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1 | =head1 NAME |
2 | |
3 | perlre - Perl regular expressions |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | This page describes the syntax of regular expressions in Perl. For a |
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8 | description of how to I<use> regular expressions in matching |
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9 | operations, plus various examples of the same, see discussions |
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10 | of C<m//>, C<s///>, C<qr//> and C<??> in L<perlop/"Regexp Quote-Like Operators">. |
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11 | |
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12 | Matching operations can have various modifiers. Modifiers |
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13 | that relate to the interpretation of the regular expression inside |
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14 | are listed below. Modifiers that alter the way a regular expression |
15 | is used by Perl are detailed in L<perlop/"Regexp Quote-Like Operators"> and |
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16 | L<perlop/"Gory details of parsing quoted constructs">. |
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17 | |
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18 | =over 4 |
19 | |
20 | =item i |
21 | |
22 | Do case-insensitive pattern matching. |
23 | |
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24 | If C<use locale> is in effect, the case map is taken from the current |
25 | locale. See L<perllocale>. |
26 | |
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27 | =item m |
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28 | |
29 | Treat string as multiple lines. That is, change "^" and "$" from matching |
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30 | the start or end of the string to matching the start or end of any |
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31 | line anywhere within the string. |
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32 | |
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33 | =item s |
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34 | |
35 | Treat string as single line. That is, change "." to match any character |
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36 | whatsoever, even a newline, which normally it would not match. |
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37 | |
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38 | The C</s> and C</m> modifiers both override the C<$*> setting. That |
39 | is, no matter what C<$*> contains, C</s> without C</m> will force |
40 | "^" to match only at the beginning of the string and "$" to match |
41 | only at the end (or just before a newline at the end) of the string. |
42 | Together, as /ms, they let the "." match any character whatsoever, |
43 | while yet allowing "^" and "$" to match, respectively, just after |
44 | and just before newlines within the string. |
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45 | |
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46 | =item x |
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47 | |
48 | Extend your pattern's legibility by permitting whitespace and comments. |
49 | |
50 | =back |
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51 | |
52 | These are usually written as "the C</x> modifier", even though the delimiter |
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53 | in question might not really be a slash. Any of these |
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54 | modifiers may also be embedded within the regular expression itself using |
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55 | the C<(?...)> construct. See below. |
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56 | |
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57 | The C</x> modifier itself needs a little more explanation. It tells |
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58 | the regular expression parser to ignore whitespace that is neither |
59 | backslashed nor within a character class. You can use this to break up |
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60 | your regular expression into (slightly) more readable parts. The C<#> |
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61 | character is also treated as a metacharacter introducing a comment, |
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62 | just as in ordinary Perl code. This also means that if you want real |
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63 | whitespace or C<#> characters in the pattern (outside a character |
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64 | class, where they are unaffected by C</x>), that you'll either have to |
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65 | escape them or encode them using octal or hex escapes. Taken together, |
66 | these features go a long way towards making Perl's regular expressions |
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67 | more readable. Note that you have to be careful not to include the |
68 | pattern delimiter in the comment--perl has no way of knowing you did |
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69 | not intend to close the pattern early. See the C-comment deletion code |
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70 | in L<perlop>. |
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71 | |
72 | =head2 Regular Expressions |
73 | |
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74 | The patterns used in Perl pattern matching derive from supplied in |
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75 | the Version 8 regex routines. (The routines are derived |
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76 | (distantly) from Henry Spencer's freely redistributable reimplementation |
77 | of the V8 routines.) See L<Version 8 Regular Expressions> for |
78 | details. |
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79 | |
80 | In particular the following metacharacters have their standard I<egrep>-ish |
81 | meanings: |
82 | |
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83 | \ Quote the next metacharacter |
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84 | ^ Match the beginning of the line |
85 | . Match any character (except newline) |
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86 | $ Match the end of the line (or before newline at the end) |
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87 | | Alternation |
88 | () Grouping |
89 | [] Character class |
90 | |
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91 | By default, the "^" character is guaranteed to match only the |
92 | beginning of the string, the "$" character only the end (or before the |
93 | newline at the end), and Perl does certain optimizations with the |
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94 | assumption that the string contains only one line. Embedded newlines |
95 | will not be matched by "^" or "$". You may, however, wish to treat a |
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96 | string as a multi-line buffer, such that the "^" will match after any |
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97 | newline within the string, and "$" will match before any newline. At the |
98 | cost of a little more overhead, you can do this by using the /m modifier |
99 | on the pattern match operator. (Older programs did this by setting C<$*>, |
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100 | but this practice is now deprecated.) |
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101 | |
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102 | To simplify multi-line substitutions, the "." character never matches a |
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103 | newline unless you use the C</s> modifier, which in effect tells Perl to pretend |
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104 | the string is a single line--even if it isn't. The C</s> modifier also |
105 | overrides the setting of C<$*>, in case you have some (badly behaved) older |
106 | code that sets it in another module. |
107 | |
108 | The following standard quantifiers are recognized: |
109 | |
110 | * Match 0 or more times |
111 | + Match 1 or more times |
112 | ? Match 1 or 0 times |
113 | {n} Match exactly n times |
114 | {n,} Match at least n times |
115 | {n,m} Match at least n but not more than m times |
116 | |
117 | (If a curly bracket occurs in any other context, it is treated |
118 | as a regular character.) The "*" modifier is equivalent to C<{0,}>, the "+" |
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119 | modifier to C<{1,}>, and the "?" modifier to C<{0,1}>. n and m are limited |
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120 | to integral values less than a preset limit defined when perl is built. |
121 | This is usually 32766 on the most common platforms. The actual limit can |
122 | be seen in the error message generated by code such as this: |
123 | |
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124 | $_ **= $_ , / {$_} / for 2 .. 42; |
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125 | |
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126 | By default, a quantified subpattern is "greedy", that is, it will match as |
127 | many times as possible (given a particular starting location) while still |
128 | allowing the rest of the pattern to match. If you want it to match the |
129 | minimum number of times possible, follow the quantifier with a "?". Note |
130 | that the meanings don't change, just the "greediness": |
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131 | |
132 | *? Match 0 or more times |
133 | +? Match 1 or more times |
134 | ?? Match 0 or 1 time |
135 | {n}? Match exactly n times |
136 | {n,}? Match at least n times |
137 | {n,m}? Match at least n but not more than m times |
138 | |
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139 | Because patterns are processed as double quoted strings, the following |
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140 | also work: |
141 | |
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142 | \t tab (HT, TAB) |
143 | \n newline (LF, NL) |
144 | \r return (CR) |
145 | \f form feed (FF) |
146 | \a alarm (bell) (BEL) |
147 | \e escape (think troff) (ESC) |
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148 | \033 octal char (think of a PDP-11) |
149 | \x1B hex char |
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150 | \x{263a} wide hex char (Unicode SMILEY) |
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151 | \c[ control char |
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152 | \N{name} named char |
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153 | \l lowercase next char (think vi) |
154 | \u uppercase next char (think vi) |
155 | \L lowercase till \E (think vi) |
156 | \U uppercase till \E (think vi) |
157 | \E end case modification (think vi) |
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158 | \Q quote (disable) pattern metacharacters till \E |
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159 | |
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160 | If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> |
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161 | and C<\U> is taken from the current locale. See L<perllocale>. For |
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162 | documentation of C<\N{name}>, see L<charnames>. |
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163 | |
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164 | You cannot include a literal C<$> or C<@> within a C<\Q> sequence. |
165 | An unescaped C<$> or C<@> interpolates the corresponding variable, |
166 | while escaping will cause the literal string C<\$> to be matched. |
167 | You'll need to write something like C<m/\Quser\E\@\Qhost/>. |
168 | |
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169 | In addition, Perl defines the following: |
170 | |
171 | \w Match a "word" character (alphanumeric plus "_") |
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172 | \W Match a non-"word" character |
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173 | \s Match a whitespace character |
174 | \S Match a non-whitespace character |
175 | \d Match a digit character |
176 | \D Match a non-digit character |
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177 | \pP Match P, named property. Use \p{Prop} for longer names. |
178 | \PP Match non-P |
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179 | \X Match eXtended Unicode "combining character sequence", |
180 | equivalent to C<(?:\PM\pM*)> |
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181 | \C Match a single C char (octet) even under utf8. |
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182 | |
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183 | A C<\w> matches a single alphanumeric character or C<_>, not a whole word. |
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184 | Use C<\w+> to match a string of Perl-identifier characters (which isn't |
185 | the same as matching an English word). If C<use locale> is in effect, the |
186 | list of alphabetic characters generated by C<\w> is taken from the |
187 | current locale. See L<perllocale>. You may use C<\w>, C<\W>, C<\s>, C<\S>, |
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188 | C<\d>, and C<\D> within character classes, but if you try to use them |
189 | as endpoints of a range, that's not a range, the "-" is understood literally. |
190 | See L<utf8> for details about C<\pP>, C<\PP>, and C<\X>. |
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191 | |
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192 | The POSIX character class syntax |
193 | |
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194 | [:class:] |
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195 | |
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196 | is also available. The available classes and their backslash |
197 | equivalents (if available) are as follows: |
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198 | |
199 | alpha |
200 | alnum |
201 | ascii |
202 | cntrl |
203 | digit \d |
204 | graph |
205 | lower |
206 | print |
207 | punct |
208 | space \s |
209 | upper |
210 | word \w |
211 | xdigit |
212 | |
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213 | For example use C<[:upper:]> to match all the uppercase characters. |
214 | Note that the C<[]> are part of the C<[::]> construct, not part of the whole |
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215 | character class. For example: |
216 | |
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217 | [01[:alpha:]%] |
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218 | |
219 | matches one, zero, any alphabetic character, and the percentage sign. |
220 | |
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221 | If the C<utf8> pragma is used, the following equivalences to Unicode |
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222 | \p{} constructs hold: |
223 | |
224 | alpha IsAlpha |
225 | alnum IsAlnum |
226 | ascii IsASCII |
227 | cntrl IsCntrl |
228 | digit IsDigit |
229 | graph IsGraph |
230 | lower IsLower |
231 | print IsPrint |
232 | punct IsPunct |
233 | space IsSpace |
234 | upper IsUpper |
235 | word IsWord |
236 | xdigit IsXDigit |
237 | |
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238 | For example C<[:lower:]> and C<\p{IsLower}> are equivalent. |
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239 | |
240 | If the C<utf8> pragma is not used but the C<locale> pragma is, the |
241 | classes correlate with the isalpha(3) interface (except for `word', |
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242 | which is a Perl extension, mirroring C<\w>). |
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243 | |
244 | The assumedly non-obviously named classes are: |
245 | |
246 | =over 4 |
247 | |
248 | =item cntrl |
249 | |
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250 | Any control character. Usually characters that don't produce output as |
251 | such but instead control the terminal somehow: for example newline and |
252 | backspace are control characters. All characters with ord() less than |
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253 | 32 are most often classified as control characters. |
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254 | |
255 | =item graph |
256 | |
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257 | Any alphanumeric or punctuation (special) character. |
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258 | |
259 | =item print |
260 | |
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261 | Any alphanumeric or punctuation (special) character or space. |
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262 | |
263 | =item punct |
264 | |
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265 | Any punctuation (special) character. |
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266 | |
267 | =item xdigit |
268 | |
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269 | Any hexadecimal digit. Though this may feel silly (/0-9a-f/i would |
270 | work just fine) it is included for completeness. |
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271 | |
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272 | =back |
273 | |
274 | You can negate the [::] character classes by prefixing the class name |
275 | with a '^'. This is a Perl extension. For example: |
276 | |
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277 | POSIX trad. Perl utf8 Perl |
278 | |
279 | [:^digit:] \D \P{IsDigit} |
280 | [:^space:] \S \P{IsSpace} |
281 | [:^word:] \W \P{IsWord} |
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282 | |
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283 | The POSIX character classes [.cc.] and [=cc=] are recognized but |
284 | B<not> supported and trying to use them will cause an error. |
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285 | |
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286 | Perl defines the following zero-width assertions: |
287 | |
288 | \b Match a word boundary |
289 | \B Match a non-(word boundary) |
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290 | \A Match only at beginning of string |
291 | \Z Match only at end of string, or before newline at the end |
292 | \z Match only at end of string |
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293 | \G Match only at pos() (e.g. at the end-of-match position |
294 | of prior m//g) |
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295 | |
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296 | A word boundary (C<\b>) is a spot between two characters |
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297 | that has a C<\w> on one side of it and a C<\W> on the other side |
298 | of it (in either order), counting the imaginary characters off the |
299 | beginning and end of the string as matching a C<\W>. (Within |
300 | character classes C<\b> represents backspace rather than a word |
301 | boundary, just as it normally does in any double-quoted string.) |
302 | The C<\A> and C<\Z> are just like "^" and "$", except that they |
303 | won't match multiple times when the C</m> modifier is used, while |
304 | "^" and "$" will match at every internal line boundary. To match |
305 | the actual end of the string and not ignore an optional trailing |
306 | newline, use C<\z>. |
307 | |
308 | The C<\G> assertion can be used to chain global matches (using |
309 | C<m//g>), as described in L<perlop/"Regexp Quote-Like Operators">. |
310 | It is also useful when writing C<lex>-like scanners, when you have |
311 | several patterns that you want to match against consequent substrings |
312 | of your string, see the previous reference. The actual location |
313 | where C<\G> will match can also be influenced by using C<pos()> as |
314 | an lvalue. See L<perlfunc/pos>. |
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315 | |
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316 | The bracketing construct C<( ... )> creates capture buffers. To |
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317 | refer to the digit'th buffer use \<digit> within the |
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318 | match. Outside the match use "$" instead of "\". (The |
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319 | \<digit> notation works in certain circumstances outside |
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320 | the match. See the warning below about \1 vs $1 for details.) |
321 | Referring back to another part of the match is called a |
322 | I<backreference>. |
323 | |
324 | There is no limit to the number of captured substrings that you may |
325 | use. However Perl also uses \10, \11, etc. as aliases for \010, |
326 | \011, etc. (Recall that 0 means octal, so \011 is the 9'th ASCII |
327 | character, a tab.) Perl resolves this ambiguity by interpreting |
328 | \10 as a backreference only if at least 10 left parentheses have |
329 | opened before it. Likewise \11 is a backreference only if at least |
330 | 11 left parentheses have opened before it. And so on. \1 through |
331 | \9 are always interpreted as backreferences." |
332 | |
333 | Examples: |
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334 | |
335 | s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words |
336 | |
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337 | if (/(.)\1/) { # find first doubled char |
338 | print "'$1' is the first doubled character\n"; |
339 | } |
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340 | |
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341 | if (/Time: (..):(..):(..)/) { # parse out values |
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342 | $hours = $1; |
343 | $minutes = $2; |
344 | $seconds = $3; |
345 | } |
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346 | |
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347 | Several special variables also refer back to portions of the previous |
348 | match. C<$+> returns whatever the last bracket match matched. |
349 | C<$&> returns the entire matched string. (At one point C<$0> did |
350 | also, but now it returns the name of the program.) C<$`> returns |
351 | everything before the matched string. And C<$'> returns everything |
352 | after the matched string. |
353 | |
354 | The numbered variables ($1, $2, $3, etc.) and the related punctuation |
355 | set (C<<$+>, C<$&>, C<$`>, and C<$'>) are all dynamically scoped |
356 | until the end of the enclosing block or until the next successful |
357 | match, whichever comes first. (See L<perlsyn/"Compound Statements">.) |
358 | |
359 | B<WARNING>: Once Perl sees that you need one of C<$&>, C<$`>, or |
360 | C<$'> anywhere in the program, it has to provide them for every |
361 | pattern match. This may substantially slow your program. Perl |
362 | uses the same mechanism to produce $1, $2, etc, so you also pay a |
363 | price for each pattern that contains capturing parentheses. (To |
364 | avoid this cost while retaining the grouping behaviour, use the |
365 | extended regular expression C<(?: ... )> instead.) But if you never |
366 | use C<$&>, C<$`> or C<$'>, then patterns I<without> capturing |
367 | parentheses will not be penalized. So avoid C<$&>, C<$'>, and C<$`> |
368 | if you can, but if you can't (and some algorithms really appreciate |
369 | them), once you've used them once, use them at will, because you've |
370 | already paid the price. As of 5.005, C<$&> is not so costly as the |
371 | other two. |
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372 | |
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373 | Backslashed metacharacters in Perl are alphanumeric, such as C<\b>, |
374 | C<\w>, C<\n>. Unlike some other regular expression languages, there |
375 | are no backslashed symbols that aren't alphanumeric. So anything |
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376 | that looks like \\, \(, \), \<, \>, \{, or \} is always |
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377 | interpreted as a literal character, not a metacharacter. This was |
378 | once used in a common idiom to disable or quote the special meanings |
379 | of regular expression metacharacters in a string that you want to |
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380 | use for a pattern. Simply quote all non-"word" characters: |
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381 | |
382 | $pattern =~ s/(\W)/\\$1/g; |
383 | |
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384 | (If C<use locale> is set, then this depends on the current locale.) |
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385 | Today it is more common to use the quotemeta() function or the C<\Q> |
386 | metaquoting escape sequence to disable all metacharacters' special |
387 | meanings like this: |
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388 | |
389 | /$unquoted\Q$quoted\E$unquoted/ |
390 | |
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391 | Beware that if you put literal backslashes (those not inside |
392 | interpolated variables) between C<\Q> and C<\E>, double-quotish |
393 | backslash interpolation may lead to confusing results. If you |
394 | I<need> to use literal backslashes within C<\Q...\E>, |
395 | consult L<perlop/"Gory details of parsing quoted constructs">. |
396 | |
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397 | =head2 Extended Patterns |
398 | |
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399 | Perl also defines a consistent extension syntax for features not |
400 | found in standard tools like B<awk> and B<lex>. The syntax is a |
401 | pair of parentheses with a question mark as the first thing within |
402 | the parentheses. The character after the question mark indicates |
403 | the extension. |
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404 | |
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405 | The stability of these extensions varies widely. Some have been |
406 | part of the core language for many years. Others are experimental |
407 | and may change without warning or be completely removed. Check |
408 | the documentation on an individual feature to verify its current |
409 | status. |
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410 | |
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411 | A question mark was chosen for this and for the minimal-matching |
412 | construct because 1) question marks are rare in older regular |
413 | expressions, and 2) whenever you see one, you should stop and |
414 | "question" exactly what is going on. That's psychology... |
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415 | |
416 | =over 10 |
417 | |
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418 | =item C<(?#text)> |
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419 | |
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420 | A comment. The text is ignored. If the C</x> modifier enables |
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421 | whitespace formatting, a simple C<#> will suffice. Note that Perl closes |
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422 | the comment as soon as it sees a C<)>, so there is no way to put a literal |
423 | C<)> in the comment. |
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424 | |
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425 | =item C<(?imsx-imsx)> |
426 | |
427 | One or more embedded pattern-match modifiers. This is particularly |
428 | useful for dynamic patterns, such as those read in from a configuration |
429 | file, read in as an argument, are specified in a table somewhere, |
430 | etc. Consider the case that some of which want to be case sensitive |
431 | and some do not. The case insensitive ones need to include merely |
432 | C<(?i)> at the front of the pattern. For example: |
433 | |
434 | $pattern = "foobar"; |
435 | if ( /$pattern/i ) { } |
436 | |
437 | # more flexible: |
438 | |
439 | $pattern = "(?i)foobar"; |
440 | if ( /$pattern/ ) { } |
441 | |
442 | Letters after a C<-> turn those modifiers off. These modifiers are |
443 | localized inside an enclosing group (if any). For example, |
444 | |
445 | ( (?i) blah ) \s+ \1 |
446 | |
447 | will match a repeated (I<including the case>!) word C<blah> in any |
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448 | case, assuming C<x> modifier, and no C<i> modifier outside this |
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449 | group. |
450 | |
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451 | =item C<(?:pattern)> |
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452 | |
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453 | =item C<(?imsx-imsx:pattern)> |
454 | |
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455 | This is for clustering, not capturing; it groups subexpressions like |
456 | "()", but doesn't make backreferences as "()" does. So |
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457 | |
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458 | @fields = split(/\b(?:a|b|c)\b/) |
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459 | |
460 | is like |
461 | |
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462 | @fields = split(/\b(a|b|c)\b/) |
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463 | |
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464 | but doesn't spit out extra fields. It's also cheaper not to capture |
465 | characters if you don't need to. |
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466 | |
19799a22 |
467 | Any letters between C<?> and C<:> act as flags modifiers as with |
468 | C<(?imsx-imsx)>. For example, |
ca9dfc88 |
469 | |
470 | /(?s-i:more.*than).*million/i |
471 | |
14218588 |
472 | is equivalent to the more verbose |
ca9dfc88 |
473 | |
474 | /(?:(?s-i)more.*than).*million/i |
475 | |
5a964f20 |
476 | =item C<(?=pattern)> |
a0d0e21e |
477 | |
19799a22 |
478 | A zero-width positive look-ahead assertion. For example, C</\w+(?=\t)/> |
a0d0e21e |
479 | matches a word followed by a tab, without including the tab in C<$&>. |
480 | |
5a964f20 |
481 | =item C<(?!pattern)> |
a0d0e21e |
482 | |
19799a22 |
483 | A zero-width negative look-ahead assertion. For example C</foo(?!bar)/> |
a0d0e21e |
484 | matches any occurrence of "foo" that isn't followed by "bar". Note |
19799a22 |
485 | however that look-ahead and look-behind are NOT the same thing. You cannot |
486 | use this for look-behind. |
7b8d334a |
487 | |
5a964f20 |
488 | If you are looking for a "bar" that isn't preceded by a "foo", C</(?!foo)bar/> |
7b8d334a |
489 | will not do what you want. That's because the C<(?!foo)> is just saying that |
490 | the next thing cannot be "foo"--and it's not, it's a "bar", so "foobar" will |
491 | match. You would have to do something like C</(?!foo)...bar/> for that. We |
492 | say "like" because there's the case of your "bar" not having three characters |
493 | before it. You could cover that this way: C</(?:(?!foo)...|^.{0,2})bar/>. |
494 | Sometimes it's still easier just to say: |
a0d0e21e |
495 | |
a3cb178b |
496 | if (/bar/ && $` !~ /foo$/) |
a0d0e21e |
497 | |
19799a22 |
498 | For look-behind see below. |
c277df42 |
499 | |
c47ff5f1 |
500 | =item C<(?<=pattern)> |
c277df42 |
501 | |
c47ff5f1 |
502 | A zero-width positive look-behind assertion. For example, C</(?<=\t)\w+/> |
19799a22 |
503 | matches a word that follows a tab, without including the tab in C<$&>. |
504 | Works only for fixed-width look-behind. |
c277df42 |
505 | |
5a964f20 |
506 | =item C<(?<!pattern)> |
c277df42 |
507 | |
19799a22 |
508 | A zero-width negative look-behind assertion. For example C</(?<!bar)foo/> |
509 | matches any occurrence of "foo" that does not follow "bar". Works |
510 | only for fixed-width look-behind. |
c277df42 |
511 | |
cc6b7395 |
512 | =item C<(?{ code })> |
c277df42 |
513 | |
19799a22 |
514 | B<WARNING>: This extended regular expression feature is considered |
515 | highly experimental, and may be changed or deleted without notice. |
c277df42 |
516 | |
19799a22 |
517 | This zero-width assertion evaluate any embedded Perl code. It |
518 | always succeeds, and its C<code> is not interpolated. Currently, |
519 | the rules to determine where the C<code> ends are somewhat convoluted. |
520 | |
521 | The C<code> is properly scoped in the following sense: If the assertion |
522 | is backtracked (compare L<"Backtracking">), all changes introduced after |
523 | C<local>ization are undone, so that |
b9ac3b5b |
524 | |
525 | $_ = 'a' x 8; |
526 | m< |
527 | (?{ $cnt = 0 }) # Initialize $cnt. |
528 | ( |
529 | a |
530 | (?{ |
531 | local $cnt = $cnt + 1; # Update $cnt, backtracking-safe. |
532 | }) |
533 | )* |
534 | aaaa |
535 | (?{ $res = $cnt }) # On success copy to non-localized |
536 | # location. |
537 | >x; |
538 | |
19799a22 |
539 | will set C<$res = 4>. Note that after the match, $cnt returns to the globally |
14218588 |
540 | introduced value, because the scopes that restrict C<local> operators |
b9ac3b5b |
541 | are unwound. |
542 | |
19799a22 |
543 | This assertion may be used as a C<(?(condition)yes-pattern|no-pattern)> |
544 | switch. If I<not> used in this way, the result of evaluation of |
545 | C<code> is put into the special variable C<$^R>. This happens |
546 | immediately, so C<$^R> can be used from other C<(?{ code })> assertions |
547 | inside the same regular expression. |
b9ac3b5b |
548 | |
19799a22 |
549 | The assignment to C<$^R> above is properly localized, so the old |
550 | value of C<$^R> is restored if the assertion is backtracked; compare |
551 | L<"Backtracking">. |
b9ac3b5b |
552 | |
19799a22 |
553 | For reasons of security, this construct is forbidden if the regular |
554 | expression involves run-time interpolation of variables, unless the |
555 | perilous C<use re 'eval'> pragma has been used (see L<re>), or the |
556 | variables contain results of C<qr//> operator (see |
557 | L<perlop/"qr/STRING/imosx">). |
871b0233 |
558 | |
14218588 |
559 | This restriction is because of the wide-spread and remarkably convenient |
19799a22 |
560 | custom of using run-time determined strings as patterns. For example: |
871b0233 |
561 | |
562 | $re = <>; |
563 | chomp $re; |
564 | $string =~ /$re/; |
565 | |
14218588 |
566 | Before Perl knew how to execute interpolated code within a pattern, |
567 | this operation was completely safe from a security point of view, |
568 | although it could raise an exception from an illegal pattern. If |
569 | you turn on the C<use re 'eval'>, though, it is no longer secure, |
570 | so you should only do so if you are also using taint checking. |
571 | Better yet, use the carefully constrained evaluation within a Safe |
572 | module. See L<perlsec> for details about both these mechanisms. |
871b0233 |
573 | |
14455d6c |
574 | =item C<(??{ code })> |
0f5d15d6 |
575 | |
19799a22 |
576 | B<WARNING>: This extended regular expression feature is considered |
577 | highly experimental, and may be changed or deleted without notice. |
9da458fc |
578 | A simplified version of the syntax may be introduced for commonly |
579 | used idioms. |
0f5d15d6 |
580 | |
19799a22 |
581 | This is a "postponed" regular subexpression. The C<code> is evaluated |
582 | at run time, at the moment this subexpression may match. The result |
583 | of evaluation is considered as a regular expression and matched as |
584 | if it were inserted instead of this construct. |
0f5d15d6 |
585 | |
428594d9 |
586 | The C<code> is not interpolated. As before, the rules to determine |
19799a22 |
587 | where the C<code> ends are currently somewhat convoluted. |
588 | |
589 | The following pattern matches a parenthesized group: |
0f5d15d6 |
590 | |
591 | $re = qr{ |
592 | \( |
593 | (?: |
594 | (?> [^()]+ ) # Non-parens without backtracking |
595 | | |
14455d6c |
596 | (??{ $re }) # Group with matching parens |
0f5d15d6 |
597 | )* |
598 | \) |
599 | }x; |
600 | |
c47ff5f1 |
601 | =item C<< (?>pattern) >> |
5a964f20 |
602 | |
19799a22 |
603 | B<WARNING>: This extended regular expression feature is considered |
604 | highly experimental, and may be changed or deleted without notice. |
605 | |
606 | An "independent" subexpression, one which matches the substring |
607 | that a I<standalone> C<pattern> would match if anchored at the given |
9da458fc |
608 | position, and it matches I<nothing other than this substring>. This |
19799a22 |
609 | construct is useful for optimizations of what would otherwise be |
610 | "eternal" matches, because it will not backtrack (see L<"Backtracking">). |
9da458fc |
611 | It may also be useful in places where the "grab all you can, and do not |
612 | give anything back" semantic is desirable. |
19799a22 |
613 | |
c47ff5f1 |
614 | For example: C<< ^(?>a*)ab >> will never match, since C<< (?>a*) >> |
19799a22 |
615 | (anchored at the beginning of string, as above) will match I<all> |
616 | characters C<a> at the beginning of string, leaving no C<a> for |
617 | C<ab> to match. In contrast, C<a*ab> will match the same as C<a+b>, |
618 | since the match of the subgroup C<a*> is influenced by the following |
619 | group C<ab> (see L<"Backtracking">). In particular, C<a*> inside |
620 | C<a*ab> will match fewer characters than a standalone C<a*>, since |
621 | this makes the tail match. |
622 | |
c47ff5f1 |
623 | An effect similar to C<< (?>pattern) >> may be achieved by writing |
19799a22 |
624 | C<(?=(pattern))\1>. This matches the same substring as a standalone |
625 | C<a+>, and the following C<\1> eats the matched string; it therefore |
c47ff5f1 |
626 | makes a zero-length assertion into an analogue of C<< (?>...) >>. |
19799a22 |
627 | (The difference between these two constructs is that the second one |
628 | uses a capturing group, thus shifting ordinals of backreferences |
629 | in the rest of a regular expression.) |
630 | |
631 | Consider this pattern: |
c277df42 |
632 | |
871b0233 |
633 | m{ \( |
634 | ( |
9da458fc |
635 | [^()]+ # x+ |
871b0233 |
636 | | |
637 | \( [^()]* \) |
638 | )+ |
639 | \) |
640 | }x |
5a964f20 |
641 | |
19799a22 |
642 | That will efficiently match a nonempty group with matching parentheses |
643 | two levels deep or less. However, if there is no such group, it |
644 | will take virtually forever on a long string. That's because there |
645 | are so many different ways to split a long string into several |
646 | substrings. This is what C<(.+)+> is doing, and C<(.+)+> is similar |
647 | to a subpattern of the above pattern. Consider how the pattern |
648 | above detects no-match on C<((()aaaaaaaaaaaaaaaaaa> in several |
649 | seconds, but that each extra letter doubles this time. This |
650 | exponential performance will make it appear that your program has |
14218588 |
651 | hung. However, a tiny change to this pattern |
5a964f20 |
652 | |
871b0233 |
653 | m{ \( |
654 | ( |
9da458fc |
655 | (?> [^()]+ ) # change x+ above to (?> x+ ) |
871b0233 |
656 | | |
657 | \( [^()]* \) |
658 | )+ |
659 | \) |
660 | }x |
c277df42 |
661 | |
c47ff5f1 |
662 | which uses C<< (?>...) >> matches exactly when the one above does (verifying |
5a964f20 |
663 | this yourself would be a productive exercise), but finishes in a fourth |
664 | the time when used on a similar string with 1000000 C<a>s. Be aware, |
665 | however, that this pattern currently triggers a warning message under |
9f1b1f2d |
666 | the C<use warnings> pragma or B<-w> switch saying it |
667 | C<"matches the null string many times">): |
c277df42 |
668 | |
c47ff5f1 |
669 | On simple groups, such as the pattern C<< (?> [^()]+ ) >>, a comparable |
19799a22 |
670 | effect may be achieved by negative look-ahead, as in C<[^()]+ (?! [^()] )>. |
c277df42 |
671 | This was only 4 times slower on a string with 1000000 C<a>s. |
672 | |
9da458fc |
673 | The "grab all you can, and do not give anything back" semantic is desirable |
674 | in many situations where on the first sight a simple C<()*> looks like |
675 | the correct solution. Suppose we parse text with comments being delimited |
676 | by C<#> followed by some optional (horizontal) whitespace. Contrary to |
4375e838 |
677 | its appearance, C<#[ \t]*> I<is not> the correct subexpression to match |
9da458fc |
678 | the comment delimiter, because it may "give up" some whitespace if |
679 | the remainder of the pattern can be made to match that way. The correct |
680 | answer is either one of these: |
681 | |
682 | (?>#[ \t]*) |
683 | #[ \t]*(?![ \t]) |
684 | |
685 | For example, to grab non-empty comments into $1, one should use either |
686 | one of these: |
687 | |
688 | / (?> \# [ \t]* ) ( .+ ) /x; |
689 | / \# [ \t]* ( [^ \t] .* ) /x; |
690 | |
691 | Which one you pick depends on which of these expressions better reflects |
692 | the above specification of comments. |
693 | |
5a964f20 |
694 | =item C<(?(condition)yes-pattern|no-pattern)> |
c277df42 |
695 | |
5a964f20 |
696 | =item C<(?(condition)yes-pattern)> |
c277df42 |
697 | |
19799a22 |
698 | B<WARNING>: This extended regular expression feature is considered |
699 | highly experimental, and may be changed or deleted without notice. |
700 | |
c277df42 |
701 | Conditional expression. C<(condition)> should be either an integer in |
702 | parentheses (which is valid if the corresponding pair of parentheses |
19799a22 |
703 | matched), or look-ahead/look-behind/evaluate zero-width assertion. |
c277df42 |
704 | |
19799a22 |
705 | For example: |
c277df42 |
706 | |
5a964f20 |
707 | m{ ( \( )? |
871b0233 |
708 | [^()]+ |
5a964f20 |
709 | (?(1) \) ) |
871b0233 |
710 | }x |
c277df42 |
711 | |
712 | matches a chunk of non-parentheses, possibly included in parentheses |
713 | themselves. |
a0d0e21e |
714 | |
a0d0e21e |
715 | =back |
716 | |
c07a80fd |
717 | =head2 Backtracking |
718 | |
35a734be |
719 | NOTE: This section presents an abstract approximation of regular |
720 | expression behavior. For a more rigorous (and complicated) view of |
721 | the rules involved in selecting a match among possible alternatives, |
722 | see L<Combining pieces together>. |
723 | |
c277df42 |
724 | A fundamental feature of regular expression matching involves the |
5a964f20 |
725 | notion called I<backtracking>, which is currently used (when needed) |
c277df42 |
726 | by all regular expression quantifiers, namely C<*>, C<*?>, C<+>, |
9da458fc |
727 | C<+?>, C<{n,m}>, and C<{n,m}?>. Backtracking is often optimized |
728 | internally, but the general principle outlined here is valid. |
c07a80fd |
729 | |
730 | For a regular expression to match, the I<entire> regular expression must |
731 | match, not just part of it. So if the beginning of a pattern containing a |
732 | quantifier succeeds in a way that causes later parts in the pattern to |
733 | fail, the matching engine backs up and recalculates the beginning |
734 | part--that's why it's called backtracking. |
735 | |
736 | Here is an example of backtracking: Let's say you want to find the |
737 | word following "foo" in the string "Food is on the foo table.": |
738 | |
739 | $_ = "Food is on the foo table."; |
740 | if ( /\b(foo)\s+(\w+)/i ) { |
741 | print "$2 follows $1.\n"; |
742 | } |
743 | |
744 | When the match runs, the first part of the regular expression (C<\b(foo)>) |
745 | finds a possible match right at the beginning of the string, and loads up |
746 | $1 with "Foo". However, as soon as the matching engine sees that there's |
747 | no whitespace following the "Foo" that it had saved in $1, it realizes its |
68dc0745 |
748 | mistake and starts over again one character after where it had the |
c07a80fd |
749 | tentative match. This time it goes all the way until the next occurrence |
750 | of "foo". The complete regular expression matches this time, and you get |
751 | the expected output of "table follows foo." |
752 | |
753 | Sometimes minimal matching can help a lot. Imagine you'd like to match |
754 | everything between "foo" and "bar". Initially, you write something |
755 | like this: |
756 | |
757 | $_ = "The food is under the bar in the barn."; |
758 | if ( /foo(.*)bar/ ) { |
759 | print "got <$1>\n"; |
760 | } |
761 | |
762 | Which perhaps unexpectedly yields: |
763 | |
764 | got <d is under the bar in the > |
765 | |
766 | That's because C<.*> was greedy, so you get everything between the |
14218588 |
767 | I<first> "foo" and the I<last> "bar". Here it's more effective |
c07a80fd |
768 | to use minimal matching to make sure you get the text between a "foo" |
769 | and the first "bar" thereafter. |
770 | |
771 | if ( /foo(.*?)bar/ ) { print "got <$1>\n" } |
772 | got <d is under the > |
773 | |
774 | Here's another example: let's say you'd like to match a number at the end |
775 | of a string, and you also want to keep the preceding part the match. |
776 | So you write this: |
777 | |
778 | $_ = "I have 2 numbers: 53147"; |
779 | if ( /(.*)(\d*)/ ) { # Wrong! |
780 | print "Beginning is <$1>, number is <$2>.\n"; |
781 | } |
782 | |
783 | That won't work at all, because C<.*> was greedy and gobbled up the |
784 | whole string. As C<\d*> can match on an empty string the complete |
785 | regular expression matched successfully. |
786 | |
8e1088bc |
787 | Beginning is <I have 2 numbers: 53147>, number is <>. |
c07a80fd |
788 | |
789 | Here are some variants, most of which don't work: |
790 | |
791 | $_ = "I have 2 numbers: 53147"; |
792 | @pats = qw{ |
793 | (.*)(\d*) |
794 | (.*)(\d+) |
795 | (.*?)(\d*) |
796 | (.*?)(\d+) |
797 | (.*)(\d+)$ |
798 | (.*?)(\d+)$ |
799 | (.*)\b(\d+)$ |
800 | (.*\D)(\d+)$ |
801 | }; |
802 | |
803 | for $pat (@pats) { |
804 | printf "%-12s ", $pat; |
805 | if ( /$pat/ ) { |
806 | print "<$1> <$2>\n"; |
807 | } else { |
808 | print "FAIL\n"; |
809 | } |
810 | } |
811 | |
812 | That will print out: |
813 | |
814 | (.*)(\d*) <I have 2 numbers: 53147> <> |
815 | (.*)(\d+) <I have 2 numbers: 5314> <7> |
816 | (.*?)(\d*) <> <> |
817 | (.*?)(\d+) <I have > <2> |
818 | (.*)(\d+)$ <I have 2 numbers: 5314> <7> |
819 | (.*?)(\d+)$ <I have 2 numbers: > <53147> |
820 | (.*)\b(\d+)$ <I have 2 numbers: > <53147> |
821 | (.*\D)(\d+)$ <I have 2 numbers: > <53147> |
822 | |
823 | As you see, this can be a bit tricky. It's important to realize that a |
824 | regular expression is merely a set of assertions that gives a definition |
825 | of success. There may be 0, 1, or several different ways that the |
826 | definition might succeed against a particular string. And if there are |
5a964f20 |
827 | multiple ways it might succeed, you need to understand backtracking to |
828 | know which variety of success you will achieve. |
c07a80fd |
829 | |
19799a22 |
830 | When using look-ahead assertions and negations, this can all get even |
54310121 |
831 | tricker. Imagine you'd like to find a sequence of non-digits not |
c07a80fd |
832 | followed by "123". You might try to write that as |
833 | |
871b0233 |
834 | $_ = "ABC123"; |
835 | if ( /^\D*(?!123)/ ) { # Wrong! |
836 | print "Yup, no 123 in $_\n"; |
837 | } |
c07a80fd |
838 | |
839 | But that isn't going to match; at least, not the way you're hoping. It |
840 | claims that there is no 123 in the string. Here's a clearer picture of |
841 | why it that pattern matches, contrary to popular expectations: |
842 | |
843 | $x = 'ABC123' ; |
844 | $y = 'ABC445' ; |
845 | |
846 | print "1: got $1\n" if $x =~ /^(ABC)(?!123)/ ; |
847 | print "2: got $1\n" if $y =~ /^(ABC)(?!123)/ ; |
848 | |
849 | print "3: got $1\n" if $x =~ /^(\D*)(?!123)/ ; |
850 | print "4: got $1\n" if $y =~ /^(\D*)(?!123)/ ; |
851 | |
852 | This prints |
853 | |
854 | 2: got ABC |
855 | 3: got AB |
856 | 4: got ABC |
857 | |
5f05dabc |
858 | You might have expected test 3 to fail because it seems to a more |
c07a80fd |
859 | general purpose version of test 1. The important difference between |
860 | them is that test 3 contains a quantifier (C<\D*>) and so can use |
861 | backtracking, whereas test 1 will not. What's happening is |
862 | that you've asked "Is it true that at the start of $x, following 0 or more |
5f05dabc |
863 | non-digits, you have something that's not 123?" If the pattern matcher had |
c07a80fd |
864 | let C<\D*> expand to "ABC", this would have caused the whole pattern to |
54310121 |
865 | fail. |
14218588 |
866 | |
c07a80fd |
867 | The search engine will initially match C<\D*> with "ABC". Then it will |
14218588 |
868 | try to match C<(?!123> with "123", which fails. But because |
c07a80fd |
869 | a quantifier (C<\D*>) has been used in the regular expression, the |
870 | search engine can backtrack and retry the match differently |
54310121 |
871 | in the hope of matching the complete regular expression. |
c07a80fd |
872 | |
5a964f20 |
873 | The pattern really, I<really> wants to succeed, so it uses the |
874 | standard pattern back-off-and-retry and lets C<\D*> expand to just "AB" this |
c07a80fd |
875 | time. Now there's indeed something following "AB" that is not |
14218588 |
876 | "123". It's "C123", which suffices. |
c07a80fd |
877 | |
14218588 |
878 | We can deal with this by using both an assertion and a negation. |
879 | We'll say that the first part in $1 must be followed both by a digit |
880 | and by something that's not "123". Remember that the look-aheads |
881 | are zero-width expressions--they only look, but don't consume any |
882 | of the string in their match. So rewriting this way produces what |
c07a80fd |
883 | you'd expect; that is, case 5 will fail, but case 6 succeeds: |
884 | |
885 | print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/ ; |
886 | print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/ ; |
887 | |
888 | 6: got ABC |
889 | |
5a964f20 |
890 | In other words, the two zero-width assertions next to each other work as though |
19799a22 |
891 | they're ANDed together, just as you'd use any built-in assertions: C</^$/> |
c07a80fd |
892 | matches only if you're at the beginning of the line AND the end of the |
893 | line simultaneously. The deeper underlying truth is that juxtaposition in |
894 | regular expressions always means AND, except when you write an explicit OR |
895 | using the vertical bar. C</ab/> means match "a" AND (then) match "b", |
896 | although the attempted matches are made at different positions because "a" |
897 | is not a zero-width assertion, but a one-width assertion. |
898 | |
19799a22 |
899 | B<WARNING>: particularly complicated regular expressions can take |
14218588 |
900 | exponential time to solve because of the immense number of possible |
9da458fc |
901 | ways they can use backtracking to try match. For example, without |
902 | internal optimizations done by the regular expression engine, this will |
903 | take a painfully long time to run: |
c07a80fd |
904 | |
9da458fc |
905 | 'aaaaaaaaaaaa' =~ /((a{0,5}){0,5}){0,5}[c]/ |
c07a80fd |
906 | |
14218588 |
907 | And if you used C<*>'s instead of limiting it to 0 through 5 matches, |
908 | then it would take forever--or until you ran out of stack space. |
c07a80fd |
909 | |
9da458fc |
910 | A powerful tool for optimizing such beasts is what is known as an |
911 | "independent group", |
c47ff5f1 |
912 | which does not backtrack (see L<C<< (?>pattern) >>>). Note also that |
9da458fc |
913 | zero-length look-ahead/look-behind assertions will not backtrack to make |
14218588 |
914 | the tail match, since they are in "logical" context: only |
915 | whether they match is considered relevant. For an example |
9da458fc |
916 | where side-effects of look-ahead I<might> have influenced the |
c47ff5f1 |
917 | following match, see L<C<< (?>pattern) >>>. |
c277df42 |
918 | |
a0d0e21e |
919 | =head2 Version 8 Regular Expressions |
920 | |
5a964f20 |
921 | In case you're not familiar with the "regular" Version 8 regex |
a0d0e21e |
922 | routines, here are the pattern-matching rules not described above. |
923 | |
54310121 |
924 | Any single character matches itself, unless it is a I<metacharacter> |
a0d0e21e |
925 | with a special meaning described here or above. You can cause |
5a964f20 |
926 | characters that normally function as metacharacters to be interpreted |
5f05dabc |
927 | literally by prefixing them with a "\" (e.g., "\." matches a ".", not any |
a0d0e21e |
928 | character; "\\" matches a "\"). A series of characters matches that |
929 | series of characters in the target string, so the pattern C<blurfl> |
930 | would match "blurfl" in the target string. |
931 | |
932 | You can specify a character class, by enclosing a list of characters |
5a964f20 |
933 | in C<[]>, which will match any one character from the list. If the |
a0d0e21e |
934 | first character after the "[" is "^", the class matches any character not |
14218588 |
935 | in the list. Within a list, the "-" character specifies a |
5a964f20 |
936 | range, so that C<a-z> represents all characters between "a" and "z", |
8a4f6ac2 |
937 | inclusive. If you want either "-" or "]" itself to be a member of a |
938 | class, put it at the start of the list (possibly after a "^"), or |
939 | escape it with a backslash. "-" is also taken literally when it is |
940 | at the end of the list, just before the closing "]". (The |
84850974 |
941 | following all specify the same class of three characters: C<[-az]>, |
942 | C<[az-]>, and C<[a\-z]>. All are different from C<[a-z]>, which |
943 | specifies a class containing twenty-six characters.) |
1209ba90 |
944 | Also, if you try to use the character classes C<\w>, C<\W>, C<\s>, |
945 | C<\S>, C<\d>, or C<\D> as endpoints of a range, that's not a range, |
946 | the "-" is understood literally. |
a0d0e21e |
947 | |
8ada0baa |
948 | Note also that the whole range idea is rather unportable between |
949 | character sets--and even within character sets they may cause results |
950 | you probably didn't expect. A sound principle is to use only ranges |
951 | that begin from and end at either alphabets of equal case ([a-e], |
952 | [A-E]), or digits ([0-9]). Anything else is unsafe. If in doubt, |
953 | spell out the character sets in full. |
954 | |
54310121 |
955 | Characters may be specified using a metacharacter syntax much like that |
a0d0e21e |
956 | used in C: "\n" matches a newline, "\t" a tab, "\r" a carriage return, |
957 | "\f" a form feed, etc. More generally, \I<nnn>, where I<nnn> is a string |
958 | of octal digits, matches the character whose ASCII value is I<nnn>. |
0f36ee90 |
959 | Similarly, \xI<nn>, where I<nn> are hexadecimal digits, matches the |
a0d0e21e |
960 | character whose ASCII value is I<nn>. The expression \cI<x> matches the |
54310121 |
961 | ASCII character control-I<x>. Finally, the "." metacharacter matches any |
a0d0e21e |
962 | character except "\n" (unless you use C</s>). |
963 | |
964 | You can specify a series of alternatives for a pattern using "|" to |
965 | separate them, so that C<fee|fie|foe> will match any of "fee", "fie", |
5a964f20 |
966 | or "foe" in the target string (as would C<f(e|i|o)e>). The |
a0d0e21e |
967 | first alternative includes everything from the last pattern delimiter |
968 | ("(", "[", or the beginning of the pattern) up to the first "|", and |
969 | the last alternative contains everything from the last "|" to the next |
14218588 |
970 | pattern delimiter. That's why it's common practice to include |
971 | alternatives in parentheses: to minimize confusion about where they |
a3cb178b |
972 | start and end. |
973 | |
5a964f20 |
974 | Alternatives are tried from left to right, so the first |
a3cb178b |
975 | alternative found for which the entire expression matches, is the one that |
976 | is chosen. This means that alternatives are not necessarily greedy. For |
628afcb5 |
977 | example: when matching C<foo|foot> against "barefoot", only the "foo" |
a3cb178b |
978 | part will match, as that is the first alternative tried, and it successfully |
979 | matches the target string. (This might not seem important, but it is |
980 | important when you are capturing matched text using parentheses.) |
981 | |
5a964f20 |
982 | Also remember that "|" is interpreted as a literal within square brackets, |
a3cb178b |
983 | so if you write C<[fee|fie|foe]> you're really only matching C<[feio|]>. |
a0d0e21e |
984 | |
14218588 |
985 | Within a pattern, you may designate subpatterns for later reference |
986 | by enclosing them in parentheses, and you may refer back to the |
987 | I<n>th subpattern later in the pattern using the metacharacter |
988 | \I<n>. Subpatterns are numbered based on the left to right order |
989 | of their opening parenthesis. A backreference matches whatever |
990 | actually matched the subpattern in the string being examined, not |
991 | the rules for that subpattern. Therefore, C<(0|0x)\d*\s\1\d*> will |
992 | match "0x1234 0x4321", but not "0x1234 01234", because subpattern |
993 | 1 matched "0x", even though the rule C<0|0x> could potentially match |
994 | the leading 0 in the second number. |
cb1a09d0 |
995 | |
19799a22 |
996 | =head2 Warning on \1 vs $1 |
cb1a09d0 |
997 | |
5a964f20 |
998 | Some people get too used to writing things like: |
cb1a09d0 |
999 | |
1000 | $pattern =~ s/(\W)/\\\1/g; |
1001 | |
1002 | This is grandfathered for the RHS of a substitute to avoid shocking the |
1003 | B<sed> addicts, but it's a dirty habit to get into. That's because in |
5f05dabc |
1004 | PerlThink, the righthand side of a C<s///> is a double-quoted string. C<\1> in |
cb1a09d0 |
1005 | the usual double-quoted string means a control-A. The customary Unix |
1006 | meaning of C<\1> is kludged in for C<s///>. However, if you get into the habit |
1007 | of doing that, you get yourself into trouble if you then add an C</e> |
1008 | modifier. |
1009 | |
5a964f20 |
1010 | s/(\d+)/ \1 + 1 /eg; # causes warning under -w |
cb1a09d0 |
1011 | |
1012 | Or if you try to do |
1013 | |
1014 | s/(\d+)/\1000/; |
1015 | |
1016 | You can't disambiguate that by saying C<\{1}000>, whereas you can fix it with |
14218588 |
1017 | C<${1}000>. The operation of interpolation should not be confused |
cb1a09d0 |
1018 | with the operation of matching a backreference. Certainly they mean two |
1019 | different things on the I<left> side of the C<s///>. |
9fa51da4 |
1020 | |
c84d73f1 |
1021 | =head2 Repeated patterns matching zero-length substring |
1022 | |
19799a22 |
1023 | B<WARNING>: Difficult material (and prose) ahead. This section needs a rewrite. |
c84d73f1 |
1024 | |
1025 | Regular expressions provide a terse and powerful programming language. As |
1026 | with most other power tools, power comes together with the ability |
1027 | to wreak havoc. |
1028 | |
1029 | A common abuse of this power stems from the ability to make infinite |
628afcb5 |
1030 | loops using regular expressions, with something as innocuous as: |
c84d73f1 |
1031 | |
1032 | 'foo' =~ m{ ( o? )* }x; |
1033 | |
1034 | The C<o?> can match at the beginning of C<'foo'>, and since the position |
1035 | in the string is not moved by the match, C<o?> would match again and again |
14218588 |
1036 | because of the C<*> modifier. Another common way to create a similar cycle |
c84d73f1 |
1037 | is with the looping modifier C<//g>: |
1038 | |
1039 | @matches = ( 'foo' =~ m{ o? }xg ); |
1040 | |
1041 | or |
1042 | |
1043 | print "match: <$&>\n" while 'foo' =~ m{ o? }xg; |
1044 | |
1045 | or the loop implied by split(). |
1046 | |
1047 | However, long experience has shown that many programming tasks may |
14218588 |
1048 | be significantly simplified by using repeated subexpressions that |
1049 | may match zero-length substrings. Here's a simple example being: |
c84d73f1 |
1050 | |
1051 | @chars = split //, $string; # // is not magic in split |
1052 | ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// / |
1053 | |
9da458fc |
1054 | Thus Perl allows such constructs, by I<forcefully breaking |
c84d73f1 |
1055 | the infinite loop>. The rules for this are different for lower-level |
1056 | loops given by the greedy modifiers C<*+{}>, and for higher-level |
1057 | ones like the C</g> modifier or split() operator. |
1058 | |
19799a22 |
1059 | The lower-level loops are I<interrupted> (that is, the loop is |
1060 | broken) when Perl detects that a repeated expression matched a |
1061 | zero-length substring. Thus |
c84d73f1 |
1062 | |
1063 | m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x; |
1064 | |
1065 | is made equivalent to |
1066 | |
1067 | m{ (?: NON_ZERO_LENGTH )* |
1068 | | |
1069 | (?: ZERO_LENGTH )? |
1070 | }x; |
1071 | |
1072 | The higher level-loops preserve an additional state between iterations: |
1073 | whether the last match was zero-length. To break the loop, the following |
1074 | match after a zero-length match is prohibited to have a length of zero. |
1075 | This prohibition interacts with backtracking (see L<"Backtracking">), |
1076 | and so the I<second best> match is chosen if the I<best> match is of |
1077 | zero length. |
1078 | |
19799a22 |
1079 | For example: |
c84d73f1 |
1080 | |
1081 | $_ = 'bar'; |
1082 | s/\w??/<$&>/g; |
1083 | |
1084 | results in C<"<><b><><a><><r><>">. At each position of the string the best |
1085 | match given by non-greedy C<??> is the zero-length match, and the I<second |
1086 | best> match is what is matched by C<\w>. Thus zero-length matches |
1087 | alternate with one-character-long matches. |
1088 | |
1089 | Similarly, for repeated C<m/()/g> the second-best match is the match at the |
1090 | position one notch further in the string. |
1091 | |
19799a22 |
1092 | The additional state of being I<matched with zero-length> is associated with |
c84d73f1 |
1093 | the matched string, and is reset by each assignment to pos(). |
9da458fc |
1094 | Zero-length matches at the end of the previous match are ignored |
1095 | during C<split>. |
c84d73f1 |
1096 | |
35a734be |
1097 | =head2 Combining pieces together |
1098 | |
1099 | Each of the elementary pieces of regular expressions which were described |
1100 | before (such as C<ab> or C<\Z>) could match at most one substring |
1101 | at the given position of the input string. However, in a typical regular |
1102 | expression these elementary pieces are combined into more complicated |
1103 | patterns using combining operators C<ST>, C<S|T>, C<S*> etc |
1104 | (in these examples C<S> and C<T> are regular subexpressions). |
1105 | |
1106 | Such combinations can include alternatives, leading to a problem of choice: |
1107 | if we match a regular expression C<a|ab> against C<"abc">, will it match |
1108 | substring C<"a"> or C<"ab">? One way to describe which substring is |
1109 | actually matched is the concept of backtracking (see L<"Backtracking">). |
1110 | However, this description is too low-level and makes you think |
1111 | in terms of a particular implementation. |
1112 | |
1113 | Another description starts with notions of "better"/"worse". All the |
1114 | substrings which may be matched by the given regular expression can be |
1115 | sorted from the "best" match to the "worst" match, and it is the "best" |
1116 | match which is chosen. This substitutes the question of "what is chosen?" |
1117 | by the question of "which matches are better, and which are worse?". |
1118 | |
1119 | Again, for elementary pieces there is no such question, since at most |
1120 | one match at a given position is possible. This section describes the |
1121 | notion of better/worse for combining operators. In the description |
1122 | below C<S> and C<T> are regular subexpressions. |
1123 | |
1124 | =over |
1125 | |
1126 | =item C<ST> |
1127 | |
1128 | Consider two possible matches, C<AB> and C<A'B'>, C<A> and C<A'> are |
1129 | substrings which can be matched by C<S>, C<B> and C<B'> are substrings |
1130 | which can be matched by C<T>. |
1131 | |
1132 | If C<A> is better match for C<S> than C<A'>, C<AB> is a better |
1133 | match than C<A'B'>. |
1134 | |
1135 | If C<A> and C<A'> coincide: C<AB> is a better match than C<AB'> if |
1136 | C<B> is better match for C<T> than C<B'>. |
1137 | |
1138 | =item C<S|T> |
1139 | |
1140 | When C<S> can match, it is a better match than when only C<T> can match. |
1141 | |
1142 | Ordering of two matches for C<S> is the same as for C<S>. Similar for |
1143 | two matches for C<T>. |
1144 | |
1145 | =item C<S{REPEAT_COUNT}> |
1146 | |
1147 | Matches as C<SSS...S> (repeated as many times as necessary). |
1148 | |
1149 | =item C<S{min,max}> |
1150 | |
1151 | Matches as C<S{max}|S{max-1}|...|S{min+1}|S{min}>. |
1152 | |
1153 | =item C<S{min,max}?> |
1154 | |
1155 | Matches as C<S{min}|S{min+1}|...|S{max-1}|S{max}>. |
1156 | |
1157 | =item C<S?>, C<S*>, C<S+> |
1158 | |
1159 | Same as C<S{0,1}>, C<S{0,BIG_NUMBER}>, C<S{1,BIG_NUMBER}> respectively. |
1160 | |
1161 | =item C<S??>, C<S*?>, C<S+?> |
1162 | |
1163 | Same as C<S{0,1}?>, C<S{0,BIG_NUMBER}?>, C<S{1,BIG_NUMBER}?> respectively. |
1164 | |
c47ff5f1 |
1165 | =item C<< (?>S) >> |
35a734be |
1166 | |
1167 | Matches the best match for C<S> and only that. |
1168 | |
1169 | =item C<(?=S)>, C<(?<=S)> |
1170 | |
1171 | Only the best match for C<S> is considered. (This is important only if |
1172 | C<S> has capturing parentheses, and backreferences are used somewhere |
1173 | else in the whole regular expression.) |
1174 | |
1175 | =item C<(?!S)>, C<(?<!S)> |
1176 | |
1177 | For this grouping operator there is no need to describe the ordering, since |
1178 | only whether or not C<S> can match is important. |
1179 | |
14455d6c |
1180 | =item C<(??{ EXPR })> |
35a734be |
1181 | |
1182 | The ordering is the same as for the regular expression which is |
1183 | the result of EXPR. |
1184 | |
1185 | =item C<(?(condition)yes-pattern|no-pattern)> |
1186 | |
1187 | Recall that which of C<yes-pattern> or C<no-pattern> actually matches is |
1188 | already determined. The ordering of the matches is the same as for the |
1189 | chosen subexpression. |
1190 | |
1191 | =back |
1192 | |
1193 | The above recipes describe the ordering of matches I<at a given position>. |
1194 | One more rule is needed to understand how a match is determined for the |
1195 | whole regular expression: a match at an earlier position is always better |
1196 | than a match at a later position. |
1197 | |
c84d73f1 |
1198 | =head2 Creating custom RE engines |
1199 | |
1200 | Overloaded constants (see L<overload>) provide a simple way to extend |
1201 | the functionality of the RE engine. |
1202 | |
1203 | Suppose that we want to enable a new RE escape-sequence C<\Y|> which |
1204 | matches at boundary between white-space characters and non-whitespace |
1205 | characters. Note that C<(?=\S)(?<!\S)|(?!\S)(?<=\S)> matches exactly |
1206 | at these positions, so we want to have each C<\Y|> in the place of the |
1207 | more complicated version. We can create a module C<customre> to do |
1208 | this: |
1209 | |
1210 | package customre; |
1211 | use overload; |
1212 | |
1213 | sub import { |
1214 | shift; |
1215 | die "No argument to customre::import allowed" if @_; |
1216 | overload::constant 'qr' => \&convert; |
1217 | } |
1218 | |
1219 | sub invalid { die "/$_[0]/: invalid escape '\\$_[1]'"} |
1220 | |
1221 | my %rules = ( '\\' => '\\', |
1222 | 'Y|' => qr/(?=\S)(?<!\S)|(?!\S)(?<=\S)/ ); |
1223 | sub convert { |
1224 | my $re = shift; |
1225 | $re =~ s{ |
1226 | \\ ( \\ | Y . ) |
1227 | } |
1228 | { $rules{$1} or invalid($re,$1) }sgex; |
1229 | return $re; |
1230 | } |
1231 | |
1232 | Now C<use customre> enables the new escape in constant regular |
1233 | expressions, i.e., those without any runtime variable interpolations. |
1234 | As documented in L<overload>, this conversion will work only over |
1235 | literal parts of regular expressions. For C<\Y|$re\Y|> the variable |
1236 | part of this regular expression needs to be converted explicitly |
1237 | (but only if the special meaning of C<\Y|> should be enabled inside $re): |
1238 | |
1239 | use customre; |
1240 | $re = <>; |
1241 | chomp $re; |
1242 | $re = customre::convert $re; |
1243 | /\Y|$re\Y|/; |
1244 | |
19799a22 |
1245 | =head1 BUGS |
1246 | |
9da458fc |
1247 | This document varies from difficult to understand to completely |
1248 | and utterly opaque. The wandering prose riddled with jargon is |
1249 | hard to fathom in several places. |
1250 | |
1251 | This document needs a rewrite that separates the tutorial content |
1252 | from the reference content. |
19799a22 |
1253 | |
1254 | =head1 SEE ALSO |
9fa51da4 |
1255 | |
9b599b2a |
1256 | L<perlop/"Regexp Quote-Like Operators">. |
1257 | |
1e66bd83 |
1258 | L<perlop/"Gory details of parsing quoted constructs">. |
1259 | |
14218588 |
1260 | L<perlfaq6>. |
1261 | |
9b599b2a |
1262 | L<perlfunc/pos>. |
1263 | |
1264 | L<perllocale>. |
1265 | |
14218588 |
1266 | I<Mastering Regular Expressions> by Jeffrey Friedl, published |
1267 | by O'Reilly and Associates. |