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1=head1 NAME
2
3perlre - Perl regular expressions
4
5=head1 DESCRIPTION
6
cb1a09d0 7This page describes the syntax of regular expressions in Perl. For a
5f05dabc 8description of how to I<use> regular expressions in matching
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9operations, plus various examples of the same, see C<m//> and C<s///> in
10L<perlop>.
11
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12The matching operations can have various modifiers. The modifiers
13which relate to the interpretation of the regular expression inside
14are listed below. For the modifiers that alter the behaviour of the
15operation, see L<perlop/"m//"> and L<perlop/"s//">.
a0d0e21e 16
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17=over 4
18
19=item i
20
21Do case-insensitive pattern matching.
22
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23If C<use locale> is in effect, the case map is taken from the current
24locale. See L<perllocale>.
25
54310121 26=item m
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27
28Treat string as multiple lines. That is, change "^" and "$" from matching
5f05dabc 29at only the very start or end of the string to the start or end of any
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30line anywhere within the string,
31
54310121 32=item s
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33
34Treat string as single line. That is, change "." to match any character
35whatsoever, even a newline, which it normally would not match.
36
54310121 37=item x
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38
39Extend your pattern's legibility by permitting whitespace and comments.
40
41=back
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42
43These are usually written as "the C</x> modifier", even though the delimiter
44in question might not actually be a slash. In fact, any of these
45modifiers may also be embedded within the regular expression itself using
46the new C<(?...)> construct. See below.
47
4633a7c4 48The C</x> modifier itself needs a little more explanation. It tells
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49the regular expression parser to ignore whitespace that is neither
50backslashed nor within a character class. You can use this to break up
4633a7c4 51your regular expression into (slightly) more readable parts. The C<#>
54310121 52character is also treated as a metacharacter introducing a comment,
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53just as in ordinary Perl code. This also means that if you want real
54whitespace or C<#> characters in the pattern that you'll have to either
55escape them or encode them using octal or hex escapes. Taken together,
56these features go a long way towards making Perl's regular expressions
57more readable. See the C comment deletion code in L<perlop>.
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58
59=head2 Regular Expressions
60
61The patterns used in pattern matching are regular expressions such as
62those supplied in the Version 8 regexp routines. (In fact, the
63routines are derived (distantly) from Henry Spencer's freely
64redistributable reimplementation of the V8 routines.)
65See L<Version 8 Regular Expressions> for details.
66
67In particular the following metacharacters have their standard I<egrep>-ish
68meanings:
69
54310121 70 \ Quote the next metacharacter
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71 ^ Match the beginning of the line
72 . Match any character (except newline)
c07a80fd 73 $ Match the end of the line (or before newline at the end)
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74 | Alternation
75 () Grouping
76 [] Character class
77
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78By default, the "^" character is guaranteed to match at only the
79beginning of the string, the "$" character at only the end (or before the
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80newline at the end) and Perl does certain optimizations with the
81assumption that the string contains only one line. Embedded newlines
82will not be matched by "^" or "$". You may, however, wish to treat a
4a6725af 83string as a multi-line buffer, such that the "^" will match after any
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84newline within the string, and "$" will match before any newline. At the
85cost of a little more overhead, you can do this by using the /m modifier
86on the pattern match operator. (Older programs did this by setting C<$*>,
5f05dabc 87but this practice is now deprecated.)
a0d0e21e 88
4a6725af 89To facilitate multi-line substitutions, the "." character never matches a
55497cff 90newline unless you use the C</s> modifier, which in effect tells Perl to pretend
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91the string is a single line--even if it isn't. The C</s> modifier also
92overrides the setting of C<$*>, in case you have some (badly behaved) older
93code that sets it in another module.
94
95The following standard quantifiers are recognized:
96
97 * Match 0 or more times
98 + Match 1 or more times
99 ? Match 1 or 0 times
100 {n} Match exactly n times
101 {n,} Match at least n times
102 {n,m} Match at least n but not more than m times
103
104(If a curly bracket occurs in any other context, it is treated
105as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+"
25f94b33 106modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited
c07a80fd 107to integral values less than 65536.
a0d0e21e 108
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109By default, a quantified subpattern is "greedy", that is, it will match as
110many times as possible (given a particular starting location) while still
111allowing the rest of the pattern to match. If you want it to match the
112minimum number of times possible, follow the quantifier with a "?". Note
113that the meanings don't change, just the "greediness":
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114
115 *? Match 0 or more times
116 +? Match 1 or more times
117 ?? Match 0 or 1 time
118 {n}? Match exactly n times
119 {n,}? Match at least n times
120 {n,m}? Match at least n but not more than m times
121
5f05dabc 122Because patterns are processed as double quoted strings, the following
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123also work:
124
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125 \t tab (HT, TAB)
126 \n newline (LF, NL)
127 \r return (CR)
128 \f form feed (FF)
129 \a alarm (bell) (BEL)
130 \e escape (think troff) (ESC)
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131 \033 octal char (think of a PDP-11)
132 \x1B hex char
a0d0e21e 133 \c[ control char
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134 \l lowercase next char (think vi)
135 \u uppercase next char (think vi)
136 \L lowercase till \E (think vi)
137 \U uppercase till \E (think vi)
138 \E end case modification (think vi)
201ecf35 139 \Q quote (disable) regexp metacharacters till \E
a0d0e21e 140
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141If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
142and <\U> is taken from the current locale. See L<perllocale>.
143
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144In addition, Perl defines the following:
145
146 \w Match a "word" character (alphanumeric plus "_")
147 \W Match a non-word character
148 \s Match a whitespace character
149 \S Match a non-whitespace character
150 \d Match a digit character
151 \D Match a non-digit character
152
153Note that C<\w> matches a single alphanumeric character, not a whole
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154word. To match a word you'd need to say C<\w+>. If C<use locale> is in
155effect, the list of alphabetic characters generated by C<\w> is taken
156from the current locale. See L<perllocale>. You may use C<\w>, C<\W>,
157C<\s>, C<\S>, C<\d>, and C<\D> within character classes (though not as
158either end of a range).
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159
160Perl defines the following zero-width assertions:
161
162 \b Match a word boundary
163 \B Match a non-(word boundary)
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164 \A Match at only beginning of string
165 \Z Match at only end of string (or before newline at the end)
a99df21c 166 \G Match only where previous m//g left off (works only with /g)
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167
168A word boundary (C<\b>) is defined as a spot between two characters that
68dc0745 169has a C<\w> on one side of it and a C<\W> on the other side of it (in
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170either order), counting the imaginary characters off the beginning and
171end of the string as matching a C<\W>. (Within character classes C<\b>
172represents backspace rather than a word boundary.) The C<\A> and C<\Z> are
173just like "^" and "$" except that they won't match multiple times when the
174C</m> modifier is used, while "^" and "$" will match at every internal line
c07a80fd 175boundary. To match the actual end of the string, not ignoring newline,
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176you can use C<\Z(?!\n)>. The C<\G> assertion can be used to chain global
177matches (using C<m//g>), as described in
e7ea3e70 178L<perlop/"Regexp Quote-Like Operators">.
a99df21c 179
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180It is also useful when writing C<lex>-like scanners, when you have several
181regexps which you want to match against consequent substrings of your
182string, see the previous reference.
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183The actual location where C<\G> will match can also be influenced
184by using C<pos()> as an lvalue. See L<perlfunc/pos>.
a0d0e21e 185
0f36ee90 186When the bracketing construct C<( ... )> is used, \E<lt>digitE<gt> matches the
cb1a09d0 187digit'th substring. Outside of the pattern, always use "$" instead of "\"
0f36ee90 188in front of the digit. (While the \E<lt>digitE<gt> notation can on rare occasion work
cb1a09d0 189outside the current pattern, this should not be relied upon. See the
0f36ee90 190WARNING below.) The scope of $E<lt>digitE<gt> (and C<$`>, C<$&>, and C<$'>)
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191extends to the end of the enclosing BLOCK or eval string, or to the next
192successful pattern match, whichever comes first. If you want to use
5f05dabc 193parentheses to delimit a subpattern (e.g., a set of alternatives) without
84dc3c4d 194saving it as a subpattern, follow the ( with a ?:.
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195
196You may have as many parentheses as you wish. If you have more
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197than 9 substrings, the variables $10, $11, ... refer to the
198corresponding substring. Within the pattern, \10, \11, etc. refer back
5f05dabc 199to substrings if there have been at least that many left parentheses before
c07a80fd 200the backreference. Otherwise (for backward compatibility) \10 is the
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201same as \010, a backspace, and \11 the same as \011, a tab. And so
202on. (\1 through \9 are always backreferences.)
203
204C<$+> returns whatever the last bracket match matched. C<$&> returns the
0f36ee90 205entire matched string. (C<$0> used to return the same thing, but not any
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206more.) C<$`> returns everything before the matched string. C<$'> returns
207everything after the matched string. Examples:
208
209 s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
210
211 if (/Time: (..):(..):(..)/) {
212 $hours = $1;
213 $minutes = $2;
214 $seconds = $3;
215 }
216
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217Once perl sees that you need one of C<$&>, C<$`> or C<$'> anywhere in
218the program, it has to provide them on each and every pattern match.
219This can slow your program down. The same mechanism that handles
220these provides for the use of $1, $2, etc., so you pay the same price
221for each regexp that contains capturing parentheses. But if you never
222use $&, etc., in your script, then regexps I<without> capturing
223parentheses won't be penalized. So avoid $&, $', and $` if you can,
224but if you can't (and some algorithms really appreciate them), once
225you've used them once, use them at will, because you've already paid
226the price.
227
a0d0e21e 228You will note that all backslashed metacharacters in Perl are
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229alphanumeric, such as C<\b>, C<\w>, C<\n>. Unlike some other regular
230expression languages, there are no backslashed symbols that aren't
231alphanumeric. So anything that looks like \\, \(, \), \E<lt>, \E<gt>,
232\{, or \} is always interpreted as a literal character, not a
233metacharacter. This was once used in a common idiom to disable or
234quote the special meanings of regular expression metacharacters in a
235string that you want to use for a pattern. Simply quote all the
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236non-alphanumeric characters:
237
238 $pattern =~ s/(\W)/\\$1/g;
239
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240Now it is much more common to see either the quotemeta() function or
241the \Q escape sequence used to disable the metacharacters special
242meanings like this:
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243
244 /$unquoted\Q$quoted\E$unquoted/
245
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246Perl defines a consistent extension syntax for regular expressions.
247The syntax is a pair of parentheses with a question mark as the first
248thing within the parentheses (this was a syntax error in older
249versions of Perl). The character after the question mark gives the
250function of the extension. Several extensions are already supported:
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251
252=over 10
253
254=item (?#text)
255
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256A comment. The text is ignored. If the C</x> switch is used to enable
257whitespace formatting, a simple C<#> will suffice.
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258
259=item (?:regexp)
260
0f36ee90 261This groups things like "()" but doesn't make backreferences like "()" does. So
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262
263 split(/\b(?:a|b|c)\b/)
264
265is like
266
267 split(/\b(a|b|c)\b/)
268
269but doesn't spit out extra fields.
270
271=item (?=regexp)
272
273A zero-width positive lookahead assertion. For example, C</\w+(?=\t)/>
274matches a word followed by a tab, without including the tab in C<$&>.
275
276=item (?!regexp)
277
278A zero-width negative lookahead assertion. For example C</foo(?!bar)/>
279matches any occurrence of "foo" that isn't followed by "bar". Note
280however that lookahead and lookbehind are NOT the same thing. You cannot
281use this for lookbehind: C</(?!foo)bar/> will not find an occurrence of
282"bar" that is preceded by something which is not "foo". That's because
283the C<(?!foo)> is just saying that the next thing cannot be "foo"--and
284it's not, it's a "bar", so "foobar" will match. You would have to do
0f36ee90 285something like C</(?!foo)...bar/> for that. We say "like" because there's
a0d0e21e 286the case of your "bar" not having three characters before it. You could
c07a80fd 287cover that this way: C</(?:(?!foo)...|^..?)bar/>. Sometimes it's still
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288easier just to say:
289
c07a80fd 290 if (/foo/ && $` =~ /bar$/)
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291
292
293=item (?imsx)
294
295One or more embedded pattern-match modifiers. This is particularly
296useful for patterns that are specified in a table somewhere, some of
297which want to be case sensitive, and some of which don't. The case
5f05dabc 298insensitive ones need to include merely C<(?i)> at the front of the
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299pattern. For example:
300
301 $pattern = "foobar";
c07a80fd 302 if ( /$pattern/i )
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303
304 # more flexible:
305
306 $pattern = "(?i)foobar";
c07a80fd 307 if ( /$pattern/ )
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308
309=back
310
311The specific choice of question mark for this and the new minimal
312matching construct was because 1) question mark is pretty rare in older
313regular expressions, and 2) whenever you see one, you should stop
314and "question" exactly what is going on. That's psychology...
315
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316=head2 Backtracking
317
318A fundamental feature of regular expression matching involves the notion
319called I<backtracking>. which is used (when needed) by all regular
320expression quantifiers, namely C<*>, C<*?>, C<+>, C<+?>, C<{n,m}>, and
321C<{n,m}?>.
322
323For a regular expression to match, the I<entire> regular expression must
324match, not just part of it. So if the beginning of a pattern containing a
325quantifier succeeds in a way that causes later parts in the pattern to
326fail, the matching engine backs up and recalculates the beginning
327part--that's why it's called backtracking.
328
329Here is an example of backtracking: Let's say you want to find the
330word following "foo" in the string "Food is on the foo table.":
331
332 $_ = "Food is on the foo table.";
333 if ( /\b(foo)\s+(\w+)/i ) {
334 print "$2 follows $1.\n";
335 }
336
337When the match runs, the first part of the regular expression (C<\b(foo)>)
338finds a possible match right at the beginning of the string, and loads up
339$1 with "Foo". However, as soon as the matching engine sees that there's
340no whitespace following the "Foo" that it had saved in $1, it realizes its
68dc0745 341mistake and starts over again one character after where it had the
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342tentative match. This time it goes all the way until the next occurrence
343of "foo". The complete regular expression matches this time, and you get
344the expected output of "table follows foo."
345
346Sometimes minimal matching can help a lot. Imagine you'd like to match
347everything between "foo" and "bar". Initially, you write something
348like this:
349
350 $_ = "The food is under the bar in the barn.";
351 if ( /foo(.*)bar/ ) {
352 print "got <$1>\n";
353 }
354
355Which perhaps unexpectedly yields:
356
357 got <d is under the bar in the >
358
359That's because C<.*> was greedy, so you get everything between the
360I<first> "foo" and the I<last> "bar". In this case, it's more effective
361to use minimal matching to make sure you get the text between a "foo"
362and the first "bar" thereafter.
363
364 if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
365 got <d is under the >
366
367Here's another example: let's say you'd like to match a number at the end
368of a string, and you also want to keep the preceding part the match.
369So you write this:
370
371 $_ = "I have 2 numbers: 53147";
372 if ( /(.*)(\d*)/ ) { # Wrong!
373 print "Beginning is <$1>, number is <$2>.\n";
374 }
375
376That won't work at all, because C<.*> was greedy and gobbled up the
377whole string. As C<\d*> can match on an empty string the complete
378regular expression matched successfully.
379
8e1088bc 380 Beginning is <I have 2 numbers: 53147>, number is <>.
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381
382Here are some variants, most of which don't work:
383
384 $_ = "I have 2 numbers: 53147";
385 @pats = qw{
386 (.*)(\d*)
387 (.*)(\d+)
388 (.*?)(\d*)
389 (.*?)(\d+)
390 (.*)(\d+)$
391 (.*?)(\d+)$
392 (.*)\b(\d+)$
393 (.*\D)(\d+)$
394 };
395
396 for $pat (@pats) {
397 printf "%-12s ", $pat;
398 if ( /$pat/ ) {
399 print "<$1> <$2>\n";
400 } else {
401 print "FAIL\n";
402 }
403 }
404
405That will print out:
406
407 (.*)(\d*) <I have 2 numbers: 53147> <>
408 (.*)(\d+) <I have 2 numbers: 5314> <7>
409 (.*?)(\d*) <> <>
410 (.*?)(\d+) <I have > <2>
411 (.*)(\d+)$ <I have 2 numbers: 5314> <7>
412 (.*?)(\d+)$ <I have 2 numbers: > <53147>
413 (.*)\b(\d+)$ <I have 2 numbers: > <53147>
414 (.*\D)(\d+)$ <I have 2 numbers: > <53147>
415
416As you see, this can be a bit tricky. It's important to realize that a
417regular expression is merely a set of assertions that gives a definition
418of success. There may be 0, 1, or several different ways that the
419definition might succeed against a particular string. And if there are
5f05dabc 420multiple ways it might succeed, you need to understand backtracking to know which variety of success you will achieve.
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421
422When using lookahead assertions and negations, this can all get even
54310121 423tricker. Imagine you'd like to find a sequence of non-digits not
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424followed by "123". You might try to write that as
425
426 $_ = "ABC123";
427 if ( /^\D*(?!123)/ ) { # Wrong!
428 print "Yup, no 123 in $_\n";
429 }
430
431But that isn't going to match; at least, not the way you're hoping. It
432claims that there is no 123 in the string. Here's a clearer picture of
433why it that pattern matches, contrary to popular expectations:
434
435 $x = 'ABC123' ;
436 $y = 'ABC445' ;
437
438 print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ;
439 print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ;
440
441 print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ;
442 print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ;
443
444This prints
445
446 2: got ABC
447 3: got AB
448 4: got ABC
449
5f05dabc 450You might have expected test 3 to fail because it seems to a more
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451general purpose version of test 1. The important difference between
452them is that test 3 contains a quantifier (C<\D*>) and so can use
453backtracking, whereas test 1 will not. What's happening is
454that you've asked "Is it true that at the start of $x, following 0 or more
5f05dabc 455non-digits, you have something that's not 123?" If the pattern matcher had
c07a80fd 456let C<\D*> expand to "ABC", this would have caused the whole pattern to
54310121 457fail.
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458The search engine will initially match C<\D*> with "ABC". Then it will
459try to match C<(?!123> with "123" which, of course, fails. But because
460a quantifier (C<\D*>) has been used in the regular expression, the
461search engine can backtrack and retry the match differently
54310121 462in the hope of matching the complete regular expression.
c07a80fd 463
54310121 464Well now,
c07a80fd 465the pattern really, I<really> wants to succeed, so it uses the
5f05dabc 466standard regexp back-off-and-retry and lets C<\D*> expand to just "AB" this
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467time. Now there's indeed something following "AB" that is not
468"123". It's in fact "C123", which suffices.
469
470We can deal with this by using both an assertion and a negation. We'll
471say that the first part in $1 must be followed by a digit, and in fact, it
472must also be followed by something that's not "123". Remember that the
473lookaheads are zero-width expressions--they only look, but don't consume
474any of the string in their match. So rewriting this way produces what
475you'd expect; that is, case 5 will fail, but case 6 succeeds:
476
477 print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ;
478 print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ;
479
480 6: got ABC
481
482In other words, the two zero-width assertions next to each other work like
483they're ANDed together, just as you'd use any builtin assertions: C</^$/>
484matches only if you're at the beginning of the line AND the end of the
485line simultaneously. The deeper underlying truth is that juxtaposition in
486regular expressions always means AND, except when you write an explicit OR
487using the vertical bar. C</ab/> means match "a" AND (then) match "b",
488although the attempted matches are made at different positions because "a"
489is not a zero-width assertion, but a one-width assertion.
490
491One warning: particularly complicated regular expressions can take
492exponential time to solve due to the immense number of possible ways they
493can use backtracking to try match. For example this will take a very long
494time to run
495
496 /((a{0,5}){0,5}){0,5}/
497
498And if you used C<*>'s instead of limiting it to 0 through 5 matches, then
499it would take literally forever--or until you ran out of stack space.
500
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501=head2 Version 8 Regular Expressions
502
503In case you're not familiar with the "regular" Version 8 regexp
504routines, here are the pattern-matching rules not described above.
505
54310121 506Any single character matches itself, unless it is a I<metacharacter>
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507with a special meaning described here or above. You can cause
508characters which normally function as metacharacters to be interpreted
5f05dabc 509literally by prefixing them with a "\" (e.g., "\." matches a ".", not any
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510character; "\\" matches a "\"). A series of characters matches that
511series of characters in the target string, so the pattern C<blurfl>
512would match "blurfl" in the target string.
513
514You can specify a character class, by enclosing a list of characters
515in C<[]>, which will match any one of the characters in the list. If the
516first character after the "[" is "^", the class matches any character not
517in the list. Within a list, the "-" character is used to specify a
518range, so that C<a-z> represents all the characters between "a" and "z",
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519inclusive. If you want "-" itself to be a member of a class, put it
520at the start or end of the list, or escape it with a backslash. (The
521following all specify the same class of three characters: C<[-az]>,
522C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which
523specifies a class containing twenty-six characters.)
a0d0e21e 524
54310121 525Characters may be specified using a metacharacter syntax much like that
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526used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return,
527"\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string
528of octal digits, matches the character whose ASCII value is I<nnn>.
0f36ee90 529Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the
a0d0e21e 530character whose ASCII value is I<nn>. The expression \cI<x> matches the
54310121 531ASCII character control-I<x>. Finally, the "." metacharacter matches any
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532character except "\n" (unless you use C</s>).
533
534You can specify a series of alternatives for a pattern using "|" to
535separate them, so that C<fee|fie|foe> will match any of "fee", "fie",
536or "foe" in the target string (as would C<f(e|i|o)e>). Note that the
537first alternative includes everything from the last pattern delimiter
538("(", "[", or the beginning of the pattern) up to the first "|", and
539the last alternative contains everything from the last "|" to the next
540pattern delimiter. For this reason, it's common practice to include
541alternatives in parentheses, to minimize confusion about where they
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542start and end. Note however that "|" is interpreted as a literal with
543square brackets, so if you write C<[fee|fie|foe]> you're really only
544matching C<[feio|]>.
a0d0e21e 545
54310121 546Within a pattern, you may designate subpatterns for later reference by
a0d0e21e 547enclosing them in parentheses, and you may refer back to the I<n>th
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548subpattern later in the pattern using the metacharacter \I<n>.
549Subpatterns are numbered based on the left to right order of their
a0d0e21e 550opening parenthesis. Note that a backreference matches whatever
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551actually matched the subpattern in the string being examined, not the
552rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will
553match "0x1234 0x4321",but not "0x1234 01234", because subpattern 1
748a9306 554actually matched "0x", even though the rule C<0|0x> could
a0d0e21e 555potentially match the leading 0 in the second number.
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556
557=head2 WARNING on \1 vs $1
558
559Some people get too used to writing things like
560
561 $pattern =~ s/(\W)/\\\1/g;
562
563This is grandfathered for the RHS of a substitute to avoid shocking the
564B<sed> addicts, but it's a dirty habit to get into. That's because in
5f05dabc 565PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in
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566the usual double-quoted string means a control-A. The customary Unix
567meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit
568of doing that, you get yourself into trouble if you then add an C</e>
569modifier.
570
571 s/(\d+)/ \1 + 1 /eg;
572
573Or if you try to do
574
575 s/(\d+)/\1000/;
576
577You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with
578C<${1}000>. Basically, the operation of interpolation should not be confused
579with the operation of matching a backreference. Certainly they mean two
580different things on the I<left> side of the C<s///>.
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581
582=head2 SEE ALSO
583
584"Mastering Regular Expressions" (see L<perlbook>) by Jeffrey Friedl.