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1 | =head1 NAME |
2 | |
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3 | perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $) |
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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 |
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11 | this document (in L<perlfaq9>: ``How do I decode or create those %-encodings |
12 | on the web'' and L<perfaq4>: ``How do I determine whether a scalar is |
13 | a number/whole/integer/float'', to be precise). |
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14 | |
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15 | =head2 How can I hope to use regular expressions without creating illegible and unmaintainable code? |
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16 | |
17 | Three techniques can make regular expressions maintainable and |
18 | understandable. |
19 | |
20 | =over 4 |
21 | |
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22 | =item Comments Outside the Regex |
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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 |
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29 | s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg; |
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30 | |
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31 | =item Comments Inside the Regex |
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32 | |
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33 | The C</x> modifier causes whitespace to be ignored in a regex pattern |
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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 | |
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73 | Either you don't have more than one line in the string you're looking at |
74 | (probably), or else you aren't using the correct modifier(s) on your |
75 | pattern (possibly). |
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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 ( <> ) { |
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99 | while ( /\b([\w'-]+)(\s+\1)+\b/gi ) { # word starts alpha |
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100 | print "Duplicate $1 at paragraph $.\n"; |
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101 | } |
102 | } |
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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 ( <> ) { |
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118 | while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries |
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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 | |
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132 | perl -0777 -ne 'print "$1\n" while /START(.*?)END/gs' file1 file2 ... |
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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 | |
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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 $. |
146 | } |
147 | |
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148 | =head2 I put a regular expression into $/ but it didn't work. What's wrong? |
149 | |
150 | $/ must be a string, not a regular expression. Awk has to be better |
151 | for something. :-) |
152 | |
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153 | Actually, you could do this if you don't mind reading the whole file |
154 | into memory: |
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155 | |
156 | undef $/; |
157 | @records = split /your_pattern/, <FH>; |
158 | |
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159 | The Net::Telnet module (available from CPAN) has the capability to |
160 | wait for a pattern in the input stream, or timeout if it doesn't |
161 | appear within a certain time. |
162 | |
163 | ## Create a file with three lines. |
164 | open FH, ">file"; |
165 | print FH "The first line\nThe second line\nThe third line\n"; |
166 | close FH; |
167 | |
168 | ## Get a read/write filehandle to it. |
169 | $fh = new FileHandle "+<file"; |
170 | |
171 | ## Attach it to a "stream" object. |
172 | use Net::Telnet; |
173 | $file = new Net::Telnet (-fhopen => $fh); |
174 | |
175 | ## Search for the second line and print out the third. |
176 | $file->waitfor('/second line\n/'); |
177 | print $file->getline; |
178 | |
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179 | =head2 How do I substitute case insensitively on the LHS while preserving case on the RHS? |
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180 | |
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181 | Here's a lovely Perlish solution by Larry Rosler. It exploits |
182 | properties of bitwise xor on ASCII strings. |
183 | |
184 | $_= "this is a TEsT case"; |
185 | |
186 | $old = 'test'; |
187 | $new = 'success'; |
188 | |
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189 | s{(\Q$old\E)} |
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190 | { uc $new | (uc $1 ^ $1) . |
191 | (uc(substr $1, -1) ^ substr $1, -1) x |
192 | (length($new) - length $1) |
193 | }egi; |
194 | |
195 | print; |
196 | |
197 | And here it is as a subroutine, modelled after the above: |
198 | |
199 | sub preserve_case($$) { |
200 | my ($old, $new) = @_; |
201 | my $mask = uc $old ^ $old; |
202 | |
203 | uc $new | $mask . |
204 | substr($mask, -1) x (length($new) - length($old)) |
205 | } |
206 | |
207 | $a = "this is a TEsT case"; |
208 | $a =~ s/(test)/preserve_case($1, "success")/egi; |
209 | print "$a\n"; |
210 | |
211 | This prints: |
212 | |
213 | this is a SUcCESS case |
214 | |
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215 | As an alternative, to keep the case of the replacement word if it is |
216 | longer than the original, you can use this code, by Jeff Pinyan: |
217 | |
218 | sub preserve_case { |
219 | my ($from, $to) = @_; |
220 | my ($lf, $lt) = map length, @_; |
221 | |
222 | if ($lt < $lf) { $from = substr $from, 0, $lt } |
223 | else { $from .= substr $to, $lf } |
224 | |
225 | return uc $to | ($from ^ uc $from); |
226 | } |
227 | |
228 | This changes the sentence to "this is a SUcCess case." |
229 | |
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230 | Just to show that C programmers can write C in any programming language, |
231 | if you prefer a more C-like solution, the following script makes the |
232 | substitution have the same case, letter by letter, as the original. |
233 | (It also happens to run about 240% slower than the Perlish solution runs.) |
234 | If the substitution has more characters than the string being substituted, |
235 | the case of the last character is used for the rest of the substitution. |
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236 | |
237 | # Original by Nathan Torkington, massaged by Jeffrey Friedl |
238 | # |
239 | sub preserve_case($$) |
240 | { |
241 | my ($old, $new) = @_; |
242 | my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc |
243 | my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new)); |
244 | my ($len) = $oldlen < $newlen ? $oldlen : $newlen; |
245 | |
246 | for ($i = 0; $i < $len; $i++) { |
247 | if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) { |
248 | $state = 0; |
249 | } elsif (lc $c eq $c) { |
250 | substr($new, $i, 1) = lc(substr($new, $i, 1)); |
251 | $state = 1; |
252 | } else { |
253 | substr($new, $i, 1) = uc(substr($new, $i, 1)); |
254 | $state = 2; |
255 | } |
256 | } |
257 | # finish up with any remaining new (for when new is longer than old) |
258 | if ($newlen > $oldlen) { |
259 | if ($state == 1) { |
260 | substr($new, $oldlen) = lc(substr($new, $oldlen)); |
261 | } elsif ($state == 2) { |
262 | substr($new, $oldlen) = uc(substr($new, $oldlen)); |
263 | } |
264 | } |
265 | return $new; |
266 | } |
267 | |
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268 | =head2 How can I make C<\w> match national character sets? |
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269 | |
270 | See L<perllocale>. |
271 | |
272 | =head2 How can I match a locale-smart version of C</[a-zA-Z]/>? |
273 | |
274 | One alphabetic character would be C</[^\W\d_]/>, no matter what locale |
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275 | you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't |
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276 | consider an underscore a letter). |
277 | |
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278 | =head2 How can I quote a variable to use in a regex? |
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279 | |
280 | The Perl parser will expand $variable and @variable references in |
281 | regular expressions unless the delimiter is a single quote. Remember, |
282 | too, that the right-hand side of a C<s///> substitution is considered |
283 | a double-quoted string (see L<perlop> for more details). Remember |
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284 | also that any regex special characters will be acted on unless you |
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285 | precede the substitution with \Q. Here's an example: |
286 | |
287 | $string = "to die?"; |
288 | $lhs = "die?"; |
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289 | $rhs = "sleep, no more"; |
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290 | |
291 | $string =~ s/\Q$lhs/$rhs/; |
292 | # $string is now "to sleep no more" |
293 | |
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294 | Without the \Q, the regex would also spuriously match "di". |
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295 | |
296 | =head2 What is C</o> really for? |
297 | |
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298 | Using a variable in a regular expression match forces a re-evaluation |
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299 | (and perhaps recompilation) each time the regular expression is |
300 | encountered. The C</o> modifier locks in the regex the first time |
301 | it's used. This always happens in a constant regular expression, and |
302 | in fact, the pattern was compiled into the internal format at the same |
303 | time your entire program was. |
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304 | |
305 | Use of C</o> is irrelevant unless variable interpolation is used in |
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306 | the pattern, and if so, the regex engine will neither know nor care |
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307 | whether the variables change after the pattern is evaluated the I<very |
308 | first> time. |
309 | |
310 | C</o> is often used to gain an extra measure of efficiency by not |
311 | performing subsequent evaluations when you know it won't matter |
312 | (because you know the variables won't change), or more rarely, when |
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313 | you don't want the regex to notice if they do. |
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314 | |
315 | For example, here's a "paragrep" program: |
316 | |
317 | $/ = ''; # paragraph mode |
318 | $pat = shift; |
319 | while (<>) { |
320 | print if /$pat/o; |
321 | } |
322 | |
323 | =head2 How do I use a regular expression to strip C style comments from a file? |
324 | |
325 | While this actually can be done, it's much harder than you'd think. |
326 | For example, this one-liner |
327 | |
328 | perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c |
329 | |
330 | will work in many but not all cases. You see, it's too simple-minded for |
331 | certain kinds of C programs, in particular, those with what appear to be |
332 | comments in quoted strings. For that, you'd need something like this, |
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333 | created by Jeffrey Friedl and later modified by Fred Curtis. |
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334 | |
335 | $/ = undef; |
336 | $_ = <>; |
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337 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs |
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338 | print; |
339 | |
340 | This could, of course, be more legibly written with the C</x> modifier, adding |
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341 | whitespace and comments. Here it is expanded, courtesy of Fred Curtis. |
342 | |
343 | s{ |
344 | /\* ## Start of /* ... */ comment |
345 | [^*]*\*+ ## Non-* followed by 1-or-more *'s |
346 | ( |
347 | [^/*][^*]*\*+ |
348 | )* ## 0-or-more things which don't start with / |
349 | ## but do end with '*' |
350 | / ## End of /* ... */ comment |
351 | |
352 | | ## OR various things which aren't comments: |
353 | |
354 | ( |
355 | " ## Start of " ... " string |
356 | ( |
357 | \\. ## Escaped char |
358 | | ## OR |
359 | [^"\\] ## Non "\ |
360 | )* |
361 | " ## End of " ... " string |
362 | |
363 | | ## OR |
364 | |
365 | ' ## Start of ' ... ' string |
366 | ( |
367 | \\. ## Escaped char |
368 | | ## OR |
369 | [^'\\] ## Non '\ |
370 | )* |
371 | ' ## End of ' ... ' string |
372 | |
373 | | ## OR |
374 | |
375 | . ## Anything other char |
376 | [^/"'\\]* ## Chars which doesn't start a comment, string or escape |
377 | ) |
378 | }{$2}gxs; |
379 | |
380 | A slight modification also removes C++ comments: |
381 | |
382 | s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs; |
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383 | |
384 | =head2 Can I use Perl regular expressions to match balanced text? |
385 | |
386 | Although Perl regular expressions are more powerful than "mathematical" |
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387 | regular expressions because they feature conveniences like backreferences |
388 | (C<\1> and its ilk), they still aren't powerful enough--with |
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389 | the possible exception of bizarre and experimental features in the |
390 | development-track releases of Perl. You still need to use non-regex |
391 | techniques to parse balanced text, such as the text enclosed between |
392 | matching parentheses or braces, for example. |
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393 | |
394 | An elaborate subroutine (for 7-bit ASCII only) to pull out balanced |
395 | and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>, |
396 | or C<(> and C<)> can be found in |
397 | http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz . |
398 | |
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399 | The C::Scan module from CPAN contains such subs for internal use, |
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400 | but they are undocumented. |
401 | |
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402 | =head2 What does it mean that regexes are greedy? How can I get around it? |
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403 | |
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404 | Most people mean that greedy regexes match as much as they can. |
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405 | Technically speaking, it's actually the quantifiers (C<?>, C<*>, C<+>, |
406 | C<{}>) that are greedy rather than the whole pattern; Perl prefers local |
407 | greed and immediate gratification to overall greed. To get non-greedy |
408 | versions of the same quantifiers, use (C<??>, C<*?>, C<+?>, C<{}?>). |
409 | |
410 | An example: |
411 | |
412 | $s1 = $s2 = "I am very very cold"; |
413 | $s1 =~ s/ve.*y //; # I am cold |
414 | $s2 =~ s/ve.*?y //; # I am very cold |
415 | |
416 | Notice how the second substitution stopped matching as soon as it |
417 | encountered "y ". The C<*?> quantifier effectively tells the regular |
418 | expression engine to find a match as quickly as possible and pass |
419 | control on to whatever is next in line, like you would if you were |
420 | playing hot potato. |
421 | |
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422 | =head2 How do I process each word on each line? |
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423 | |
424 | Use the split function: |
425 | |
426 | while (<>) { |
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427 | foreach $word ( split ) { |
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428 | # do something with $word here |
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429 | } |
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430 | } |
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431 | |
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432 | Note that this isn't really a word in the English sense; it's just |
433 | chunks of consecutive non-whitespace characters. |
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434 | |
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435 | To work with only alphanumeric sequences (including underscores), you |
436 | might consider |
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437 | |
438 | while (<>) { |
439 | foreach $word (m/(\w+)/g) { |
440 | # do something with $word here |
441 | } |
442 | } |
443 | |
444 | =head2 How can I print out a word-frequency or line-frequency summary? |
445 | |
446 | To do this, you have to parse out each word in the input stream. We'll |
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447 | pretend that by word you mean chunk of alphabetics, hyphens, or |
448 | apostrophes, rather than the non-whitespace chunk idea of a word given |
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449 | in the previous question: |
450 | |
451 | while (<>) { |
452 | while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'" |
453 | $seen{$1}++; |
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454 | } |
455 | } |
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456 | while ( ($word, $count) = each %seen ) { |
457 | print "$count $word\n"; |
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458 | } |
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459 | |
460 | If you wanted to do the same thing for lines, you wouldn't need a |
461 | regular expression: |
462 | |
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463 | while (<>) { |
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464 | $seen{$_}++; |
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465 | } |
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466 | while ( ($line, $count) = each %seen ) { |
467 | print "$count $line"; |
468 | } |
469 | |
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470 | If you want these output in a sorted order, see L<perlfaq4>: ``How do I |
471 | sort a hash (optionally by value instead of key)?''. |
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472 | |
473 | =head2 How can I do approximate matching? |
474 | |
475 | See the module String::Approx available from CPAN. |
476 | |
477 | =head2 How do I efficiently match many regular expressions at once? |
478 | |
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479 | The following is extremely inefficient: |
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480 | |
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481 | # slow but obvious way |
482 | @popstates = qw(CO ON MI WI MN); |
483 | while (defined($line = <>)) { |
484 | for $state (@popstates) { |
485 | if ($line =~ /\b$state\b/i) { |
486 | print $line; |
487 | last; |
488 | } |
489 | } |
490 | } |
491 | |
492 | That's because Perl has to recompile all those patterns for each of |
493 | the lines of the file. As of the 5.005 release, there's a much better |
494 | approach, one which makes use of the new C<qr//> operator: |
495 | |
496 | # use spiffy new qr// operator, with /i flag even |
497 | use 5.005; |
498 | @popstates = qw(CO ON MI WI MN); |
499 | @poppats = map { qr/\b$_\b/i } @popstates; |
500 | while (defined($line = <>)) { |
501 | for $patobj (@poppats) { |
502 | print $line if $line =~ /$patobj/; |
503 | } |
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504 | } |
505 | |
506 | =head2 Why don't word-boundary searches with C<\b> work for me? |
507 | |
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508 | Two common misconceptions are that C<\b> is a synonym for C<\s+> and |
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509 | that it's the edge between whitespace characters and non-whitespace |
510 | characters. Neither is correct. C<\b> is the place between a C<\w> |
511 | character and a C<\W> character (that is, C<\b> is the edge of a |
512 | "word"). It's a zero-width assertion, just like C<^>, C<$>, and all |
513 | the other anchors, so it doesn't consume any characters. L<perlre> |
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514 | describes the behavior of all the regex metacharacters. |
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515 | |
516 | Here are examples of the incorrect application of C<\b>, with fixes: |
517 | |
518 | "two words" =~ /(\w+)\b(\w+)/; # WRONG |
519 | "two words" =~ /(\w+)\s+(\w+)/; # right |
520 | |
521 | " =matchless= text" =~ /\b=(\w+)=\b/; # WRONG |
522 | " =matchless= text" =~ /=(\w+)=/; # right |
523 | |
524 | Although they may not do what you thought they did, C<\b> and C<\B> |
525 | can still be quite useful. For an example of the correct use of |
526 | C<\b>, see the example of matching duplicate words over multiple |
527 | lines. |
528 | |
529 | An example of using C<\B> is the pattern C<\Bis\B>. This will find |
530 | occurrences of "is" on the insides of words only, as in "thistle", but |
531 | not "this" or "island". |
532 | |
533 | =head2 Why does using $&, $`, or $' slow my program down? |
534 | |
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535 | Once Perl sees that you need one of these variables anywhere in |
536 | the program, it provides them on each and every pattern match. |
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537 | The same mechanism that handles these provides for the use of $1, $2, |
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538 | etc., so you pay the same price for each regex that contains capturing |
a6dd486b |
539 | parentheses. If you never use $&, etc., in your script, then regexes |
65acb1b1 |
540 | I<without> capturing parentheses won't be penalized. So avoid $&, $', |
541 | and $` if you can, but if you can't, once you've used them at all, use |
542 | them at will because you've already paid the price. Remember that some |
543 | algorithms really appreciate them. As of the 5.005 release. the $& |
544 | variable is no longer "expensive" the way the other two are. |
68dc0745 |
545 | |
546 | =head2 What good is C<\G> in a regular expression? |
547 | |
5d43e42d |
548 | The notation C<\G> is used in a match or substitution in conjunction with |
549 | the C</g> modifier to anchor the regular expression to the point just past |
550 | where the last match occurred, i.e. the pos() point. A failed match resets |
551 | the position of C<\G> unless the C</c> modifier is in effect. C<\G> can be |
552 | used in a match without the C</g> modifier; it acts the same (i.e. still |
553 | anchors at the pos() point) but of course only matches once and does not |
554 | update pos(), as non-C</g> expressions never do. C<\G> in an expression |
555 | applied to a target string that has never been matched against a C</g> |
556 | expression before or has had its pos() reset is functionally equivalent to |
557 | C<\A>, which matches at the beginning of the string. |
68dc0745 |
558 | |
559 | For example, suppose you had a line of text quoted in standard mail |
c47ff5f1 |
560 | and Usenet notation, (that is, with leading C<< > >> characters), and |
561 | you want change each leading C<< > >> into a corresponding C<:>. You |
68dc0745 |
562 | could do so in this way: |
563 | |
564 | s/^(>+)/':' x length($1)/gem; |
565 | |
566 | Or, using C<\G>, the much simpler (and faster): |
567 | |
568 | s/\G>/:/g; |
569 | |
570 | A more sophisticated use might involve a tokenizer. The following |
571 | lex-like example is courtesy of Jeffrey Friedl. It did not work in |
c90c0ff4 |
572 | 5.003 due to bugs in that release, but does work in 5.004 or better. |
573 | (Note the use of C</c>, which prevents a failed match with C</g> from |
574 | resetting the search position back to the beginning of the string.) |
68dc0745 |
575 | |
576 | while (<>) { |
577 | chomp; |
578 | PARSER: { |
c90c0ff4 |
579 | m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; }; |
580 | m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; }; |
581 | m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; }; |
582 | m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; }; |
68dc0745 |
583 | } |
584 | } |
585 | |
586 | Of course, that could have been written as |
587 | |
588 | while (<>) { |
589 | chomp; |
590 | PARSER: { |
c90c0ff4 |
591 | if ( /\G( \d+\b )/gcx { |
68dc0745 |
592 | print "number: $1\n"; |
593 | redo PARSER; |
594 | } |
c90c0ff4 |
595 | if ( /\G( \w+ )/gcx { |
68dc0745 |
596 | print "word: $1\n"; |
597 | redo PARSER; |
598 | } |
c90c0ff4 |
599 | if ( /\G( \s+ )/gcx { |
68dc0745 |
600 | print "space: $1\n"; |
601 | redo PARSER; |
602 | } |
c90c0ff4 |
603 | if ( /\G( [^\w\d]+ )/gcx { |
68dc0745 |
604 | print "other: $1\n"; |
605 | redo PARSER; |
606 | } |
607 | } |
608 | } |
609 | |
a6dd486b |
610 | but then you lose the vertical alignment of the regular expressions. |
68dc0745 |
611 | |
d92eb7b0 |
612 | =head2 Are Perl regexes DFAs or NFAs? Are they POSIX compliant? |
68dc0745 |
613 | |
614 | While it's true that Perl's regular expressions resemble the DFAs |
615 | (deterministic finite automata) of the egrep(1) program, they are in |
46fc3d4c |
616 | fact implemented as NFAs (non-deterministic finite automata) to allow |
68dc0745 |
617 | backtracking and backreferencing. And they aren't POSIX-style either, |
618 | because those guarantee worst-case behavior for all cases. (It seems |
619 | that some people prefer guarantees of consistency, even when what's |
620 | guaranteed is slowness.) See the book "Mastering Regular Expressions" |
621 | (from O'Reilly) by Jeffrey Friedl for all the details you could ever |
622 | hope to know on these matters (a full citation appears in |
623 | L<perlfaq2>). |
624 | |
625 | =head2 What's wrong with using grep or map in a void context? |
626 | |
92c2ed05 |
627 | Both grep and map build a return list, regardless of their context. |
628 | This means you're making Perl go to the trouble of building up a |
629 | return list that you then just ignore. That's no way to treat a |
630 | programming language, you insensitive scoundrel! |
68dc0745 |
631 | |
54310121 |
632 | =head2 How can I match strings with multibyte characters? |
68dc0745 |
633 | |
634 | This is hard, and there's no good way. Perl does not directly support |
635 | wide characters. It pretends that a byte and a character are |
636 | synonymous. The following set of approaches was offered by Jeffrey |
637 | Friedl, whose article in issue #5 of The Perl Journal talks about this |
638 | very matter. |
639 | |
fc36a67e |
640 | Let's suppose you have some weird Martian encoding where pairs of |
641 | ASCII uppercase letters encode single Martian letters (i.e. the two |
642 | bytes "CV" make a single Martian letter, as do the two bytes "SG", |
643 | "VS", "XX", etc.). Other bytes represent single characters, just like |
644 | ASCII. |
68dc0745 |
645 | |
fc36a67e |
646 | So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the |
647 | nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'. |
68dc0745 |
648 | |
649 | Now, say you want to search for the single character C</GX/>. Perl |
fc36a67e |
650 | doesn't know about Martian, so it'll find the two bytes "GX" in the "I |
651 | am CVSGXX!" string, even though that character isn't there: it just |
652 | looks like it is because "SG" is next to "XX", but there's no real |
653 | "GX". This is a big problem. |
68dc0745 |
654 | |
655 | Here are a few ways, all painful, to deal with it: |
656 | |
3fe9a6f1 |
657 | $martian =~ s/([A-Z][A-Z])/ $1 /g; # Make sure adjacent ``martian'' bytes |
68dc0745 |
658 | # are no longer adjacent. |
659 | print "found GX!\n" if $martian =~ /GX/; |
660 | |
661 | Or like this: |
662 | |
663 | @chars = $martian =~ m/([A-Z][A-Z]|[^A-Z])/g; |
664 | # above is conceptually similar to: @chars = $text =~ m/(.)/g; |
665 | # |
666 | foreach $char (@chars) { |
667 | print "found GX!\n", last if $char eq 'GX'; |
668 | } |
669 | |
670 | Or like this: |
671 | |
672 | while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded |
54310121 |
673 | print "found GX!\n", last if $1 eq 'GX'; |
68dc0745 |
674 | } |
675 | |
676 | Or like this: |
677 | |
65acb1b1 |
678 | die "sorry, Perl doesn't (yet) have Martian support )-:\n"; |
68dc0745 |
679 | |
46fc3d4c |
680 | There are many double- (and multi-) byte encodings commonly used these |
68dc0745 |
681 | days. Some versions of these have 1-, 2-, 3-, and 4-byte characters, |
682 | all mixed. |
683 | |
65acb1b1 |
684 | =head2 How do I match a pattern that is supplied by the user? |
685 | |
686 | Well, if it's really a pattern, then just use |
687 | |
688 | chomp($pattern = <STDIN>); |
689 | if ($line =~ /$pattern/) { } |
690 | |
a6dd486b |
691 | Alternatively, since you have no guarantee that your user entered |
65acb1b1 |
692 | a valid regular expression, trap the exception this way: |
693 | |
694 | if (eval { $line =~ /$pattern/ }) { } |
695 | |
a6dd486b |
696 | If all you really want to search for a string, not a pattern, |
65acb1b1 |
697 | then you should either use the index() function, which is made for |
698 | string searching, or if you can't be disabused of using a pattern |
699 | match on a non-pattern, then be sure to use C<\Q>...C<\E>, documented |
700 | in L<perlre>. |
701 | |
702 | $pattern = <STDIN>; |
703 | |
704 | open (FILE, $input) or die "Couldn't open input $input: $!; aborting"; |
705 | while (<FILE>) { |
706 | print if /\Q$pattern\E/; |
707 | } |
708 | close FILE; |
709 | |
68dc0745 |
710 | =head1 AUTHOR AND COPYRIGHT |
711 | |
65acb1b1 |
712 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
713 | All rights reserved. |
714 | |
715 | When included as part of the Standard Version of Perl, or as part of |
716 | its complete documentation whether printed or otherwise, this work |
d92eb7b0 |
717 | may be distributed only under the terms of Perl's Artistic License. |
5a964f20 |
718 | Any distribution of this file or derivatives thereof I<outside> |
719 | of that package require that special arrangements be made with |
720 | copyright holder. |
721 | |
722 | Irrespective of its distribution, all code examples in this file |
723 | are hereby placed into the public domain. You are permitted and |
724 | encouraged to use this code in your own programs for fun |
725 | or for profit as you see fit. A simple comment in the code giving |
726 | credit would be courteous but is not required. |