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.
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.
26 The rules determining what it is are quite simple: if the character
27 following the backslash is an ASCII punctuation (non-word) character (that is,
28 anything that is not a letter, digit or underscore), then the backslash just
29 takes away the special meaning (if any) of the character following it.
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
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
35 you have warnings turned on, Perl will issue a warning if you use such a
38 It is however guaranteed that backslash or escape sequences never have a
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
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
51 There is one exception. If you use an alphanumerical character as the
52 delimiter of your pattern (which you probably shouldn't do for readability
53 reasons), you will have to escape the delimiter if you want to match
54 it. Perl won't warn then. See also L<perlop/Gory details of parsing
60 =head2 All the sequences and escapes
62 Those not usable within a bracketed character class (like C<[\da-z]>) are marked
65 \000 Octal escape sequence. See also \o{}.
66 \1 Absolute backreference. Not in [].
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 [].
72 \C Single octet, even under UTF-8. Not in [].
73 \d Character class for digits.
74 \D Character class for non-digits.
76 \E Turn off \Q, \L and \U processing. Not in [].
78 \g{}, \g1 Named, absolute or relative backreference. Not in []
79 \G Pos assertion. Not in [].
80 \h Character class for horizontal whitespace.
81 \H Character class for non horizontal whitespace.
82 \k{}, \k<>, \k'' Named backreference. Not in [].
83 \K Keep the stuff left of \K. Not in [].
84 \l Lowercase next character. Not in [].
85 \L Lowercase till \E. Not in [].
86 \n (Logical) newline character.
87 \N Any character but newline. Experimental. Not in [].
88 \N{} Named or numbered (Unicode) character.
89 \o{} Octal escape sequence.
90 \p{}, \pP Character with the given Unicode property.
91 \P{}, \PP Character without the given Unicode property.
92 \Q Quotemeta till \E. Not in [].
94 \R Generic new line. Not in [].
95 \s Character class for whitespace.
96 \S Character class for non whitespace.
98 \u Titlecase next character. Not in [].
99 \U Uppercase till \E. Not in [].
100 \v Character class for vertical whitespace.
101 \V Character class for non vertical whitespace.
102 \w Character class for word characters.
103 \W Character class for non-word characters.
104 \x{}, \x00 Hexadecimal escape sequence.
105 \X Unicode "extended grapheme cluster". Not in [].
106 \z End of string. Not in [].
107 \Z End of string. Not in [].
109 =head2 Character Escapes
111 =head3 Fixed characters
113 A handful of characters have a dedicated I<character escape>. The following
114 table shows them, along with their ASCII code points (in decimal and hex),
115 their ASCII name, the control escape on ASCII platforms and a short
116 description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
118 Seq. Code Point ASCII Cntrl Description.
120 \a 7 07 BEL \cG alarm or bell
121 \b 8 08 BS \cH backspace [1]
122 \e 27 1B ESC \c[ escape character
123 \f 12 0C FF \cL form feed
124 \n 10 0A LF \cJ line feed [2]
125 \r 13 0D CR \cM carriage return
132 C<\b> is the backspace character only inside a character class. Outside a
133 character class, C<\b> is a word/non-word boundary.
137 C<\n> matches a logical newline. Perl will convert between C<\n> and your
138 OS's native newline character when reading from or writing to text files.
144 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
146 =head3 Control characters
148 C<\c> is used to denote a control character; the character following C<\c>
149 determines the value of the construct. For example the value of C<\cA> is
150 C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
151 The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
152 list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
153 L<perlebcdic/OPERATOR DIFFERENCES>.
155 Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
156 string) is not valid. The backslash must be followed by another character.
157 That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
159 To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
160 C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
162 Mnemonic: I<c>ontrol character.
166 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
168 =head3 Named or numbered characters
170 Unicode characters have a Unicode name and numeric ordinal value. Use the
171 C<\N{}> construct to specify a character by either of these values.
173 To specify by name, the name of the character goes between the curly braces.
174 In this case, you have to C<use charnames> to load the Unicode names of the
175 characters, otherwise Perl will complain.
177 To specify by Unicode ordinal number, use the form
178 C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
179 hexadecimal that gives the ordinal number that Unicode has assigned to the
180 desired character. It is customary (but not required) to use leading zeros to
181 pad the number to 4 digits. Thus C<\N{U+0041}> means
182 C<Latin Capital Letter A>, and you will rarely see it written without the two
183 leading zeros. C<\N{U+0041}> means "A" even on EBCDIC machines (where the
184 ordinal value of "A" is not 0x41).
186 It is even possible to give your own names to characters, and even to short
187 sequences of characters. For details, see L<charnames>.
189 (There is an expanded internal form that you may see in debug output:
190 C<\N{U+I<wide hex character>.I<wide hex character>...}>.
191 The C<...> means any number of these I<wide hex character>s separated by dots.
192 This represents the sequence formed by the characters. This is an internal
193 form only, subject to change, and you should not try to use it yourself.)
195 Mnemonic: I<N>amed character.
197 Note that a character that is expressed as a named or numbered character is
198 considered as a character without special meaning by the regex engine, and will
203 use charnames ':full'; # Loads the Unicode names.
204 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
206 use charnames 'Cyrillic'; # Loads Cyrillic names.
207 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
211 There are two forms of octal escapes. Each is used to specify a character by
212 its ordinal, specified in octal notation.
214 One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
215 represent one or more octal digits. It can be used for any Unicode character.
217 It was introduced to avoid the potential problems with the other form,
218 available in all Perls. That form consists of a backslash followed by three
219 octal digits. One problem with this form is that it can look exactly like an
220 old-style backreference (see
221 L</Disambiguation rules between old-style octal escapes and backreferences>
222 below.) You can avoid this by making the first of the three digits always a
223 zero, but that makes \077 the largest ordinal unambiguously specifiable by this
226 In some contexts, a backslash followed by two or even one octal digits may be
227 interpreted as an octal escape, sometimes with a warning, and because of some
228 bugs, sometimes with surprising results. Also, if you are creating a regex
229 out of smaller snippets concatentated together, and you use fewer than three
230 digits, the beginning of one snippet may be interpreted as adding digits to the
231 ending of the snippet before it. See L</Absolute referencing> for more
232 discussion and examples of the snippet problem.
234 Note that a character that is expressed as an octal escape is considered
235 as a character without special meaning by the regex engine, and will match
238 To summarize, the C<\o{}> form is always safe to use, and the other form is
239 safe to use for ordinals up through \077 when you use exactly three digits to
242 Mnemonic: I<0>ctal or I<o>ctal.
244 =head4 Examples (assuming an ASCII platform)
247 $str =~ /\o{120}/; # Match, "\120" is "P".
248 $str =~ /\120/; # Same.
249 $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
250 $str =~ /\120+/; # Same.
251 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
252 /\o{23073}/ # Black foreground, white background smiling face.
253 /\o{4801234567}/ # Raises a warning, and yields chr(4)
255 =head4 Disambiguation rules between old-style octal escapes and backreferences
257 Octal escapes of the C<\000> form outside of bracketed character classes
258 potentially clash with old-style backreferences. (see L</Absolute referencing>
259 below). They both consist of a backslash followed by numbers. So Perl has to
260 use heuristics to determine whether it is a backreference or an octal escape.
261 Perl uses the following rules to disambiguate:
267 If the backslash is followed by a single digit, it's a backreference.
271 If the first digit following the backslash is a 0, it's an octal escape.
275 If the number following the backslash is N (in decimal), and Perl already has
276 seen N capture groups, Perl will consider this to be a backreference.
277 Otherwise, it will consider it to be an octal escape. Note that if N has more
278 than three digits, Perl only takes the first three for the octal escape;
279 the rest are matched as is.
284 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
285 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
286 # and \1000 is seen as \100 (a '@') and a '0'
290 You can the force a backreference interpretation always by using the C<\g{...}>
291 form. You can the force an octal interpretation always by using the C<\o{...}>
292 form, or for numbers up through \077 (= 63 decimal), by using three digits,
293 beginning with a "0".
295 =head3 Hexadecimal escapes
297 Like octal escapes, there are two forms of hexadecimal escapes, but both start
298 with the same thing, C<\x>. This is followed by either exactly two hexadecimal
299 digits forming a number, or a hexadecimal number of arbitrary length surrounded
300 by curly braces. The hexadecimal number is the code point of the character you
303 Note that a character that is expressed as one of these escapes is considered
304 as a character without special meaning by the regex engine, and will match
307 Mnemonic: heI<x>adecimal.
309 =head4 Examples (assuming an ASCII platform)
312 $str =~ /\x50/; # Match, "\x50" is "P".
313 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
314 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
316 /\x{2603}\x{2602}/ # Snowman with an umbrella.
317 # The Unicode character 2603 is a snowman,
318 # the Unicode character 2602 is an umbrella.
319 /\x{263B}/ # Black smiling face.
320 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
324 A number of backslash sequences have to do with changing the character,
325 or characters following them. C<\l> will lowercase the character following
326 it, while C<\u> will uppercase (or, more accurately, titlecase) the
327 character following it. (They perform similar functionality as the
328 functions C<lcfirst> and C<ucfirst>).
330 To uppercase or lowercase several characters, one might want to use
331 C<\L> or C<\U>, which will lowercase/uppercase all characters following
332 them, until either the end of the pattern, or the next occurrence of
333 C<\E>, whatever comes first. They perform similar functionality as the
334 functions C<lc> and C<uc> do.
336 C<\Q> is used to escape all characters following, up to the next C<\E>
337 or the end of the pattern. C<\Q> adds a backslash to any character that
338 isn't a letter, digit or underscore. This will ensure that any character
339 between C<\Q> and C<\E> is matched literally, and will not be interpreted
340 by the regexp engine.
342 Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
348 $miranda = "(Miranda)";
349 $str =~ /\u$sid/; # Matches 'Sid'
350 $str =~ /\L$greg/; # Matches 'greg'
351 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
352 # had been written as /\(Miranda\)/
354 =head2 Character classes
356 Perl regular expressions have a large range of character classes. Some of
357 the character classes are written as a backslash sequence. We will briefly
358 discuss those here; full details of character classes can be found in
361 C<\w> is a character class that matches any single I<word> character (letters,
362 digits, underscore). C<\d> is a character class that matches any decimal digit,
363 while the character class C<\s> matches any whitespace character.
364 New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
365 and vertical whitespace characters.
367 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
368 character classes that match any character that isn't a word character,
369 digit, whitespace, horizontal whitespace nor vertical whitespace.
371 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
373 =head3 Unicode classes
375 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
376 match a character that matches the given Unicode property; properties
377 include things like "letter", or "thai character". Capitalizing the
378 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
379 that doesn't match the given Unicode property. For more details, see
380 L<perlrecharclass/Backslash sequences> and
381 L<perlunicode/Unicode Character Properties>.
383 Mnemonic: I<p>roperty.
387 If capturing parenthesis are used in a regular expression, we can refer
388 to the part of the source string that was matched, and match exactly the
389 same thing. There are three ways of referring to such I<backreference>:
390 absolutely, relatively, and by name.
392 =for later add link to perlrecapture
394 =head3 Absolute referencing
396 Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
397 is a positive (unsigned) decimal number of any length is an absolute reference
398 to a capturing group.
400 I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
401 been matched by that set of parentheses. Thus C<\g1> refers to the first
402 capture group in the regex.
404 The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
405 which avoids ambiguity when building a regex by concatenating shorter
406 strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
407 C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
408 probably not what you intended.
410 In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
411 least I<N> capturing groups, or else I<N> will be considered an octal escape
412 (but something like C<\18> is the same as C<\0018>, that is the octal escape
413 C<"\001"> followed by a literal digit C<"8">).
419 /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
420 /(\w+) \1/; # Same thing; written old-style
421 /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
424 =head3 Relative referencing
426 C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
427 be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
430 The big advantage of this form is that it makes it much easier to write
431 patterns with references that can be interpolated in larger patterns,
432 even if the larger pattern also contains capture groups.
439 \g{-1} # Refers to group 3 (B)
440 \g{-3} # Refers to group 1 (A)
442 /x; # Matches "ABBA".
444 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
445 /$qr$qr/ # Matches 'ababcdcd'.
447 =head3 Named referencing
449 C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
450 named capture group, dispensing completely with having to think about capture
453 To be compatible with .Net regular expressions, C<\g{name}> may also be
454 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
456 To prevent any ambiguity, I<name> must not start with a digit nor contain a
461 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
462 /(?<word>\w+) \k{word}/ # Same.
463 /(?<word>\w+) \k<word>/ # Same.
464 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
465 # Match a four letter palindrome (e.g. "ABBA")
469 Assertions are conditions that have to be true; they don't actually
470 match parts of the substring. There are six assertions that are written as
477 C<\A> only matches at the beginning of the string. If the C</m> modifier
478 isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
479 modifier is used, then C</^/> matches internal newlines, but the meaning
480 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
481 of the string regardless whether the C</m> modifier is used.
485 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
486 used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
487 end of the string, or before the newline at the end of the string. If the
488 C</m> modifier is used, then C</$/> matches at internal newlines, but the
489 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
490 the end of the string (or just before a trailing newline) regardless whether
491 the C</m> modifier is used.
493 C<\z> is just like C<\Z>, except that it will not match before a trailing
494 newline. C<\z> will only match at the end of the string - regardless of the
495 modifiers used, and not before a newline.
499 C<\G> is usually only used in combination with the C</g> modifier. If the
500 C</g> modifier is used (and the match is done in scalar context), Perl will
501 remember where in the source string the last match ended, and the next time,
502 it will start the match from where it ended the previous time.
504 C<\G> matches the point where the previous match ended, or the beginning
505 of the string if there was no previous match.
507 =for later add link to perlremodifiers
513 C<\b> matches at any place between a word and a non-word character; C<\B>
514 matches at any place between characters where C<\b> doesn't match. C<\b>
515 and C<\B> assume there's a non-word character before the beginning and after
516 the end of the source string; so C<\b> will match at the beginning (or end)
517 of the source string if the source string begins (or ends) with a word
518 character. Otherwise, C<\B> will match.
520 Mnemonic: I<b>oundary.
526 "cat" =~ /\Acat/; # Match.
527 "cat" =~ /cat\Z/; # Match.
528 "cat\n" =~ /cat\Z/; # Match.
529 "cat\n" =~ /cat\z/; # No match.
531 "cat" =~ /\bcat\b/; # Matches.
532 "cats" =~ /\bcat\b/; # No match.
533 "cat" =~ /\bcat\B/; # No match.
534 "cats" =~ /\bcat\B/; # Match.
536 while ("cat dog" =~ /(\w+)/g) {
537 print $1; # Prints 'catdog'
539 while ("cat dog" =~ /\G(\w+)/g) {
540 print $1; # Prints 'cat'
545 Here we document the backslash sequences that don't fall in one of the
546 categories above. They are:
552 C<\C> always matches a single octet, even if the source string is encoded
553 in UTF-8 format, and the character to be matched is a multi-octet character.
554 C<\C> was introduced in perl 5.6.
560 This is new in perl 5.10.0. Anything that is matched left of C<\K> is
561 not included in C<$&> - and will not be replaced if the pattern is
562 used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
563 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
569 This is a new experimental feature in perl 5.12.0. It matches any character
570 that is not a newline. It is a short-hand for writing C<[^\n]>, and is
571 identical to the C<.> metasymbol, except under the C</s> flag, which changes
572 the meaning of C<.>, but not C<\N>.
574 Note that C<\N{...}> can mean a
575 L<named or numbered character|/Named or numbered characters>.
577 Mnemonic: Complement of I<\n>.
582 C<\R> matches a I<generic newline>, that is, anything that is considered
583 a newline by Unicode. This includes all characters matched by C<\v>
584 (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
585 (carriage return followed by a line feed, aka the network newline, or
586 the newline used in Windows text files). C<\R> is equivalent to
587 C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
588 character, it cannot be put inside a bracketed character class; C</[\R]/> is an
589 error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
591 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
592 and more importantly because Unicode recommends such a regular expression
593 metacharacter, and suggests C<\R> as the notation.
598 This matches a Unicode I<extended grapheme cluster>.
600 C<\X> matches quite well what normal (non-Unicode-programmer) usage
601 would consider a single character. As an example, consider a G with some sort
602 of diacritic mark, such as an arrow. There is no such single character in
603 Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
604 UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
605 were a single character.
607 Mnemonic: eI<X>tended Unicode character.
613 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
615 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
616 $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
618 "\n" =~ /^\R$/; # Match, \n is a generic newline.
619 "\r" =~ /^\R$/; # Match, \r is a generic newline.
620 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
622 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.