to |
174 | spend reinventing a triangular wheel, then there are several things | |
175 | you can try before you give up in frustration: | |
176 | ||
177 | =over 4 | |
178 | ||
179 | =item * Solve the balanced text problem from another question in L<perlfaq6> | |
180 | ||
181 | =item * Try the recursive regex features in Perl 5.10 and later. See L<perlre> | |
182 | ||
183 | =item * Try defining a grammar using Perl 5.10's C<(?DEFINE)> feature. | |
184 | ||
185 | =item * Break the problem down into sub-problems instead of trying to use a single regex | |
186 | ||
187 | =item * Convince everyone not to use XML or HTML in the first place | |
188 | ||
189 | =back | |
190 | ||
191 | Good luck! | |
192 | ||
68dc0745 | 193 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
d74e8afc ITB |
194 | X<$/, regexes in> X<$INPUT_RECORD_SEPARATOR, regexes in> |
195 | X<$RS, regexes in> | |
68dc0745 | 196 | |
109f0441 | 197 | $/ has to be a string. You can use these examples if you really need to |
c195e131 | 198 | do this. |
49d635f9 | 199 | |
28b41a80 RGS |
200 | If you have File::Stream, this is easy. |
201 | ||
ac9dac7f RGS |
202 | use File::Stream; |
203 | ||
204 | my $stream = File::Stream->new( | |
205 | $filehandle, | |
206 | separator => qr/\s*,\s*/, | |
207 | ); | |
28b41a80 | 208 | |
ac9dac7f | 209 | print "$_\n" while <$stream>; |
28b41a80 RGS |
210 | |
211 | If you don't have File::Stream, you have to do a little more work. | |
212 | ||
109f0441 | 213 | You can use the four-argument form of sysread to continually add to |
197aec24 | 214 | a buffer. After you add to the buffer, you check if you have a |
49d635f9 RGS |
215 | complete line (using your regular expression). |
216 | ||
ac9dac7f RGS |
217 | local $_ = ""; |
218 | while( sysread FH, $_, 8192, length ) { | |
109f0441 | 219 | while( s/^((?s).*?)your_pattern// ) { |
ac9dac7f RGS |
220 | my $record = $1; |
221 | # do stuff here. | |
222 | } | |
223 | } | |
197aec24 | 224 | |
109f0441 S |
225 | You can do the same thing with foreach and a match using the |
226 | c flag and the \G anchor, if you do not mind your entire file | |
227 | being in memory at the end. | |
197aec24 | 228 | |
ac9dac7f RGS |
229 | local $_ = ""; |
230 | while( sysread FH, $_, 8192, length ) { | |
231 | foreach my $record ( m/\G((?s).*?)your_pattern/gc ) { | |
232 | # do stuff here. | |
233 | } | |
234 | substr( $_, 0, pos ) = "" if pos; | |
235 | } | |
68dc0745 | 236 | |
3fe9a6f1 | 237 | |
a6dd486b | 238 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
d74e8afc ITB |
239 | X<replace, case preserving> X<substitute, case preserving> |
240 | X<substitution, case preserving> X<s, case preserving> | |
68dc0745 | 241 | |
d92eb7b0 GS |
242 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
243 | properties of bitwise xor on ASCII strings. | |
244 | ||
ac9dac7f | 245 | $_= "this is a TEsT case"; |
d92eb7b0 | 246 | |
ac9dac7f RGS |
247 | $old = 'test'; |
248 | $new = 'success'; | |
d92eb7b0 | 249 | |
ac9dac7f RGS |
250 | s{(\Q$old\E)} |
251 | { uc $new | (uc $1 ^ $1) . | |
252 | (uc(substr $1, -1) ^ substr $1, -1) x | |
253 | (length($new) - length $1) | |
254 | }egi; | |
d92eb7b0 | 255 | |
ac9dac7f | 256 | print; |
d92eb7b0 | 257 | |
8305e449 | 258 | And here it is as a subroutine, modeled after the above: |
d92eb7b0 | 259 | |
ac9dac7f RGS |
260 | sub preserve_case($$) { |
261 | my ($old, $new) = @_; | |
262 | my $mask = uc $old ^ $old; | |
d92eb7b0 | 263 | |
ac9dac7f RGS |
264 | uc $new | $mask . |
265 | substr($mask, -1) x (length($new) - length($old)) | |
d92eb7b0 GS |
266 | } |
267 | ||
109f0441 S |
268 | $string = "this is a TEsT case"; |
269 | $string =~ s/(test)/preserve_case($1, "success")/egi; | |
270 | print "$string\n"; | |
d92eb7b0 GS |
271 | |
272 | This prints: | |
273 | ||
ac9dac7f | 274 | this is a SUcCESS case |
d92eb7b0 | 275 | |
74b9445a JP |
276 | As an alternative, to keep the case of the replacement word if it is |
277 | longer than the original, you can use this code, by Jeff Pinyan: | |
278 | ||
ac9dac7f RGS |
279 | sub preserve_case { |
280 | my ($from, $to) = @_; | |
281 | my ($lf, $lt) = map length, @_; | |
7207e29d | 282 | |
ac9dac7f RGS |
283 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
284 | else { $from .= substr $to, $lf } | |
7207e29d | 285 | |
ac9dac7f RGS |
286 | return uc $to | ($from ^ uc $from); |
287 | } | |
74b9445a JP |
288 | |
289 | This changes the sentence to "this is a SUcCess case." | |
290 | ||
d92eb7b0 GS |
291 | Just to show that C programmers can write C in any programming language, |
292 | if you prefer a more C-like solution, the following script makes the | |
293 | substitution have the same case, letter by letter, as the original. | |
294 | (It also happens to run about 240% slower than the Perlish solution runs.) | |
295 | If the substitution has more characters than the string being substituted, | |
296 | the case of the last character is used for the rest of the substitution. | |
68dc0745 | 297 | |
ac9dac7f RGS |
298 | # Original by Nathan Torkington, massaged by Jeffrey Friedl |
299 | # | |
300 | sub preserve_case($$) | |
301 | { | |
302 | my ($old, $new) = @_; | |
303 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc | |
304 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); | |
305 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; | |
306 | ||
307 | for ($i = 0; $i < $len; $i++) { | |
308 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { | |
309 | $state = 0; | |
310 | } elsif (lc $c eq $c) { | |
311 | substr($new, $i, 1) = lc(substr($new, $i, 1)); | |
312 | $state = 1; | |
313 | } else { | |
314 | substr($new, $i, 1) = uc(substr($new, $i, 1)); | |
315 | $state = 2; | |
316 | } | |
317 | } | |
318 | # finish up with any remaining new (for when new is longer than old) | |
319 | if ($newlen > $oldlen) { | |
320 | if ($state == 1) { | |
321 | substr($new, $oldlen) = lc(substr($new, $oldlen)); | |
322 | } elsif ($state == 2) { | |
323 | substr($new, $oldlen) = uc(substr($new, $oldlen)); | |
324 | } | |
325 | } | |
326 | return $new; | |
327 | } | |
68dc0745 | 328 | |
5a964f20 | 329 | =head2 How can I make C<\w> match national character sets? |
d74e8afc | 330 | X<\w> |
68dc0745 | 331 | |
49d635f9 RGS |
332 | Put C<use locale;> in your script. The \w character class is taken |
333 | from the current locale. | |
334 | ||
335 | See L<perllocale> for details. | |
68dc0745 | 336 | |
337 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? | |
d74e8afc | 338 | X<alpha> |
68dc0745 | 339 | |
49d635f9 RGS |
340 | You can use the POSIX character class syntax C</[[:alpha:]]/> |
341 | documented in L<perlre>. | |
342 | ||
343 | No matter which locale you are in, the alphabetic characters are | |
344 | the characters in \w without the digits and the underscore. | |
345 | As a regex, that looks like C</[^\W\d_]/>. Its complement, | |
197aec24 RGS |
346 | the non-alphabetics, is then everything in \W along with |
347 | the digits and the underscore, or C</[\W\d_]/>. | |
68dc0745 | 348 | |
d92eb7b0 | 349 | =head2 How can I quote a variable to use in a regex? |
d74e8afc | 350 | X<regex, escaping> X<regexp, escaping> X<regular expression, escaping> |
68dc0745 | 351 | |
352 | The Perl parser will expand $variable and @variable references in | |
353 | regular expressions unless the delimiter is a single quote. Remember, | |
79a522f5 | 354 | too, that the right-hand side of a C<s///> substitution is considered |
68dc0745 | 355 | a double-quoted string (see L<perlop> for more details). Remember |
d92eb7b0 | 356 | also that any regex special characters will be acted on unless you |
68dc0745 | 357 | precede the substitution with \Q. Here's an example: |
358 | ||
ac9dac7f RGS |
359 | $string = "Placido P. Octopus"; |
360 | $regex = "P."; | |
68dc0745 | 361 | |
ac9dac7f RGS |
362 | $string =~ s/$regex/Polyp/; |
363 | # $string is now "Polypacido P. Octopus" | |
68dc0745 | 364 | |
c83084d1 MJD |
365 | Because C<.> is special in regular expressions, and can match any |
366 | single character, the regex C<P.> here has matched the <Pl> in the | |
367 | original string. | |
368 | ||
369 | To escape the special meaning of C<.>, we use C<\Q>: | |
370 | ||
ac9dac7f RGS |
371 | $string = "Placido P. Octopus"; |
372 | $regex = "P."; | |
c83084d1 | 373 | |
ac9dac7f RGS |
374 | $string =~ s/\Q$regex/Polyp/; |
375 | # $string is now "Placido Polyp Octopus" | |
c83084d1 MJD |
376 | |
377 | The use of C<\Q> causes the <.> in the regex to be treated as a | |
378 | regular character, so that C<P.> matches a C<P> followed by a dot. | |
68dc0745 | 379 | |
380 | =head2 What is C</o> really for? | |
ee891a00 | 381 | X</o, regular expressions> X<compile, regular expressions> |
68dc0745 | 382 | |
ee891a00 | 383 | (contributed by brian d foy) |
68dc0745 | 384 | |
ee891a00 RGS |
385 | The C</o> option for regular expressions (documented in L<perlop> and |
386 | L<perlreref>) tells Perl to compile the regular expression only once. | |
387 | This is only useful when the pattern contains a variable. Perls 5.6 | |
388 | and later handle this automatically if the pattern does not change. | |
68dc0745 | 389 | |
ee891a00 RGS |
390 | Since the match operator C<m//>, the substitution operator C<s///>, |
391 | and the regular expression quoting operator C<qr//> are double-quotish | |
392 | constructs, you can interpolate variables into the pattern. See the | |
393 | answer to "How can I quote a variable to use in a regex?" for more | |
394 | details. | |
68dc0745 | 395 | |
ee891a00 RGS |
396 | This example takes a regular expression from the argument list and |
397 | prints the lines of input that match it: | |
68dc0745 | 398 | |
ee891a00 | 399 | my $pattern = shift @ARGV; |
109f0441 | 400 | |
ee891a00 RGS |
401 | while( <> ) { |
402 | print if m/$pattern/; | |
403 | } | |
404 | ||
405 | Versions of Perl prior to 5.6 would recompile the regular expression | |
406 | for each iteration, even if C<$pattern> had not changed. The C</o> | |
407 | would prevent this by telling Perl to compile the pattern the first | |
408 | time, then reuse that for subsequent iterations: | |
409 | ||
410 | my $pattern = shift @ARGV; | |
109f0441 | 411 | |
ee891a00 RGS |
412 | while( <> ) { |
413 | print if m/$pattern/o; # useful for Perl < 5.6 | |
414 | } | |
415 | ||
416 | In versions 5.6 and later, Perl won't recompile the regular expression | |
417 | if the variable hasn't changed, so you probably don't need the C</o> | |
418 | option. It doesn't hurt, but it doesn't help either. If you want any | |
419 | version of Perl to compile the regular expression only once even if | |
420 | the variable changes (thus, only using its initial value), you still | |
421 | need the C</o>. | |
422 | ||
423 | You can watch Perl's regular expression engine at work to verify for | |
424 | yourself if Perl is recompiling a regular expression. The C<use re | |
425 | 'debug'> pragma (comes with Perl 5.005 and later) shows the details. | |
426 | With Perls before 5.6, you should see C<re> reporting that its | |
427 | compiling the regular expression on each iteration. With Perl 5.6 or | |
428 | later, you should only see C<re> report that for the first iteration. | |
429 | ||
430 | use re 'debug'; | |
109f0441 | 431 | |
ee891a00 RGS |
432 | $regex = 'Perl'; |
433 | foreach ( qw(Perl Java Ruby Python) ) { | |
434 | print STDERR "-" x 73, "\n"; | |
435 | print STDERR "Trying $_...\n"; | |
436 | print STDERR "\t$_ is good!\n" if m/$regex/; | |
437 | } | |
68dc0745 | 438 | |
439 | =head2 How do I use a regular expression to strip C style comments from a file? | |
440 | ||
441 | While this actually can be done, it's much harder than you'd think. | |
442 | For example, this one-liner | |
443 | ||
ac9dac7f | 444 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
68dc0745 | 445 | |
446 | will work in many but not all cases. You see, it's too simple-minded for | |
447 | certain kinds of C programs, in particular, those with what appear to be | |
448 | comments in quoted strings. For that, you'd need something like this, | |
d92eb7b0 | 449 | created by Jeffrey Friedl and later modified by Fred Curtis. |
68dc0745 | 450 | |
ac9dac7f RGS |
451 | $/ = undef; |
452 | $_ = <>; | |
453 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse; | |
454 | print; | |
68dc0745 | 455 | |
456 | This could, of course, be more legibly written with the C</x> modifier, adding | |
d92eb7b0 GS |
457 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
458 | ||
459 | s{ | |
460 | /\* ## Start of /* ... */ comment | |
461 | [^*]*\*+ ## Non-* followed by 1-or-more *'s | |
462 | ( | |
463 | [^/*][^*]*\*+ | |
464 | )* ## 0-or-more things which don't start with / | |
465 | ## but do end with '*' | |
466 | / ## End of /* ... */ comment | |
467 | ||
468 | | ## OR various things which aren't comments: | |
469 | ||
470 | ( | |
471 | " ## Start of " ... " string | |
472 | ( | |
473 | \\. ## Escaped char | |
474 | | ## OR | |
475 | [^"\\] ## Non "\ | |
476 | )* | |
477 | " ## End of " ... " string | |
478 | ||
479 | | ## OR | |
480 | ||
481 | ' ## Start of ' ... ' string | |
482 | ( | |
483 | \\. ## Escaped char | |
484 | | ## OR | |
485 | [^'\\] ## Non '\ | |
486 | )* | |
487 | ' ## End of ' ... ' string | |
488 | ||
489 | | ## OR | |
490 | ||
491 | . ## Anything other char | |
492 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape | |
493 | ) | |
c98c5709 | 494 | }{defined $2 ? $2 : ""}gxse; |
d92eb7b0 | 495 | |
109f0441 S |
496 | A slight modification also removes C++ comments, possibly spanning multiple lines |
497 | using a continuation character: | |
d92eb7b0 | 498 | |
109f0441 | 499 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//([^\\]|[^\n][\n]?)*?\n|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $3 ? $3 : ""#gse; |
68dc0745 | 500 | |
501 | =head2 Can I use Perl regular expressions to match balanced text? | |
d74e8afc | 502 | X<regex, matching balanced test> X<regexp, matching balanced test> |
109f0441 S |
503 | X<regular expression, matching balanced test> X<possessive> X<PARNO> |
504 | X<Text::Balanced> X<Regexp::Common> X<backtracking> X<recursion> | |
505 | ||
506 | (contributed by brian d foy) | |
507 | ||
508 | Your first try should probably be the C<Text::Balanced> module, which | |
509 | is in the Perl standard library since Perl 5.8. It has a variety of | |
510 | functions to deal with tricky text. The C<Regexp::Common> module can | |
511 | also help by providing canned patterns you can use. | |
512 | ||
513 | As of Perl 5.10, you can match balanced text with regular expressions | |
514 | using recursive patterns. Before Perl 5.10, you had to resort to | |
515 | various tricks such as using Perl code in C<(??{})> sequences. | |
516 | ||
517 | Here's an example using a recursive regular expression. The goal is to | |
518 | capture all of the text within angle brackets, including the text in | |
519 | nested angle brackets. This sample text has two "major" groups: a | |
520 | group with one level of nesting and a group with two levels of | |
521 | nesting. There are five total groups in angle brackets: | |
522 | ||
523 | I have some <brackets in <nested brackets> > and | |
524 | <another group <nested once <nested twice> > > | |
525 | and that's it. | |
526 | ||
527 | The regular expression to match the balanced text uses two new (to | |
528 | Perl 5.10) regular expression features. These are covered in L<perlre> | |
529 | and this example is a modified version of one in that documentation. | |
530 | ||
589a5df2 | 531 | First, adding the new possessive C<+> to any quantifier finds the |
109f0441 S |
532 | longest match and does not backtrack. That's important since you want |
533 | to handle any angle brackets through the recursion, not backtracking. | |
534 | The group C<< [^<>]++ >> finds one or more non-angle brackets without | |
535 | backtracking. | |
536 | ||
537 | Second, the new C<(?PARNO)> refers to the sub-pattern in the | |
538 | particular capture buffer given by C<PARNO>. In the following regex, | |
539 | the first capture buffer finds (and remembers) the balanced text, and | |
540 | you need that same pattern within the first buffer to get past the | |
541 | nested text. That's the recursive part. The C<(?1)> uses the pattern | |
542 | in the outer capture buffer as an independent part of the regex. | |
543 | ||
544 | Putting it all together, you have: | |
545 | ||
546 | #!/usr/local/bin/perl5.10.0 | |
547 | ||
548 | my $string =<<"HERE"; | |
549 | I have some <brackets in <nested brackets> > and | |
550 | <another group <nested once <nested twice> > > | |
551 | and that's it. | |
552 | HERE | |
553 | ||
554 | my @groups = $string =~ m/ | |
555 | ( # start of capture buffer 1 | |
556 | < # match an opening angle bracket | |
557 | (?: | |
558 | [^<>]++ # one or more non angle brackets, non backtracking | |
559 | | | |
560 | (?1) # found < or >, so recurse to capture buffer 1 | |
561 | )* | |
562 | > # match a closing angle bracket | |
563 | ) # end of capture buffer 1 | |
564 | /xg; | |
565 | ||
566 | $" = "\n\t"; | |
567 | print "Found:\n\t@groups\n"; | |
568 | ||
569 | The output shows that Perl found the two major groups: | |
570 | ||
571 | Found: | |
572 | <brackets in <nested brackets> > | |
573 | <another group <nested once <nested twice> > > | |
574 | ||
575 | With a little extra work, you can get the all of the groups in angle | |
576 | brackets even if they are in other angle brackets too. Each time you | |
577 | get a balanced match, remove its outer delimiter (that's the one you | |
578 | just matched so don't match it again) and add it to a queue of strings | |
579 | to process. Keep doing that until you get no matches: | |
580 | ||
581 | #!/usr/local/bin/perl5.10.0 | |
582 | ||
583 | my @queue =<<"HERE"; | |
584 | I have some <brackets in <nested brackets> > and | |
585 | <another group <nested once <nested twice> > > | |
586 | and that's it. | |
587 | HERE | |
588 | ||
589 | my $regex = qr/ | |
590 | ( # start of bracket 1 | |
591 | < # match an opening angle bracket | |
592 | (?: | |
593 | [^<>]++ # one or more non angle brackets, non backtracking | |
594 | | | |
595 | (?1) # recurse to bracket 1 | |
596 | )* | |
597 | > # match a closing angle bracket | |
598 | ) # end of bracket 1 | |
599 | /x; | |
600 | ||
601 | $" = "\n\t"; | |
602 | ||
603 | while( @queue ) | |
604 | { | |
605 | my $string = shift @queue; | |
606 | ||
607 | my @groups = $string =~ m/$regex/g; | |
608 | print "Found:\n\t@groups\n\n" if @groups; | |
609 | ||
610 | unshift @queue, map { s/^<//; s/>$//; $_ } @groups; | |
611 | } | |
612 | ||
613 | The output shows all of the groups. The outermost matches show up | |
614 | first and the nested matches so up later: | |
615 | ||
616 | Found: | |
617 | <brackets in <nested brackets> > | |
618 | <another group <nested once <nested twice> > > | |
619 | ||
620 | Found: | |
621 | <nested brackets> | |
622 | ||
623 | Found: | |
624 | <nested once <nested twice> > | |
625 | ||
626 | Found: | |
627 | <nested twice> | |
68dc0745 | 628 | |
d92eb7b0 | 629 | =head2 What does it mean that regexes are greedy? How can I get around it? |
d74e8afc | 630 | X<greedy> X<greediness> |
68dc0745 | 631 | |
d92eb7b0 | 632 | Most people mean that greedy regexes match as much as they can. |
68dc0745 | 633 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
634 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local | |
635 | greed and immediate gratification to overall greed. To get non-greedy | |
636 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). | |
637 | ||
638 | An example: | |
639 | ||
ac9dac7f RGS |
640 | $s1 = $s2 = "I am very very cold"; |
641 | $s1 =~ s/ve.*y //; # I am cold | |
642 | $s2 =~ s/ve.*?y //; # I am very cold | |
68dc0745 | 643 | |
644 | Notice how the second substitution stopped matching as soon as it | |
645 | encountered "y ". The C<*?> quantifier effectively tells the regular | |
646 | expression engine to find a match as quickly as possible and pass | |
647 | control on to whatever is next in line, like you would if you were | |
648 | playing hot potato. | |
649 | ||
f9ac83b8 | 650 | =head2 How do I process each word on each line? |
d74e8afc | 651 | X<word> |
68dc0745 | 652 | |
653 | Use the split function: | |
654 | ||
ac9dac7f RGS |
655 | while (<>) { |
656 | foreach $word ( split ) { | |
657 | # do something with $word here | |
658 | } | |
197aec24 | 659 | } |
68dc0745 | 660 | |
54310121 | 661 | Note that this isn't really a word in the English sense; it's just |
662 | chunks of consecutive non-whitespace characters. | |
68dc0745 | 663 | |
f1cbbd6e GS |
664 | To work with only alphanumeric sequences (including underscores), you |
665 | might consider | |
68dc0745 | 666 | |
ac9dac7f RGS |
667 | while (<>) { |
668 | foreach $word (m/(\w+)/g) { | |
669 | # do something with $word here | |
670 | } | |
68dc0745 | 671 | } |
68dc0745 | 672 | |
673 | =head2 How can I print out a word-frequency or line-frequency summary? | |
674 | ||
675 | To do this, you have to parse out each word in the input stream. We'll | |
54310121 | 676 | pretend that by word you mean chunk of alphabetics, hyphens, or |
677 | apostrophes, rather than the non-whitespace chunk idea of a word given | |
68dc0745 | 678 | in the previous question: |
679 | ||
ac9dac7f RGS |
680 | while (<>) { |
681 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" | |
682 | $seen{$1}++; | |
683 | } | |
54310121 | 684 | } |
ac9dac7f RGS |
685 | |
686 | while ( ($word, $count) = each %seen ) { | |
687 | print "$count $word\n"; | |
688 | } | |
68dc0745 | 689 | |
690 | If you wanted to do the same thing for lines, you wouldn't need a | |
691 | regular expression: | |
692 | ||
ac9dac7f RGS |
693 | while (<>) { |
694 | $seen{$_}++; | |
695 | } | |
696 | ||
697 | while ( ($line, $count) = each %seen ) { | |
698 | print "$count $line"; | |
699 | } | |
68dc0745 | 700 | |
b432a672 AL |
701 | If you want these output in a sorted order, see L<perlfaq4>: "How do I |
702 | sort a hash (optionally by value instead of key)?". | |
68dc0745 | 703 | |
704 | =head2 How can I do approximate matching? | |
d74e8afc | 705 | X<match, approximate> X<matching, approximate> |
68dc0745 | 706 | |
707 | See the module String::Approx available from CPAN. | |
708 | ||
709 | =head2 How do I efficiently match many regular expressions at once? | |
d74e8afc ITB |
710 | X<regex, efficiency> X<regexp, efficiency> |
711 | X<regular expression, efficiency> | |
68dc0745 | 712 | |
7678cced RGS |
713 | ( contributed by brian d foy ) |
714 | ||
6670e5e7 | 715 | Avoid asking Perl to compile a regular expression every time |
f12f5f55 | 716 | you want to match it. In this example, perl must recompile |
109f0441 | 717 | the regular expression for every iteration of the C<foreach> |
7678cced RGS |
718 | loop since it has no way to know what $pattern will be. |
719 | ||
ac9dac7f | 720 | @patterns = qw( foo bar baz ); |
6670e5e7 | 721 | |
ac9dac7f RGS |
722 | LINE: while( <DATA> ) |
723 | { | |
6670e5e7 | 724 | foreach $pattern ( @patterns ) |
7678cced | 725 | { |
ac9dac7f RGS |
726 | if( /\b$pattern\b/i ) |
727 | { | |
728 | print; | |
729 | next LINE; | |
730 | } | |
731 | } | |
7678cced | 732 | } |
68dc0745 | 733 | |
109f0441 | 734 | The C<qr//> operator showed up in perl 5.005. It compiles a |
7678cced RGS |
735 | regular expression, but doesn't apply it. When you use the |
736 | pre-compiled version of the regex, perl does less work. In | |
109f0441 | 737 | this example, I inserted a C<map> to turn each pattern into |
f12f5f55 | 738 | its pre-compiled form. The rest of the script is the same, |
7678cced RGS |
739 | but faster. |
740 | ||
ac9dac7f | 741 | @patterns = map { qr/\b$_\b/i } qw( foo bar baz ); |
7678cced | 742 | |
ac9dac7f RGS |
743 | LINE: while( <> ) |
744 | { | |
6670e5e7 | 745 | foreach $pattern ( @patterns ) |
7678cced | 746 | { |
109f0441 S |
747 | if( /$pattern/ ) |
748 | { | |
749 | print; | |
750 | next LINE; | |
751 | } | |
ac9dac7f | 752 | } |
7678cced | 753 | } |
6670e5e7 | 754 | |
7678cced | 755 | In some cases, you may be able to make several patterns into |
f12f5f55 | 756 | a single regular expression. Beware of situations that require |
7678cced | 757 | backtracking though. |
65acb1b1 | 758 | |
7678cced RGS |
759 | $regex = join '|', qw( foo bar baz ); |
760 | ||
ac9dac7f RGS |
761 | LINE: while( <> ) |
762 | { | |
7678cced RGS |
763 | print if /\b(?:$regex)\b/i; |
764 | } | |
765 | ||
109f0441 S |
766 | For more details on regular expression efficiency, see I<Mastering |
767 | Regular Expressions> by Jeffrey Freidl. He explains how regular | |
7678cced | 768 | expressions engine work and why some patterns are surprisingly |
6670e5e7 | 769 | inefficient. Once you understand how perl applies regular |
7678cced | 770 | expressions, you can tune them for individual situations. |
68dc0745 | 771 | |
772 | =head2 Why don't word-boundary searches with C<\b> work for me? | |
d74e8afc | 773 | X<\b> |
68dc0745 | 774 | |
7678cced RGS |
775 | (contributed by brian d foy) |
776 | ||
777 | Ensure that you know what \b really does: it's the boundary between a | |
778 | word character, \w, and something that isn't a word character. That | |
779 | thing that isn't a word character might be \W, but it can also be the | |
780 | start or end of the string. | |
781 | ||
782 | It's not (not!) the boundary between whitespace and non-whitespace, | |
783 | and it's not the stuff between words we use to create sentences. | |
784 | ||
785 | In regex speak, a word boundary (\b) is a "zero width assertion", | |
786 | meaning that it doesn't represent a character in the string, but a | |
787 | condition at a certain position. | |
788 | ||
789 | For the regular expression, /\bPerl\b/, there has to be a word | |
790 | boundary before the "P" and after the "l". As long as something other | |
791 | than a word character precedes the "P" and succeeds the "l", the | |
792 | pattern will match. These strings match /\bPerl\b/. | |
793 | ||
794 | "Perl" # no word char before P or after l | |
795 | "Perl " # same as previous (space is not a word char) | |
796 | "'Perl'" # the ' char is not a word char | |
797 | "Perl's" # no word char before P, non-word char after "l" | |
798 | ||
799 | These strings do not match /\bPerl\b/. | |
800 | ||
801 | "Perl_" # _ is a word char! | |
802 | "Perler" # no word char before P, but one after l | |
6670e5e7 | 803 | |
7678cced | 804 | You don't have to use \b to match words though. You can look for |
d7f8936a | 805 | non-word characters surrounded by word characters. These strings |
7678cced RGS |
806 | match the pattern /\b'\b/. |
807 | ||
808 | "don't" # the ' char is surrounded by "n" and "t" | |
809 | "qep'a'" # the ' char is surrounded by "p" and "a" | |
6670e5e7 | 810 | |
7678cced | 811 | These strings do not match /\b'\b/. |
68dc0745 | 812 | |
7678cced | 813 | "foo'" # there is no word char after non-word ' |
6670e5e7 | 814 | |
7678cced RGS |
815 | You can also use the complement of \b, \B, to specify that there |
816 | should not be a word boundary. | |
68dc0745 | 817 | |
7678cced RGS |
818 | In the pattern /\Bam\B/, there must be a word character before the "a" |
819 | and after the "m". These patterns match /\Bam\B/: | |
68dc0745 | 820 | |
7678cced RGS |
821 | "llama" # "am" surrounded by word chars |
822 | "Samuel" # same | |
6670e5e7 | 823 | |
7678cced | 824 | These strings do not match /\Bam\B/ |
68dc0745 | 825 | |
7678cced RGS |
826 | "Sam" # no word boundary before "a", but one after "m" |
827 | "I am Sam" # "am" surrounded by non-word chars | |
68dc0745 | 828 | |
68dc0745 | 829 | |
830 | =head2 Why does using $&, $`, or $' slow my program down? | |
d74e8afc | 831 | X<$MATCH> X<$&> X<$POSTMATCH> X<$'> X<$PREMATCH> X<$`> |
68dc0745 | 832 | |
571e049f | 833 | (contributed by Anno Siegel) |
68dc0745 | 834 | |
571e049f | 835 | Once Perl sees that you need one of these variables anywhere in the |
b68463f7 RGS |
836 | program, it provides them on each and every pattern match. That means |
837 | that on every pattern match the entire string will be copied, part of it | |
838 | to $`, part to $&, and part to $'. Thus the penalty is most severe with | |
839 | long strings and patterns that match often. Avoid $&, $', and $` if you | |
840 | can, but if you can't, once you've used them at all, use them at will | |
841 | because you've already paid the price. Remember that some algorithms | |
842 | really appreciate them. As of the 5.005 release, the $& variable is no | |
843 | longer "expensive" the way the other two are. | |
844 | ||
845 | Since Perl 5.6.1 the special variables @- and @+ can functionally replace | |
846 | $`, $& and $'. These arrays contain pointers to the beginning and end | |
847 | of each match (see perlvar for the full story), so they give you | |
848 | essentially the same information, but without the risk of excessive | |
849 | string copying. | |
6670e5e7 | 850 | |
109f0441 S |
851 | Perl 5.10 added three specials, C<${^MATCH}>, C<${^PREMATCH}>, and |
852 | C<${^POSTMATCH}> to do the same job but without the global performance | |
853 | penalty. Perl 5.10 only sets these variables if you compile or execute the | |
854 | regular expression with the C</p> modifier. | |
855 | ||
68dc0745 | 856 | =head2 What good is C<\G> in a regular expression? |
d74e8afc | 857 | X<\G> |
68dc0745 | 858 | |
49d635f9 RGS |
859 | You use the C<\G> anchor to start the next match on the same |
860 | string where the last match left off. The regular | |
861 | expression engine cannot skip over any characters to find | |
862 | the next match with this anchor, so C<\G> is similar to the | |
863 | beginning of string anchor, C<^>. The C<\G> anchor is typically | |
ee891a00 | 864 | used with the C<g> flag. It uses the value of C<pos()> |
49d635f9 | 865 | as the position to start the next match. As the match |
ee891a00 | 866 | operator makes successive matches, it updates C<pos()> with the |
49d635f9 RGS |
867 | position of the next character past the last match (or the |
868 | first character of the next match, depending on how you like | |
ee891a00 | 869 | to look at it). Each string has its own C<pos()> value. |
49d635f9 | 870 | |
ee891a00 | 871 | Suppose you want to match all of consecutive pairs of digits |
49d635f9 RGS |
872 | in a string like "1122a44" and stop matching when you |
873 | encounter non-digits. You want to match C<11> and C<22> but | |
874 | the letter <a> shows up between C<22> and C<44> and you want | |
875 | to stop at C<a>. Simply matching pairs of digits skips over | |
876 | the C<a> and still matches C<44>. | |
877 | ||
878 | $_ = "1122a44"; | |
879 | my @pairs = m/(\d\d)/g; # qw( 11 22 44 ) | |
880 | ||
ee891a00 | 881 | If you use the C<\G> anchor, you force the match after C<22> to |
49d635f9 RGS |
882 | start with the C<a>. The regular expression cannot match |
883 | there since it does not find a digit, so the next match | |
884 | fails and the match operator returns the pairs it already | |
885 | found. | |
886 | ||
887 | $_ = "1122a44"; | |
888 | my @pairs = m/\G(\d\d)/g; # qw( 11 22 ) | |
889 | ||
890 | You can also use the C<\G> anchor in scalar context. You | |
891 | still need the C<g> flag. | |
892 | ||
893 | $_ = "1122a44"; | |
894 | while( m/\G(\d\d)/g ) | |
895 | { | |
896 | print "Found $1\n"; | |
897 | } | |
197aec24 | 898 | |
ee891a00 | 899 | After the match fails at the letter C<a>, perl resets C<pos()> |
49d635f9 RGS |
900 | and the next match on the same string starts at the beginning. |
901 | ||
902 | $_ = "1122a44"; | |
903 | while( m/\G(\d\d)/g ) | |
904 | { | |
905 | print "Found $1\n"; | |
906 | } | |
907 | ||
908 | print "Found $1 after while" if m/(\d\d)/g; # finds "11" | |
909 | ||
ee891a00 RGS |
910 | You can disable C<pos()> resets on fail with the C<c> flag, documented |
911 | in L<perlop> and L<perlreref>. Subsequent matches start where the last | |
912 | successful match ended (the value of C<pos()>) even if a match on the | |
913 | same string has failed in the meantime. In this case, the match after | |
914 | the C<while()> loop starts at the C<a> (where the last match stopped), | |
915 | and since it does not use any anchor it can skip over the C<a> to find | |
916 | C<44>. | |
49d635f9 RGS |
917 | |
918 | $_ = "1122a44"; | |
919 | while( m/\G(\d\d)/gc ) | |
920 | { | |
921 | print "Found $1\n"; | |
922 | } | |
923 | ||
924 | print "Found $1 after while" if m/(\d\d)/g; # finds "44" | |
925 | ||
926 | Typically you use the C<\G> anchor with the C<c> flag | |
927 | when you want to try a different match if one fails, | |
928 | such as in a tokenizer. Jeffrey Friedl offers this example | |
929 | which works in 5.004 or later. | |
68dc0745 | 930 | |
ac9dac7f RGS |
931 | while (<>) { |
932 | chomp; | |
933 | PARSER: { | |
934 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; | |
935 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; | |
936 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; | |
937 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; | |
938 | } | |
939 | } | |
68dc0745 | 940 | |
ee891a00 | 941 | For each line, the C<PARSER> loop first tries to match a series |
49d635f9 RGS |
942 | of digits followed by a word boundary. This match has to |
943 | start at the place the last match left off (or the beginning | |
197aec24 | 944 | of the string on the first match). Since C<m/ \G( \d+\b |
49d635f9 RGS |
945 | )/gcx> uses the C<c> flag, if the string does not match that |
946 | regular expression, perl does not reset pos() and the next | |
947 | match starts at the same position to try a different | |
948 | pattern. | |
68dc0745 | 949 | |
d92eb7b0 | 950 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
d74e8afc | 951 | X<DFA> X<NFA> X<POSIX> |
68dc0745 | 952 | |
953 | While it's true that Perl's regular expressions resemble the DFAs | |
954 | (deterministic finite automata) of the egrep(1) program, they are in | |
46fc3d4c | 955 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 | 956 | backtracking and backreferencing. And they aren't POSIX-style either, |
957 | because those guarantee worst-case behavior for all cases. (It seems | |
958 | that some people prefer guarantees of consistency, even when what's | |
959 | guaranteed is slowness.) See the book "Mastering Regular Expressions" | |
960 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever | |
961 | hope to know on these matters (a full citation appears in | |
962 | L<perlfaq2>). | |
963 | ||
788611b6 | 964 | =head2 What's wrong with using grep in a void context? |
d74e8afc | 965 | X<grep> |
68dc0745 | 966 | |
788611b6 A |
967 | The problem is that grep builds a return list, regardless of the context. |
968 | This means you're making Perl go to the trouble of building a list that | |
969 | you then just throw away. If the list is large, you waste both time and space. | |
970 | If your intent is to iterate over the list, then use a for loop for this | |
f05bbc40 | 971 | purpose. |
68dc0745 | 972 | |
788611b6 A |
973 | In perls older than 5.8.1, map suffers from this problem as well. |
974 | But since 5.8.1, this has been fixed, and map is context aware - in void | |
975 | context, no lists are constructed. | |
976 | ||
54310121 | 977 | =head2 How can I match strings with multibyte characters? |
d74e8afc | 978 | X<regex, and multibyte characters> X<regexp, and multibyte characters> |
ac9dac7f | 979 | X<regular expression, and multibyte characters> X<martian> X<encoding, Martian> |
68dc0745 | 980 | |
d9d154f2 JH |
981 | Starting from Perl 5.6 Perl has had some level of multibyte character |
982 | support. Perl 5.8 or later is recommended. Supported multibyte | |
fe854a6f | 983 | character repertoires include Unicode, and legacy encodings |
d9d154f2 JH |
984 | through the Encode module. See L<perluniintro>, L<perlunicode>, |
985 | and L<Encode>. | |
986 | ||
987 | If you are stuck with older Perls, you can do Unicode with the | |
988 | C<Unicode::String> module, and character conversions using the | |
989 | C<Unicode::Map8> and C<Unicode::Map> modules. If you are using | |
990 | Japanese encodings, you might try using the jperl 5.005_03. | |
991 | ||
992 | Finally, the following set of approaches was offered by Jeffrey | |
993 | Friedl, whose article in issue #5 of The Perl Journal talks about | |
994 | this very matter. | |
68dc0745 | 995 | |
fc36a67e | 996 | Let's suppose you have some weird Martian encoding where pairs of |
997 | ASCII uppercase letters encode single Martian letters (i.e. the two | |
998 | bytes "CV" make a single Martian letter, as do the two bytes "SG", | |
999 | "VS", "XX", etc.). Other bytes represent single characters, just like | |
1000 | ASCII. | |
68dc0745 | 1001 | |
fc36a67e | 1002 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
1003 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. | |
68dc0745 | 1004 | |
1005 | Now, say you want to search for the single character C</GX/>. Perl | |
fc36a67e | 1006 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
1007 | am CVSGXX!" string, even though that character isn't there: it just | |
1008 | looks like it is because "SG" is next to "XX", but there's no real | |
1009 | "GX". This is a big problem. | |
68dc0745 | 1010 | |
1011 | Here are a few ways, all painful, to deal with it: | |
1012 | ||
ac9dac7f RGS |
1013 | # Make sure adjacent "martian" bytes are no longer adjacent. |
1014 | $martian =~ s/([A-Z][A-Z])/ $1 /g; | |
1015 | ||
1016 | print "found GX!\n" if $martian =~ /GX/; | |
68dc0745 | 1017 | |
1018 | Or like this: | |
1019 | ||
ac9dac7f RGS |
1020 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
1021 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; | |
1022 | # | |
1023 | foreach $char (@chars) { | |
1024 | print "found GX!\n", last if $char eq 'GX'; | |
1025 | } | |
68dc0745 | 1026 | |
1027 | Or like this: | |
1028 | ||
ac9dac7f RGS |
1029 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded |
1030 | print "found GX!\n", last if $1 eq 'GX'; | |
1031 | } | |
68dc0745 | 1032 | |
49d635f9 | 1033 | Here's another, slightly less painful, way to do it from Benjamin |
c98c5709 | 1034 | Goldberg, who uses a zero-width negative look-behind assertion. |
49d635f9 | 1035 | |
c98c5709 | 1036 | print "found GX!\n" if $martian =~ m/ |
ac9dac7f RGS |
1037 | (?<![A-Z]) |
1038 | (?:[A-Z][A-Z])*? | |
1039 | GX | |
c98c5709 | 1040 | /x; |
197aec24 | 1041 | |
49d635f9 | 1042 | This succeeds if the "martian" character GX is in the string, and fails |
c98c5709 RGS |
1043 | otherwise. If you don't like using (?<!), a zero-width negative |
1044 | look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]). | |
49d635f9 RGS |
1045 | |
1046 | It does have the drawback of putting the wrong thing in $-[0] and $+[0], | |
1047 | but this usually can be worked around. | |
68dc0745 | 1048 | |
ac9dac7f RGS |
1049 | =head2 How do I match a regular expression that's in a variable? |
1050 | X<regex, in variable> X<eval> X<regex> X<quotemeta> X<\Q, regex> | |
1051 | X<\E, regex>, X<qr//> | |
65acb1b1 | 1052 | |
ac9dac7f | 1053 | (contributed by brian d foy) |
65acb1b1 | 1054 | |
ac9dac7f RGS |
1055 | We don't have to hard-code patterns into the match operator (or |
1056 | anything else that works with regular expressions). We can put the | |
1057 | pattern in a variable for later use. | |
65acb1b1 | 1058 | |
ac9dac7f RGS |
1059 | The match operator is a double quote context, so you can interpolate |
1060 | your variable just like a double quoted string. In this case, you | |
1061 | read the regular expression as user input and store it in C<$regex>. | |
1062 | Once you have the pattern in C<$regex>, you use that variable in the | |
1063 | match operator. | |
65acb1b1 | 1064 | |
ac9dac7f | 1065 | chomp( my $regex = <STDIN> ); |
65acb1b1 | 1066 | |
ac9dac7f | 1067 | if( $string =~ m/$regex/ ) { ... } |
65acb1b1 | 1068 | |
ac9dac7f RGS |
1069 | Any regular expression special characters in C<$regex> are still |
1070 | special, and the pattern still has to be valid or Perl will complain. | |
1071 | For instance, in this pattern there is an unpaired parenthesis. | |
65acb1b1 | 1072 | |
ac9dac7f RGS |
1073 | my $regex = "Unmatched ( paren"; |
1074 | ||
1075 | "Two parens to bind them all" =~ m/$regex/; | |
1076 | ||
1077 | When Perl compiles the regular expression, it treats the parenthesis | |
1078 | as the start of a memory match. When it doesn't find the closing | |
1079 | parenthesis, it complains: | |
1080 | ||
1081 | Unmatched ( in regex; marked by <-- HERE in m/Unmatched ( <-- HERE paren/ at script line 3. | |
1082 | ||
1083 | You can get around this in several ways depending on our situation. | |
1084 | First, if you don't want any of the characters in the string to be | |
1085 | special, you can escape them with C<quotemeta> before you use the string. | |
1086 | ||
1087 | chomp( my $regex = <STDIN> ); | |
1088 | $regex = quotemeta( $regex ); | |
1089 | ||
1090 | if( $string =~ m/$regex/ ) { ... } | |
1091 | ||
1092 | You can also do this directly in the match operator using the C<\Q> | |
1093 | and C<\E> sequences. The C<\Q> tells Perl where to start escaping | |
1094 | special characters, and the C<\E> tells it where to stop (see L<perlop> | |
1095 | for more details). | |
1096 | ||
1097 | chomp( my $regex = <STDIN> ); | |
1098 | ||
1099 | if( $string =~ m/\Q$regex\E/ ) { ... } | |
1100 | ||
1101 | Alternately, you can use C<qr//>, the regular expression quote operator (see | |
1102 | L<perlop> for more details). It quotes and perhaps compiles the pattern, | |
1103 | and you can apply regular expression flags to the pattern. | |
1104 | ||
1105 | chomp( my $input = <STDIN> ); | |
1106 | ||
1107 | my $regex = qr/$input/is; | |
1108 | ||
1109 | $string =~ m/$regex/ # same as m/$input/is; | |
1110 | ||
1111 | You might also want to trap any errors by wrapping an C<eval> block | |
1112 | around the whole thing. | |
1113 | ||
1114 | chomp( my $input = <STDIN> ); | |
1115 | ||
1116 | eval { | |
1117 | if( $string =~ m/\Q$input\E/ ) { ... } | |
1118 | }; | |
1119 | warn $@ if $@; | |
1120 | ||
1121 | Or... | |
1122 | ||
1123 | my $regex = eval { qr/$input/is }; | |
1124 | if( defined $regex ) { | |
1125 | $string =~ m/$regex/; | |
1126 | } | |
1127 | else { | |
1128 | warn $@; | |
1129 | } | |
65acb1b1 | 1130 | |
68dc0745 | 1131 | =head1 AUTHOR AND COPYRIGHT |
1132 | ||
8d2e243f | 1133 | Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and |
7678cced | 1134 | other authors as noted. All rights reserved. |
5a964f20 | 1135 | |
5a7beb56 JH |
1136 | This documentation is free; you can redistribute it and/or modify it |
1137 | under the same terms as Perl itself. | |
5a964f20 TC |
1138 | |
1139 | Irrespective of its distribution, all code examples in this file | |
1140 | are hereby placed into the public domain. You are permitted and | |
1141 | encouraged to use this code in your own programs for fun | |
1142 | or for profit as you see fit. A simple comment in the code giving | |
1143 | credit would be courteous but is not required. |