3 perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
7 The top level documentation about Perl regular expressions
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.
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.
22 In a regular expression, the backslash can perform one of two tasks:
23 it either takes away the special meaning of the character following it
24 (for instance, C<\|> matches a vertical bar, it's not an alternation),
25 or it is the start of a backslash or escape sequence.
27 The rules determining what it is are quite simple: if the character
28 following the backslash is a punctuation (non-word) character (that is,
29 anything that is not a letter, digit or underscore), then the backslash
30 just takes away the special meaning (if any) of the character following
33 If the character following the backslash is a letter or a digit, then the
34 sequence may be special; if so, it's listed below. A few letters have not
35 been used yet, and escaping them with a backslash is safe for now, but a
36 future version of Perl may assign a special meaning to it. However, if you
37 have warnings turned on, Perl will issue a warning if you use such a sequence.
40 It is however garanteed that backslash or escape sequences never have a
41 punctuation character following the backslash, not now, and not in a future
42 version of Perl 5. So it is safe to put a backslash in front of a non-word
45 Note that the backslash itself is special; if you want to match a backslash,
46 you have to escape the backslash with a backslash: C</\\/> matches a single
53 There is one exception. If you use an alphanumerical character as the
54 delimiter of your pattern (which you probably shouldn't do for readability
55 reasons), you will have to escape the delimiter if you want to match
56 it. Perl won't warn then. See also L<perlop/Gory details of parsing
62 =head2 All the sequences and escapes
64 \000 Octal escape sequence.
65 \1 Absolute backreference.
67 \A Beginning of string.
68 \b Word/non-word boundary. (Backspace in a char class).
69 \B Not a word/non-word boundary.
70 \cX Control-X (X can be any ASCII character).
71 \C Single octet, even under UTF-8.
72 \d Character class for digits.
73 \D Character class for non-digits.
75 \E Turn off \Q, \L and \U processing.
77 \g{}, \g1 Named, absolute or relative backreference.
79 \h Character class for horizontal white space.
80 \H Character class for non horizontal white space.
81 \k{}, \k<>, \k'' Named backreference.
82 \K Keep the stuff left of \K.
83 \l Lowercase next character.
85 \n (Logical) newline character.
86 \N{} Named (Unicode) character.
87 \p{}, \pP Character with a Unicode property.
88 \P{}, \PP Character without a Unicode property.
92 \s Character class for white space.
93 \S Character class for non white space.
95 \u Titlecase next character.
97 \v Character class for vertical white space.
98 \V Character class for non vertical white space.
99 \w Character class for word characters.
100 \W Character class for non-word characters.
101 \x{}, \x00 Hexadecimal escape sequence.
102 \X Extended Unicode "combining character sequence".
106 =head2 Character Escapes
108 =head3 Fixed characters
110 A handful of characters have a dedidated I<character escape>. The following
111 table shows them, along with their code points (in decimal and hex), their
112 ASCII name, the control escape (see below) and a short description.
114 Seq. Code Point ASCII Cntr Description.
116 \a 7 07 BEL \cG alarm or bell
117 \b 8 08 BS \cH backspace [1]
118 \e 27 1B ESC \c[ escape character
119 \f 12 0C FF \cL form feed
120 \n 10 0A LF \cJ line feed [2]
121 \r 13 0D CR \cM carriage return
128 C<\b> is only the backspace character inside a character class. Outside a
129 character class, C<\b> is a word/non-word boundary.
133 C<\n> matches a logical newline. Perl will convert between C<\n> and your
134 OSses native newline character when reading from or writing to text files.
140 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
142 =head3 Control characters
144 C<\c> is used to denote a control character; the character following C<\c>
145 is the name of the control character. For instance, C</\cM/> matches the
146 character I<control-M> (a carriage return, code point 13). The case of the
147 character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same
150 Mnemonic: I<c>ontrol character.
154 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
156 =head3 Named characters
158 All Unicode characters have a Unicode name, and characters in various scripts
159 have names as well. It is even possible to give your own names to characters.
160 You can use a character by name by using the C<\N{}> construct; the name of
161 the character goes between the curly braces. You do have to C<use charnames>
162 to load the names of the characters, otherwise Perl will complain you use
163 a name it doesn't know about. For more details, see L<charnames>.
165 Mnemonic: I<N>amed character.
169 use charnames ':full'; # Loads the Unicode names.
170 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
172 use charnames 'Cyrillic'; # Loads Cyrillic names.
173 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
177 Octal escapes consist of a backslash followed by two or three octal digits
178 matching the code point of the character you want to use. This allows for
179 522 characters (C<\00> up to C<\777>) that can be expressed this way.
180 Enough in pre-Unicode days, but most Unicode characters cannot be escaped
183 Note that a character that is expressed as an octal escape is considered
184 as a character without special meaning by the regex engine, and will match
190 $str =~ /\120/; # Match, "\120" is "P".
191 $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
192 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
196 Octal escapes potentially clash with backreferences. They both consist
197 of a backslash followed by numbers. So Perl has to use heuristics to
198 determine whether it is a backreference or an octal escape. Perl uses
205 If the backslash is followed by a single digit, it's a backrefence.
209 If the first digit following the backslash is a 0, it's an octal escape.
213 If the number following the backslash is N (decimal), and Perl already has
214 seen N capture groups, Perl will consider this to be a backreference.
215 Otherwise, it will consider it to be an octal escape. Note that if N > 999,
216 Perl only takes the first three digits for the octal escape; the rest is
222 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
223 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
224 # and \1000 is seen as \100 (a '@') and a '0'.
228 =head3 Hexadecimal escapes
230 Hexadecimal escapes start with C<\x> and are then either followed by
231 two digit hexadecimal number, or a hexadecimal number of arbitrary length
232 surrounded by curly braces. The hexadecimal number is the code point of
233 the character you want to express.
235 Note that a character that is expressed as a hexadecimal escape is considered
236 as a character without special meaning by the regex engine, and will match
239 Mnemonic: heI<x>adecimal.
244 $str =~ /\x50/; # Match, "\x50" is "P".
245 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
246 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
248 /\x{2603}\x{2602}/ # Snowman with an umbrella.
249 # The Unicode character 2603 is a snowman,
250 # the Unicode character 2602 is an umbrella.
251 /\x{263B}/ # Black smiling face.
252 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
256 A number of backslash sequences have to do with changing the character,
257 or characters following them. C<\l> will lowercase the character following
258 it, while C<\u> will uppercase (or, more accurately, titlecase) the
259 character following it. (They perform similar functionality as the
260 functions C<lcfirst> and C<ucfirst>).
262 To uppercase or lowercase several characters, one might want to use
263 C<\L> or C<\U>, which will lowercase/uppercase all characters following
264 them, until either the end of the pattern, or the next occurance of
265 C<\E>, whatever comes first. They perform similar functionality as the
266 functions C<lc> and C<uc> do.
268 C<\Q> is used to escape all characters following, up to the next C<\E>
269 or the end of the pattern. C<\Q> adds a backslash to any character that
270 isn't a letter, digit or underscore. This will ensure that any character
271 between C<\Q> and C<\E> is matched literally, and will not be interpreted
272 by the regexp engine.
274 Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
280 $miranda = "(Miranda)";
281 $str =~ /\u$sid/; # Matches 'Sid'
282 $str =~ /\L$greg/; # Matches 'greg'
283 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
284 # had been written as /\(Miranda\)/
286 =head2 Character classes
288 Perl regular expressions have a large range of character classes. Some of
289 the character classes are written as a backslash sequence. We will briefly
290 discuss those here; full details of character classes can be found in
293 C<\w> is a character class that matches any I<word> character (letters,
294 digits, underscore). C<\d> is a character class that matches any digit,
295 while the character class C<\s> matches any white space character.
296 New in perl 5.10 are the classes C<\h> and C<\v> which match horizontal
297 and vertical white space characters.
299 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
300 character classes that match any character that isn't a word character,
301 digit, white space, horizontal white space or vertical white space.
303 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
305 =head3 Unicode classes
307 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
308 match a character that matches the given Unicode property; properties
309 include things like "letter", or "thai character". Capitalizing the
310 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
311 that doesn't match the given Unicode property. For more details, see
312 L<perlrecharclass/Backslashed sequences> and
313 L<perlunicode/Unicode Character Properties>.
315 Mnemonic: I<p>roperty.
320 If capturing parenthesis are used in a regular expression, we can refer
321 to the part of the source string that was matched, and match exactly the
322 same thing. (Full details are discussed in L<perlrecapture>). There are
323 three ways of refering to such I<backreference>: absolutely, relatively,
326 =head3 Absolute referencing
328 A backslash sequence that starts with a backslash and is followed by a
329 number is an absolute reference (but be aware of the caveat mentioned above).
330 If the number is I<N>, it refers to the Nth set of parenthesis - whatever
331 has been matched by that set of parenthesis has to be matched by the C<\N>
336 /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
337 /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
340 =head3 Relative referencing
342 New in perl 5.10 is different way of refering to capture buffers: C<\g>.
343 C<\g> takes a number as argument, with the number in curly braces (the
344 braces are optional). If the number (N) does not have a sign, it's a reference
345 to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
346 C<\g> always refers to a capture group and will never be seen as an octal
347 escape). If the number is negative, the reference is relative, refering to
348 the Nth group before the C<\g{-N}>.
350 The big advantage of C<\g{-N}> is that it makes it much easier to write
351 patterns with references that can be interpolated in larger patterns,
352 even if the larger pattern also contains capture groups.
361 \g{-1} # Refers to buffer 3 (B)
362 \g{-3} # Refers to buffer 1 (A)
364 /x; # Matches "ABBA".
366 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
367 /$qr$qr/ # Matches 'ababcdcd'.
369 =head3 Named referencing
371 Also new in perl 5.10 is the use of named capture buffers, which can be
372 referred to by name. This is done with C<\g{name}>, which is a
373 backreference to the capture buffer with the name I<name>.
375 To be compatible with .Net regular expressions, C<\g{name}> may also be
376 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
378 Note that C<\g{}> has the potential to be ambiguous, as it could be a named
379 reference, or an absolute or relative reference (if its argument is numeric).
380 However, names are not allowed to start with digits, nor are allowed to
381 contain a hyphen, so there is no ambiguity.
385 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
386 /(?<word>\w+) \k{word}/ # Same.
387 /(?<word>\w+) \k<word>/ # Same.
388 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
389 # Match a four letter palindrome (e.g. "ABBA")
393 Assertions are conditions that have to be true -- they don't actually
394 match parts of the substring. There are six assertions that are written as
401 C<\A> only matches at the beginning of the string. If the C</m> modifier
402 isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
403 modifier is used, then C</^/> matches internal newlines, but the meaning
404 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
405 of the string regardless whether the C</m> modifier is used.
409 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
410 used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
411 end of the string, or before the newline at the end of the string. If the
412 C</m> modifier is used, then C</$/> matches at internal newlines, but the
413 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
414 the end of the string (or just before a trailing newline) regardless whether
415 the C</m> modifier is used.
417 C<\z> is just like C<\Z>, except that it will not match before a trailing
418 newline. C<\z> will only match at the end of the string - regardless of the
419 modifiers used, and not before a newline.
423 C<\G> is usually only used in combination with the C</g> modifier. If the
424 C</g> modifier is used (and the match is done in scalar context), Perl will
425 remember where in the source string the last match ended, and the next time,
426 it will start the match from where it ended the previous time.
428 C<\G> matches the point where the previous match ended, or the beginning
429 of the string if there was no previous match. See also L<perlremodifiers>.
435 C<\b> matches at any place between a word and a non-word character; C<\B>
436 matches at any place between characters where C<\b> doesn't match. C<\b>
437 and C<\B> assume there's a non-word character before the beginning and after
438 the end of the source string; so C<\b> will match at the beginning (or end)
439 of the source string if the source string begins (or ends) with a word
440 character. Otherwise, C<\B> will match.
442 Mnemonic: I<b>oundary.
448 "cat" =~ /\Acat/; # Match.
449 "cat" =~ /cat\Z/; # Match.
450 "cat\n" =~ /cat\Z/; # Match.
451 "cat\n" =~ /cat\z/; # No match.
453 "cat" =~ /\bcat\b/; # Matches.
454 "cats" =~ /\bcat\b/; # No match.
455 "cat" =~ /\bcat\B/; # No match.
456 "cats" =~ /\bcat\B/; # Match.
458 while ("cat dog" =~ /(\w+)/g) {
459 print $1; # Prints 'catdog'
461 while ("cat dog" =~ /\G(\w+)/g) {
462 print $1; # Prints 'cat'
467 Here we document the backslash sequences that don't fall in one of the
468 categories above. They are:
474 C<\C> always matches a single octet, even if the source string is encoded
475 in UTF-8 format, and the character to be matched is a multi-octet character.
476 C<\C> was introduced in perl 5.6.
482 This is new in perl 5.10. Anything that is matched left of C<\K> is
483 not included in C<$&> - and will not be replaced if the pattern is
484 used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
485 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
491 C<\R> matches a I<generic newline>, that is, anything that is considered
492 a newline by Unicode. This includes all characters matched by C<\v>
493 (vertical white space), and the multi character sequence C<"\x0D\x0A">
494 (carriage return followed by a line feed, aka the network newline, or
495 the newline used in Windows text files). C<\R> is equivalent with
496 C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a more than one character,
497 it cannot be put inside a bracketed character class; C</[\R]/> is an error.
498 C<\R> is introduced in perl 5.10.
500 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>.
504 This matches an extended Unicode I<combining character sequence>, and
505 is equivalent to C<< (?>\PM\pM*) >>. C<\PM> matches any character that is
506 not considered a Unicode mark character, while C<\pM> matches any character
507 that is considered a Unicode mark character; so C<\X> matches any non
508 mark character followed by zero or more mark characters. Mark characters
509 include (but are not restricted to) I<combining characters> and
512 Mnemonic: eI<X>tended Unicode character.
518 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
520 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
521 $str =~ s/(.)\K\1//g; # Delete duplicated characters.
523 "\n" =~ /^\R$/; # Match, \n is a generic newline.
524 "\r" =~ /^\R$/; # Match, \r is a generic newline.
525 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
527 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.