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1 | =head1 NAME |
2 | ||
3 | perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | The top level documentation about Perl regular expressions | |
8 | is found in L<perlre>. | |
9 | ||
10 | This document describes all backslash and escape sequences. After | |
11 | explaining the role of the backslash, it lists all the sequences that have | |
12 | a special meaning in Perl regular expressions (in alphabetical order), | |
13 | then describes each of them. | |
14 | ||
15 | Most sequences are described in detail in different documents; the primary | |
16 | purpose of this document is to have a quick reference guide describing all | |
17 | backslash and escape sequences. | |
18 | ||
8a118206 RGS |
19 | =head2 The backslash |
20 | ||
21 | In a regular expression, the backslash can perform one of two tasks: | |
22 | it either takes away the special meaning of the character following it | |
23 | (for instance, C<\|> matches a vertical bar, it's not an alternation), | |
24 | or it is the start of a backslash or escape sequence. | |
25 | ||
26 | The rules determining what it is are quite simple: if the character | |
df225385 | 27 | following the backslash is an ASCII punctuation (non-word) character (that is, |
b6538e4f TC |
28 | anything that is not a letter, digit, or underscore), then the backslash just |
29 | takes away any special meaning of the character following it. | |
df225385 KW |
30 | |
31 | If the character following the backslash is an ASCII letter or an ASCII digit, | |
32 | then the sequence may be special; if so, it's listed below. A few letters have | |
6b46370c KW |
33 | not been used yet, so escaping them with a backslash doesn't change them to be |
34 | special. A future version of Perl may assign a special meaning to them, so if | |
b6538e4f | 35 | you have warnings turned on, Perl issues a warning if you use such a |
6b46370c | 36 | sequence. [1]. |
8a118206 | 37 | |
e2cb52ee | 38 | It is however guaranteed that backslash or escape sequences never have a |
8a118206 RGS |
39 | punctuation character following the backslash, not now, and not in a future |
40 | version of Perl 5. So it is safe to put a backslash in front of a non-word | |
41 | character. | |
42 | ||
43 | Note that the backslash itself is special; if you want to match a backslash, | |
44 | you have to escape the backslash with a backslash: C</\\/> matches a single | |
45 | backslash. | |
46 | ||
47 | =over 4 | |
48 | ||
49 | =item [1] | |
50 | ||
b6538e4f | 51 | There is one exception. If you use an alphanumeric character as the |
8a118206 | 52 | delimiter of your pattern (which you probably shouldn't do for readability |
b6538e4f | 53 | reasons), you have to escape the delimiter if you want to match |
8a118206 RGS |
54 | it. Perl won't warn then. See also L<perlop/Gory details of parsing |
55 | quoted constructs>. | |
56 | ||
57 | =back | |
58 | ||
59 | ||
60 | =head2 All the sequences and escapes | |
61 | ||
df225385 KW |
62 | Those not usable within a bracketed character class (like C<[\da-z]>) are marked |
63 | as C<Not in [].> | |
64 | ||
f0a2b745 | 65 | \000 Octal escape sequence. See also \o{}. |
df225385 | 66 | \1 Absolute backreference. Not in []. |
8a118206 | 67 | \a Alarm or bell. |
df225385 KW |
68 | \A Beginning of string. Not in []. |
69 | \b Word/non-word boundary. (Backspace in []). | |
70 | \B Not a word/non-word boundary. Not in []. | |
f321be7e | 71 | \cX Control-X. |
5ed061ff KW |
72 | \C Single octet, even under UTF-8. Not in []. |
73 | (Deprecated) | |
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74 | \d Character class for digits. |
75 | \D Character class for non-digits. | |
76 | \e Escape character. | |
df225385 | 77 | \E Turn off \Q, \L and \U processing. Not in []. |
8a118206 | 78 | \f Form feed. |
628253b8 | 79 | \F Foldcase till \E. Not in []. |
f321be7e SK |
80 | \g{}, \g1 Named, absolute or relative backreference. |
81 | Not in []. | |
df225385 | 82 | \G Pos assertion. Not in []. |
418e7b04 KW |
83 | \h Character class for horizontal whitespace. |
84 | \H Character class for non horizontal whitespace. | |
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85 | \k{}, \k<>, \k'' Named backreference. Not in []. |
86 | \K Keep the stuff left of \K. Not in []. | |
87 | \l Lowercase next character. Not in []. | |
88 | \L Lowercase till \E. Not in []. | |
8a118206 | 89 | \n (Logical) newline character. |
4e5e0888 | 90 | \N Any character but newline. Not in []. |
fb121860 | 91 | \N{} Named or numbered (Unicode) character or sequence. |
f0a2b745 | 92 | \o{} Octal escape sequence. |
e1b711da KW |
93 | \p{}, \pP Character with the given Unicode property. |
94 | \P{}, \PP Character without the given Unicode property. | |
736fe711 KW |
95 | \Q Quote (disable) pattern metacharacters till \E. Not |
96 | in []. | |
8a118206 | 97 | \r Return character. |
df225385 | 98 | \R Generic new line. Not in []. |
418e7b04 KW |
99 | \s Character class for whitespace. |
100 | \S Character class for non whitespace. | |
8a118206 | 101 | \t Tab character. |
df225385 KW |
102 | \u Titlecase next character. Not in []. |
103 | \U Uppercase till \E. Not in []. | |
418e7b04 KW |
104 | \v Character class for vertical whitespace. |
105 | \V Character class for non vertical whitespace. | |
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106 | \w Character class for word characters. |
107 | \W Character class for non-word characters. | |
108 | \x{}, \x00 Hexadecimal escape sequence. | |
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109 | \X Unicode "extended grapheme cluster". Not in []. |
110 | \z End of string. Not in []. | |
111 | \Z End of string. Not in []. | |
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112 | |
113 | =head2 Character Escapes | |
114 | ||
115 | =head3 Fixed characters | |
116 | ||
e2cb52ee | 117 | A handful of characters have a dedicated I<character escape>. The following |
58151fe4 | 118 | table shows them, along with their ASCII code points (in decimal and hex), |
4948b50f KW |
119 | their ASCII name, the control escape on ASCII platforms and a short |
120 | description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.) | |
8a118206 | 121 | |
4948b50f | 122 | Seq. Code Point ASCII Cntrl Description. |
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123 | Dec Hex |
124 | \a 7 07 BEL \cG alarm or bell | |
125 | \b 8 08 BS \cH backspace [1] | |
126 | \e 27 1B ESC \c[ escape character | |
127 | \f 12 0C FF \cL form feed | |
128 | \n 10 0A LF \cJ line feed [2] | |
129 | \r 13 0D CR \cM carriage return | |
130 | \t 9 09 TAB \cI tab | |
131 | ||
132 | =over 4 | |
133 | ||
134 | =item [1] | |
135 | ||
301ba1af | 136 | C<\b> is the backspace character only inside a character class. Outside a |
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137 | character class, C<\b> is a word/non-word boundary. |
138 | ||
139 | =item [2] | |
140 | ||
b6538e4f | 141 | C<\n> matches a logical newline. Perl converts between C<\n> and your |
f6993e9e | 142 | OS's native newline character when reading from or writing to text files. |
8a118206 RGS |
143 | |
144 | =back | |
145 | ||
146 | =head4 Example | |
147 | ||
148 | $str =~ /\t/; # Matches if $str contains a (horizontal) tab. | |
149 | ||
150 | =head3 Control characters | |
151 | ||
152 | C<\c> is used to denote a control character; the character following C<\c> | |
4948b50f KW |
153 | determines the value of the construct. For example the value of C<\cA> is |
154 | C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc. | |
155 | The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete | |
156 | list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in | |
157 | L<perlebcdic/OPERATOR DIFFERENCES>. | |
158 | ||
159 | Note that C<\c\> alone at the end of a regular expression (or doubled-quoted | |
160 | string) is not valid. The backslash must be followed by another character. | |
161 | That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>. | |
162 | ||
163 | To write platform-independent code, you must use C<\N{I<NAME>}> instead, like | |
164 | C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>. | |
8a118206 RGS |
165 | |
166 | Mnemonic: I<c>ontrol character. | |
167 | ||
168 | =head4 Example | |
169 | ||
170 | $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K). | |
171 | ||
fb121860 | 172 | =head3 Named or numbered characters and character sequences |
8a118206 | 173 | |
17148a1a KW |
174 | Unicode characters have a Unicode name and numeric code point (ordinal) |
175 | value. Use the | |
e526e8bb | 176 | C<\N{}> construct to specify a character by either of these values. |
fb121860 | 177 | Certain sequences of characters also have names. |
e526e8bb | 178 | |
fb121860 | 179 | To specify by name, the name of the character or character sequence goes |
fbb93542 | 180 | between the curly braces. |
e526e8bb | 181 | |
b6538e4f TC |
182 | To specify a character by Unicode code point, use the form C<\N{U+I<code |
183 | point>}>, where I<code point> is a number in hexadecimal that gives the | |
17148a1a | 184 | code point that Unicode has assigned to the desired character. It is |
b6538e4f TC |
185 | customary but not required to use leading zeros to pad the number to 4 |
186 | digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will | |
187 | rarely see it written without the two leading zeros. C<\N{U+0041}> means | |
188 | "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41). | |
e526e8bb | 189 | |
fb121860 KW |
190 | It is even possible to give your own names to characters and character |
191 | sequences. For details, see L<charnames>. | |
8a118206 | 192 | |
8c37f1d0 | 193 | (There is an expanded internal form that you may see in debug output: |
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194 | C<\N{U+I<code point>.I<code point>...}>. |
195 | The C<...> means any number of these I<code point>s separated by dots. | |
8c37f1d0 KW |
196 | This represents the sequence formed by the characters. This is an internal |
197 | form only, subject to change, and you should not try to use it yourself.) | |
198 | ||
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199 | Mnemonic: I<N>amed character. |
200 | ||
b6538e4f TC |
201 | Note that a character or character sequence expressed as a named |
202 | or numbered character is considered a character without special | |
fb121860 | 203 | meaning by the regex engine, and will match "as is". |
df225385 | 204 | |
8a118206 RGS |
205 | =head4 Example |
206 | ||
8a118206 RGS |
207 | $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character |
208 | ||
209 | use charnames 'Cyrillic'; # Loads Cyrillic names. | |
210 | $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA". | |
211 | ||
212 | =head3 Octal escapes | |
213 | ||
f0a2b745 | 214 | There are two forms of octal escapes. Each is used to specify a character by |
17148a1a | 215 | its code point specified in octal notation. |
f0a2b745 KW |
216 | |
217 | One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots | |
218 | represent one or more octal digits. It can be used for any Unicode character. | |
219 | ||
220 | It was introduced to avoid the potential problems with the other form, | |
221 | available in all Perls. That form consists of a backslash followed by three | |
222 | octal digits. One problem with this form is that it can look exactly like an | |
223 | old-style backreference (see | |
224 | L</Disambiguation rules between old-style octal escapes and backreferences> | |
225 | below.) You can avoid this by making the first of the three digits always a | |
9645299c | 226 | zero, but that makes \077 the largest code point specifiable. |
f0a2b745 KW |
227 | |
228 | In some contexts, a backslash followed by two or even one octal digits may be | |
229 | interpreted as an octal escape, sometimes with a warning, and because of some | |
230 | bugs, sometimes with surprising results. Also, if you are creating a regex | |
c69ca1d4 | 231 | out of smaller snippets concatenated together, and you use fewer than three |
f0a2b745 KW |
232 | digits, the beginning of one snippet may be interpreted as adding digits to the |
233 | ending of the snippet before it. See L</Absolute referencing> for more | |
234 | discussion and examples of the snippet problem. | |
8a118206 | 235 | |
b6538e4f TC |
236 | Note that a character expressed as an octal escape is considered |
237 | a character without special meaning by the regex engine, and will match | |
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238 | "as is". |
239 | ||
f0a2b745 | 240 | To summarize, the C<\o{}> form is always safe to use, and the other form is |
17148a1a | 241 | safe to use for code points through \077 when you use exactly three digits to |
f0a2b745 | 242 | specify them. |
8a118206 | 243 | |
f0a2b745 | 244 | Mnemonic: I<0>ctal or I<o>ctal. |
8a118206 | 245 | |
f0a2b745 | 246 | =head4 Examples (assuming an ASCII platform) |
8a118206 | 247 | |
f0a2b745 KW |
248 | $str = "Perl"; |
249 | $str =~ /\o{120}/; # Match, "\120" is "P". | |
250 | $str =~ /\120/; # Same. | |
f321be7e SK |
251 | $str =~ /\o{120}+/; # Match, "\120" is "P", |
252 | # it's repeated at least once. | |
f0a2b745 KW |
253 | $str =~ /\120+/; # Same. |
254 | $str =~ /P\053/; # No match, "\053" is "+" and taken literally. | |
255 | /\o{23073}/ # Black foreground, white background smiling face. | |
f321be7e | 256 | /\o{4801234567}/ # Raises a warning, and yields chr(4). |
f0a2b745 KW |
257 | |
258 | =head4 Disambiguation rules between old-style octal escapes and backreferences | |
259 | ||
260 | Octal escapes of the C<\000> form outside of bracketed character classes | |
f321be7e | 261 | potentially clash with old-style backreferences (see L</Absolute referencing> |
f0a2b745 KW |
262 | below). They both consist of a backslash followed by numbers. So Perl has to |
263 | use heuristics to determine whether it is a backreference or an octal escape. | |
264 | Perl uses the following rules to disambiguate: | |
8a118206 RGS |
265 | |
266 | =over 4 | |
267 | ||
268 | =item 1 | |
269 | ||
353c6505 | 270 | If the backslash is followed by a single digit, it's a backreference. |
8a118206 RGS |
271 | |
272 | =item 2 | |
273 | ||
274 | If the first digit following the backslash is a 0, it's an octal escape. | |
275 | ||
276 | =item 3 | |
277 | ||
b6538e4f TC |
278 | If the number following the backslash is N (in decimal), and Perl already |
279 | has seen N capture groups, Perl considers this a backreference. Otherwise, | |
280 | it considers it an octal escape. If N has more than three digits, Perl | |
281 | takes only the first three for the octal escape; the rest are matched as is. | |
8a118206 RGS |
282 | |
283 | my $pat = "(" x 999; | |
284 | $pat .= "a"; | |
285 | $pat .= ")" x 999; | |
286 | /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups. | |
287 | /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups | |
f321be7e | 288 | # and \1000 is seen as \100 (a '@') and a '0'. |
8a118206 RGS |
289 | |
290 | =back | |
291 | ||
17148a1a | 292 | You can force a backreference interpretation always by using the C<\g{...}> |
f0a2b745 KW |
293 | form. You can the force an octal interpretation always by using the C<\o{...}> |
294 | form, or for numbers up through \077 (= 63 decimal), by using three digits, | |
295 | beginning with a "0". | |
296 | ||
8a118206 RGS |
297 | =head3 Hexadecimal escapes |
298 | ||
f0a2b745 KW |
299 | Like octal escapes, there are two forms of hexadecimal escapes, but both start |
300 | with the same thing, C<\x>. This is followed by either exactly two hexadecimal | |
301 | digits forming a number, or a hexadecimal number of arbitrary length surrounded | |
302 | by curly braces. The hexadecimal number is the code point of the character you | |
303 | want to express. | |
8a118206 | 304 | |
b6538e4f TC |
305 | Note that a character expressed as one of these escapes is considered a |
306 | character without special meaning by the regex engine, and will match | |
8a118206 RGS |
307 | "as is". |
308 | ||
309 | Mnemonic: heI<x>adecimal. | |
310 | ||
9f5650a8 | 311 | =head4 Examples (assuming an ASCII platform) |
8a118206 RGS |
312 | |
313 | $str = "Perl"; | |
314 | $str =~ /\x50/; # Match, "\x50" is "P". | |
f822d0dd | 315 | $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once |
8a118206 RGS |
316 | $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally. |
317 | ||
318 | /\x{2603}\x{2602}/ # Snowman with an umbrella. | |
319 | # The Unicode character 2603 is a snowman, | |
320 | # the Unicode character 2602 is an umbrella. | |
321 | /\x{263B}/ # Black smiling face. | |
322 | /\x{263b}/ # Same, the hex digits A - F are case insensitive. | |
323 | ||
324 | =head2 Modifiers | |
325 | ||
326 | A number of backslash sequences have to do with changing the character, | |
327 | or characters following them. C<\l> will lowercase the character following | |
5f2b17ca | 328 | it, while C<\u> will uppercase (or, more accurately, titlecase) the |
b6538e4f TC |
329 | character following it. They provide functionality similar to the |
330 | functions C<lcfirst> and C<ucfirst>. | |
8a118206 RGS |
331 | |
332 | To uppercase or lowercase several characters, one might want to use | |
333 | C<\L> or C<\U>, which will lowercase/uppercase all characters following | |
b6538e4f | 334 | them, until either the end of the pattern or the next occurrence of |
17148a1a | 335 | C<\E>, whichever comes first. They provide functionality similar to what |
b6538e4f | 336 | the functions C<lc> and C<uc> provide. |
8a118206 | 337 | |
736fe711 KW |
338 | C<\Q> is used to quote (disable) pattern metacharacters, up to the next |
339 | C<\E> or the end of the pattern. C<\Q> adds a backslash to any character | |
340 | that could have special meaning to Perl. In the ASCII range, it quotes | |
341 | every character that isn't a letter, digit, or underscore. See | |
342 | L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII | |
343 | code points. Using this ensures that any character between C<\Q> and | |
344 | C<\E> will be matched literally, not interpreted as a metacharacter by | |
345 | the regex engine. | |
8a118206 | 346 | |
628253b8 BF |
347 | C<\F> can be used to casefold all characters following, up to the next C<\E> |
348 | or the end of the pattern. It provides the functionality similar to | |
349 | the C<fc> function. | |
350 | ||
351 | Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd. | |
8a118206 RGS |
352 | |
353 | =head4 Examples | |
354 | ||
355 | $sid = "sid"; | |
356 | $greg = "GrEg"; | |
357 | $miranda = "(Miranda)"; | |
358 | $str =~ /\u$sid/; # Matches 'Sid' | |
359 | $str =~ /\L$greg/; # Matches 'greg' | |
360 | $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern | |
361 | # had been written as /\(Miranda\)/ | |
362 | ||
363 | =head2 Character classes | |
364 | ||
365 | Perl regular expressions have a large range of character classes. Some of | |
366 | the character classes are written as a backslash sequence. We will briefly | |
367 | discuss those here; full details of character classes can be found in | |
368 | L<perlrecharclass>. | |
369 | ||
d35dd6c6 KW |
370 | C<\w> is a character class that matches any single I<word> character |
371 | (letters, digits, Unicode marks, and connector punctuation (like the | |
372 | underscore)). C<\d> is a character class that matches any decimal | |
373 | digit, while the character class C<\s> matches any whitespace character. | |
99d59c4d | 374 | New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal |
418e7b04 | 375 | and vertical whitespace characters. |
cfaf538b KW |
376 | |
377 | The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies | |
9645299c KW |
378 | depending on various pragma and regular expression modifiers. It is |
379 | possible to restrict the match to the ASCII range by using the C</a> | |
380 | regular expression modifier. See L<perlrecharclass>. | |
8a118206 RGS |
381 | |
382 | The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are | |
e486b3cc KW |
383 | character classes that match, respectively, any character that isn't a |
384 | word character, digit, whitespace, horizontal whitespace, or vertical | |
385 | whitespace. | |
8a118206 RGS |
386 | |
387 | Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical. | |
388 | ||
389 | =head3 Unicode classes | |
390 | ||
391 | C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to | |
392 | match a character that matches the given Unicode property; properties | |
393 | include things like "letter", or "thai character". Capitalizing the | |
394 | sequence to C<\PP> and C<\P{Property}> make the sequence match a character | |
395 | that doesn't match the given Unicode property. For more details, see | |
4948b50f | 396 | L<perlrecharclass/Backslash sequences> and |
8a118206 RGS |
397 | L<perlunicode/Unicode Character Properties>. |
398 | ||
399 | Mnemonic: I<p>roperty. | |
400 | ||
8a118206 RGS |
401 | =head2 Referencing |
402 | ||
403 | If capturing parenthesis are used in a regular expression, we can refer | |
404 | to the part of the source string that was matched, and match exactly the | |
1843fd28 RGS |
405 | same thing. There are three ways of referring to such I<backreference>: |
406 | absolutely, relatively, and by name. | |
407 | ||
408 | =for later add link to perlrecapture | |
8a118206 RGS |
409 | |
410 | =head3 Absolute referencing | |
411 | ||
c27a5cfe | 412 | Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N> |
d8b950dc | 413 | is a positive (unsigned) decimal number of any length is an absolute reference |
c27a5cfe KW |
414 | to a capturing group. |
415 | ||
8e4698ef KW |
416 | I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has |
417 | been matched by that set of parentheses. Thus C<\g1> refers to the first | |
c27a5cfe KW |
418 | capture group in the regex. |
419 | ||
420 | The C<\gI<N>> form can be equivalently written as C<\g{I<N>}> | |
421 | which avoids ambiguity when building a regex by concatenating shorter | |
d8b950dc KW |
422 | strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained |
423 | C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is | |
424 | probably not what you intended. | |
c27a5cfe KW |
425 | |
426 | In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at | |
b6538e4f TC |
427 | least I<N> capturing groups, or else I<N> is considered an octal escape |
428 | (but something like C<\18> is the same as C<\0018>; that is, the octal escape | |
c27a5cfe KW |
429 | C<"\001"> followed by a literal digit C<"8">). |
430 | ||
431 | Mnemonic: I<g>roup. | |
8a118206 RGS |
432 | |
433 | =head4 Examples | |
434 | ||
c27a5cfe | 435 | /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat"). |
f321be7e | 436 | /(\w+) \1/; # Same thing; written old-style. |
c27a5cfe | 437 | /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA"). |
8a118206 RGS |
438 | |
439 | ||
440 | =head3 Relative referencing | |
441 | ||
c27a5cfe KW |
442 | C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can |
443 | be written as C<\g{-I<N>>.) It refers to the I<N>th group before the | |
444 | C<\g{-I<N>}>. | |
8a118206 | 445 | |
c27a5cfe | 446 | The big advantage of this form is that it makes it much easier to write |
8a118206 RGS |
447 | patterns with references that can be interpolated in larger patterns, |
448 | even if the larger pattern also contains capture groups. | |
449 | ||
8a118206 RGS |
450 | =head4 Examples |
451 | ||
c27a5cfe KW |
452 | /(A) # Group 1 |
453 | ( # Group 2 | |
454 | (B) # Group 3 | |
455 | \g{-1} # Refers to group 3 (B) | |
456 | \g{-3} # Refers to group 1 (A) | |
8a118206 RGS |
457 | ) |
458 | /x; # Matches "ABBA". | |
459 | ||
460 | my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc. | |
461 | /$qr$qr/ # Matches 'ababcdcd'. | |
462 | ||
463 | =head3 Named referencing | |
464 | ||
d8b950dc KW |
465 | C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a |
466 | named capture group, dispensing completely with having to think about capture | |
467 | buffer positions. | |
8a118206 RGS |
468 | |
469 | To be compatible with .Net regular expressions, C<\g{name}> may also be | |
470 | written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>. | |
471 | ||
d8b950dc KW |
472 | To prevent any ambiguity, I<name> must not start with a digit nor contain a |
473 | hyphen. | |
8a118206 RGS |
474 | |
475 | =head4 Examples | |
476 | ||
477 | /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat") | |
478 | /(?<word>\w+) \k{word}/ # Same. | |
479 | /(?<word>\w+) \k<word>/ # Same. | |
480 | /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/ | |
481 | # Match a four letter palindrome (e.g. "ABBA") | |
482 | ||
483 | =head2 Assertions | |
484 | ||
ac036724 | 485 | Assertions are conditions that have to be true; they don't actually |
8a118206 RGS |
486 | match parts of the substring. There are six assertions that are written as |
487 | backslash sequences. | |
488 | ||
489 | =over 4 | |
490 | ||
491 | =item \A | |
492 | ||
493 | C<\A> only matches at the beginning of the string. If the C</m> modifier | |
1726f7e8 | 494 | isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m> |
8a118206 RGS |
495 | modifier is used, then C</^/> matches internal newlines, but the meaning |
496 | of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning | |
497 | of the string regardless whether the C</m> modifier is used. | |
498 | ||
499 | =item \z, \Z | |
500 | ||
501 | C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't | |
b6538e4f TC |
502 | used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the |
503 | end of the string, or one before the newline at the end of the string. If the | |
8a118206 RGS |
504 | C</m> modifier is used, then C</$/> matches at internal newlines, but the |
505 | meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at | |
506 | the end of the string (or just before a trailing newline) regardless whether | |
507 | the C</m> modifier is used. | |
508 | ||
b6538e4f TC |
509 | C<\z> is just like C<\Z>, except that it does not match before a trailing |
510 | newline. C<\z> matches at the end of the string only, regardless of the | |
511 | modifiers used, and not just before a newline. It is how to anchor the | |
512 | match to the true end of the string under all conditions. | |
8a118206 RGS |
513 | |
514 | =item \G | |
515 | ||
b6538e4f TC |
516 | C<\G> is usually used only in combination with the C</g> modifier. If the |
517 | C</g> modifier is used and the match is done in scalar context, Perl | |
518 | remembers where in the source string the last match ended, and the next time, | |
8a118206 RGS |
519 | it will start the match from where it ended the previous time. |
520 | ||
b6538e4f TC |
521 | C<\G> matches the point where the previous match on that string ended, |
522 | or the beginning of that string if there was no previous match. | |
1843fd28 RGS |
523 | |
524 | =for later add link to perlremodifiers | |
8a118206 RGS |
525 | |
526 | Mnemonic: I<G>lobal. | |
527 | ||
528 | =item \b, \B | |
529 | ||
530 | C<\b> matches at any place between a word and a non-word character; C<\B> | |
531 | matches at any place between characters where C<\b> doesn't match. C<\b> | |
532 | and C<\B> assume there's a non-word character before the beginning and after | |
533 | the end of the source string; so C<\b> will match at the beginning (or end) | |
534 | of the source string if the source string begins (or ends) with a word | |
b6538e4f TC |
535 | character. Otherwise, C<\B> will match. |
536 | ||
537 | Do not use something like C<\b=head\d\b> and expect it to match the | |
538 | beginning of a line. It can't, because for there to be a boundary before | |
539 | the non-word "=", there must be a word character immediately previous. | |
540 | All boundary determinations look for word characters alone, not for | |
541 | non-words characters nor for string ends. It may help to understand how | |
542 | <\b> and <\B> work by equating them as follows: | |
543 | ||
544 | \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w)) | |
545 | \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w)) | |
8a118206 RGS |
546 | |
547 | Mnemonic: I<b>oundary. | |
548 | ||
549 | =back | |
550 | ||
551 | =head4 Examples | |
552 | ||
553 | "cat" =~ /\Acat/; # Match. | |
554 | "cat" =~ /cat\Z/; # Match. | |
555 | "cat\n" =~ /cat\Z/; # Match. | |
556 | "cat\n" =~ /cat\z/; # No match. | |
557 | ||
558 | "cat" =~ /\bcat\b/; # Matches. | |
559 | "cats" =~ /\bcat\b/; # No match. | |
560 | "cat" =~ /\bcat\B/; # No match. | |
561 | "cats" =~ /\bcat\B/; # Match. | |
562 | ||
563 | while ("cat dog" =~ /(\w+)/g) { | |
564 | print $1; # Prints 'catdog' | |
565 | } | |
566 | while ("cat dog" =~ /\G(\w+)/g) { | |
567 | print $1; # Prints 'cat' | |
568 | } | |
569 | ||
570 | =head2 Misc | |
571 | ||
572 | Here we document the backslash sequences that don't fall in one of the | |
b6538e4f | 573 | categories above. These are: |
8a118206 RGS |
574 | |
575 | =over 4 | |
576 | ||
577 | =item \C | |
578 | ||
37ea023e DM |
579 | (Deprecated.) C<\C> always matches a single octet, even if the source |
580 | string is encoded | |
8a118206 | 581 | in UTF-8 format, and the character to be matched is a multi-octet character. |
69a6e56c | 582 | This is very dangerous, because it violates |
b6538e4f | 583 | the logical character abstraction and can cause UTF-8 sequences to become malformed. |
8a118206 | 584 | |
37ea023e DM |
585 | Use C<utf8::encode()> instead. |
586 | ||
8a118206 RGS |
587 | Mnemonic: oI<C>tet. |
588 | ||
589 | =item \K | |
590 | ||
b6538e4f TC |
591 | This appeared in perl 5.10.0. Anything matched left of C<\K> is |
592 | not included in C<$&>, and will not be replaced if the pattern is | |
593 | used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x> | |
8a118206 RGS |
594 | instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>. |
595 | ||
596 | Mnemonic: I<K>eep. | |
597 | ||
df225385 KW |
598 | =item \N |
599 | ||
2171640d | 600 | This feature, available starting in v5.12, matches any character |
b6538e4f | 601 | that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is |
b3b85878 KW |
602 | identical to the C<.> metasymbol, except under the C</s> flag, which changes |
603 | the meaning of C<.>, but not C<\N>. | |
df225385 | 604 | |
e526e8bb | 605 | Note that C<\N{...}> can mean a |
fb121860 KW |
606 | L<named or numbered character |
607 | |/Named or numbered characters and character sequences>. | |
df225385 KW |
608 | |
609 | Mnemonic: Complement of I<\n>. | |
610 | ||
8a118206 | 611 | =item \R |
6b46370c | 612 | X<\R> |
8a118206 | 613 | |
b6538e4f TC |
614 | C<\R> matches a I<generic newline>; that is, anything considered a |
615 | linebreak sequence by Unicode. This includes all characters matched by | |
616 | C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A"> | |
617 | (carriage return followed by a line feed, sometimes called the network | |
618 | newline; it's the end of line sequence used in Microsoft text files opened | |
1978b668 | 619 | in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The |
040ac264 FC |
620 | reason it doesn't backtrack is that the sequence is considered |
621 | inseparable. That means that | |
1978b668 KW |
622 | |
623 | "\x0D\x0A" =~ /^\R\x0A$/ # No match | |
624 | ||
625 | fails, because the C<\R> matches the entire string, and won't backtrack | |
626 | to match just the C<"\x0D">.) Since | |
b6538e4f TC |
627 | C<\R> can match a sequence of more than one character, it cannot be put |
628 | inside a bracketed character class; C</[\R]/> is an error; use C<\v> | |
629 | instead. C<\R> was introduced in perl 5.10.0. | |
8a118206 | 630 | |
8129baca KW |
631 | Note that this does not respect any locale that might be in effect; it |
632 | matches according to the platform's native character set. | |
633 | ||
10fdd326 JH |
634 | Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>, |
635 | and more importantly because Unicode recommends such a regular expression | |
b6538e4f | 636 | metacharacter, and suggests C<\R> as its notation. |
8a118206 RGS |
637 | |
638 | =item \X | |
6b46370c | 639 | X<\X> |
8a118206 | 640 | |
0111a78f | 641 | This matches a Unicode I<extended grapheme cluster>. |
8a118206 | 642 | |
10fdd326 | 643 | C<\X> matches quite well what normal (non-Unicode-programmer) usage |
0111a78f | 644 | would consider a single character. As an example, consider a G with some sort |
c670e63a | 645 | of diacritic mark, such as an arrow. There is no such single character in |
df225385 | 646 | Unicode, but one can be composed by using a G followed by a Unicode "COMBINING |
c670e63a KW |
647 | UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it |
648 | were a single character. | |
10fdd326 | 649 | |
aa9e685b KW |
650 | The match is greedy and non-backtracking, so that the cluster is never |
651 | broken up into smaller components. | |
652 | ||
8a118206 RGS |
653 | Mnemonic: eI<X>tended Unicode character. |
654 | ||
655 | =back | |
656 | ||
657 | =head4 Examples | |
658 | ||
f822d0dd | 659 | $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz' |
d8b950dc | 660 | $str =~ s/(.)\K\g1//g; # Delete duplicated characters. |
8a118206 RGS |
661 | |
662 | "\n" =~ /^\R$/; # Match, \n is a generic newline. | |
663 | "\r" =~ /^\R$/; # Match, \r is a generic newline. | |
664 | "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline. | |
665 | ||
b6538e4f | 666 | "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above. |
8a118206 RGS |
667 | |
668 | =cut |