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.
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 \p{}, \pP Character with the given Unicode property.
90 \P{}, \PP Character without the given Unicode property.
91 \Q Quotemeta till \E. Not in [].
93 \R Generic new line. Not in [].
94 \s Character class for whitespace.
95 \S Character class for non whitespace.
97 \u Titlecase next character. Not in [].
98 \U Uppercase till \E. Not in [].
99 \v Character class for vertical whitespace.
100 \V Character class for non vertical whitespace.
101 \w Character class for word characters.
102 \W Character class for non-word characters.
103 \x{}, \x00 Hexadecimal escape sequence.
104 \X Unicode "extended grapheme cluster". Not in [].
105 \z End of string. Not in [].
106 \Z End of string. Not in [].
108 =head2 Character Escapes
110 =head3 Fixed characters
112 A handful of characters have a dedicated I<character escape>. The following
113 table shows them, along with their ASCII code points (in decimal and hex),
114 their ASCII name, the control escape on ASCII platforms and a short
115 description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
117 Seq. Code Point ASCII Cntrl Description.
119 \a 7 07 BEL \cG alarm or bell
120 \b 8 08 BS \cH backspace [1]
121 \e 27 1B ESC \c[ escape character
122 \f 12 0C FF \cL form feed
123 \n 10 0A LF \cJ line feed [2]
124 \r 13 0D CR \cM carriage return
131 C<\b> is only the backspace character inside a character class. Outside a
132 character class, C<\b> is a word/non-word boundary.
136 C<\n> matches a logical newline. Perl will convert between C<\n> and your
137 OSses native newline character when reading from or writing to text files.
143 $str =~ /\t/; # Matches if $str contains a (horizontal) tab.
145 =head3 Control characters
147 C<\c> is used to denote a control character; the character following C<\c>
148 determines the value of the construct. For example the value of C<\cA> is
149 C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
150 The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
151 list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
152 L<perlebcdic/OPERATOR DIFFERENCES>.
154 Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
155 string) is not valid. The backslash must be followed by another character.
156 That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
158 To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
159 C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
161 Mnemonic: I<c>ontrol character.
165 $str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
167 =head3 Named or numbered characters
169 All Unicode characters have a Unicode name and numeric ordinal value. Use the
170 C<\N{}> construct to specify a character by either of these values.
172 To specify by name, the name of the character goes between the curly braces.
173 In this case, you have to C<use charnames> to load the Unicode names of the
174 characters, otherwise Perl will complain.
176 To specify by Unicode ordinal number, use the form
177 C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
178 hexadecimal that gives the ordinal number that Unicode has assigned to the
179 desired character. It is customary (but not required) to use leading zeros to
180 pad the number to 4 digits. Thus C<\N{U+0041}> means
181 C<Latin Capital Letter A>, and you will rarely see it written without the two
182 leading zeros. C<\N{U+0041}> means C<A> even on EBCDIC machines (where the
183 ordinal value of C<A> is not 0x41).
185 It is even possible to give your own names to characters, and even to short
186 sequences of characters. For details, see L<charnames>.
188 (There is an expanded internal form that you may see in debug output:
189 C<\N{U+I<wide hex character>.I<wide hex character>...}>.
190 The C<...> means any number of these I<wide hex character>s separated by dots.
191 This represents the sequence formed by the characters. This is an internal
192 form only, subject to change, and you should not try to use it yourself.)
194 Mnemonic: I<N>amed character.
196 Note that a character that is expressed as a named or numbered character is
197 considered as a character without special meaning by the regex engine, and will
202 use charnames ':full'; # Loads the Unicode names.
203 $str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
205 use charnames 'Cyrillic'; # Loads Cyrillic names.
206 $str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
210 Octal escapes consist of a backslash followed by two or three octal digits
211 matching the code point of the character you want to use. This allows for
212 512 characters (C<\00> up to C<\777>) that can be expressed this way (but
213 anything above C<\377> is deprecated).
214 Enough in pre-Unicode days, but most Unicode characters cannot be escaped
217 Note that a character that is expressed as an octal escape is considered
218 as a character without special meaning by the regex engine, and will match
221 =head4 Examples (assuming an ASCII platform)
224 $str =~ /\120/; # Match, "\120" is "P".
225 $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
226 $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
230 Octal escapes potentially clash with backreferences. They both consist
231 of a backslash followed by numbers. So Perl has to use heuristics to
232 determine whether it is a backreference or an octal escape. Perl uses
239 If the backslash is followed by a single digit, it's a backreference.
243 If the first digit following the backslash is a 0, it's an octal escape.
247 If the number following the backslash is N (decimal), and Perl already has
248 seen N capture groups, Perl will consider this to be a backreference.
249 Otherwise, it will consider it to be an octal escape. Note that if N > 999,
250 Perl only takes the first three digits for the octal escape; the rest is
256 /^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
257 /^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
258 # and \1000 is seen as \100 (a '@') and a '0'.
262 =head3 Hexadecimal escapes
264 Hexadecimal escapes start with C<\x> and are then either followed by a
265 two digit hexadecimal number, or a hexadecimal number of arbitrary length
266 surrounded by curly braces. The hexadecimal number is the code point of
267 the character you want to express.
269 Note that a character that is expressed as a hexadecimal escape is considered
270 as a character without special meaning by the regex engine, and will match
273 Mnemonic: heI<x>adecimal.
275 =head4 Examples (assuming an ASCII platform)
278 $str =~ /\x50/; # Match, "\x50" is "P".
279 $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
280 $str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
282 /\x{2603}\x{2602}/ # Snowman with an umbrella.
283 # The Unicode character 2603 is a snowman,
284 # the Unicode character 2602 is an umbrella.
285 /\x{263B}/ # Black smiling face.
286 /\x{263b}/ # Same, the hex digits A - F are case insensitive.
290 A number of backslash sequences have to do with changing the character,
291 or characters following them. C<\l> will lowercase the character following
292 it, while C<\u> will uppercase (or, more accurately, titlecase) the
293 character following it. (They perform similar functionality as the
294 functions C<lcfirst> and C<ucfirst>).
296 To uppercase or lowercase several characters, one might want to use
297 C<\L> or C<\U>, which will lowercase/uppercase all characters following
298 them, until either the end of the pattern, or the next occurrence of
299 C<\E>, whatever comes first. They perform similar functionality as the
300 functions C<lc> and C<uc> do.
302 C<\Q> is used to escape all characters following, up to the next C<\E>
303 or the end of the pattern. C<\Q> adds a backslash to any character that
304 isn't a letter, digit or underscore. This will ensure that any character
305 between C<\Q> and C<\E> is matched literally, and will not be interpreted
306 by the regexp engine.
308 Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
314 $miranda = "(Miranda)";
315 $str =~ /\u$sid/; # Matches 'Sid'
316 $str =~ /\L$greg/; # Matches 'greg'
317 $str =~ /\Q$miranda\E/; # Matches '(Miranda)', as if the pattern
318 # had been written as /\(Miranda\)/
320 =head2 Character classes
322 Perl regular expressions have a large range of character classes. Some of
323 the character classes are written as a backslash sequence. We will briefly
324 discuss those here; full details of character classes can be found in
327 C<\w> is a character class that matches any single I<word> character (letters,
328 digits, underscore). C<\d> is a character class that matches any decimal digit,
329 while the character class C<\s> matches any whitespace character.
330 New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
331 and vertical whitespace characters.
333 The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
334 character classes that match any character that isn't a word character,
335 digit, whitespace, horizontal whitespace nor vertical whitespace.
337 Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
339 =head3 Unicode classes
341 C<\pP> (where C<P> is a single letter) and C<\p{Property}> are used to
342 match a character that matches the given Unicode property; properties
343 include things like "letter", or "thai character". Capitalizing the
344 sequence to C<\PP> and C<\P{Property}> make the sequence match a character
345 that doesn't match the given Unicode property. For more details, see
346 L<perlrecharclass/Backslash sequences> and
347 L<perlunicode/Unicode Character Properties>.
349 Mnemonic: I<p>roperty.
354 If capturing parenthesis are used in a regular expression, we can refer
355 to the part of the source string that was matched, and match exactly the
356 same thing. There are three ways of referring to such I<backreference>:
357 absolutely, relatively, and by name.
359 =for later add link to perlrecapture
361 =head3 Absolute referencing
363 A backslash sequence that starts with a backslash and is followed by a
364 number is an absolute reference (but be aware of the caveat mentioned above).
365 If the number is I<N>, it refers to the Nth set of parentheses - whatever
366 has been matched by that set of parenthesis has to be matched by the C<\N>
371 /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
372 /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
375 =head3 Relative referencing
377 New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>.
378 C<\g> takes a number as argument, with the number in curly braces (the
379 braces are optional). If the number (N) does not have a sign, it's a reference
380 to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
381 C<\g> always refers to a capture group and will never be seen as an octal
382 escape). If the number is negative, the reference is relative, referring to
383 the Nth group before the C<\g{-N}>.
385 The big advantage of C<\g{-N}> is that it makes it much easier to write
386 patterns with references that can be interpolated in larger patterns,
387 even if the larger pattern also contains capture groups.
396 \g{-1} # Refers to buffer 3 (B)
397 \g{-3} # Refers to buffer 1 (A)
399 /x; # Matches "ABBA".
401 my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches 'abab', 'cdcd', etc.
402 /$qr$qr/ # Matches 'ababcdcd'.
404 =head3 Named referencing
406 Also new in perl 5.10.0 is the use of named capture buffers, which can be
407 referred to by name. This is done with C<\g{name}>, which is a
408 backreference to the capture buffer with the name I<name>.
410 To be compatible with .Net regular expressions, C<\g{name}> may also be
411 written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
413 Note that C<\g{}> has the potential to be ambiguous, as it could be a named
414 reference, or an absolute or relative reference (if its argument is numeric).
415 However, names are not allowed to start with digits, nor are they allowed to
416 contain a hyphen, so there is no ambiguity.
420 /(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
421 /(?<word>\w+) \k{word}/ # Same.
422 /(?<word>\w+) \k<word>/ # Same.
423 /(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
424 # Match a four letter palindrome (e.g. "ABBA")
428 Assertions are conditions that have to be true; they don't actually
429 match parts of the substring. There are six assertions that are written as
436 C<\A> only matches at the beginning of the string. If the C</m> modifier
437 isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
438 modifier is used, then C</^/> matches internal newlines, but the meaning
439 of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
440 of the string regardless whether the C</m> modifier is used.
444 C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
445 used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
446 end of the string, or before the newline at the end of the string. If the
447 C</m> modifier is used, then C</$/> matches at internal newlines, but the
448 meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
449 the end of the string (or just before a trailing newline) regardless whether
450 the C</m> modifier is used.
452 C<\z> is just like C<\Z>, except that it will not match before a trailing
453 newline. C<\z> will only match at the end of the string - regardless of the
454 modifiers used, and not before a newline.
458 C<\G> is usually only used in combination with the C</g> modifier. If the
459 C</g> modifier is used (and the match is done in scalar context), Perl will
460 remember where in the source string the last match ended, and the next time,
461 it will start the match from where it ended the previous time.
463 C<\G> matches the point where the previous match ended, or the beginning
464 of the string if there was no previous match.
466 =for later add link to perlremodifiers
472 C<\b> matches at any place between a word and a non-word character; C<\B>
473 matches at any place between characters where C<\b> doesn't match. C<\b>
474 and C<\B> assume there's a non-word character before the beginning and after
475 the end of the source string; so C<\b> will match at the beginning (or end)
476 of the source string if the source string begins (or ends) with a word
477 character. Otherwise, C<\B> will match.
479 Mnemonic: I<b>oundary.
485 "cat" =~ /\Acat/; # Match.
486 "cat" =~ /cat\Z/; # Match.
487 "cat\n" =~ /cat\Z/; # Match.
488 "cat\n" =~ /cat\z/; # No match.
490 "cat" =~ /\bcat\b/; # Matches.
491 "cats" =~ /\bcat\b/; # No match.
492 "cat" =~ /\bcat\B/; # No match.
493 "cats" =~ /\bcat\B/; # Match.
495 while ("cat dog" =~ /(\w+)/g) {
496 print $1; # Prints 'catdog'
498 while ("cat dog" =~ /\G(\w+)/g) {
499 print $1; # Prints 'cat'
504 Here we document the backslash sequences that don't fall in one of the
505 categories above. They are:
511 C<\C> always matches a single octet, even if the source string is encoded
512 in UTF-8 format, and the character to be matched is a multi-octet character.
513 C<\C> was introduced in perl 5.6.
519 This is new in perl 5.10.0. Anything that is matched left of C<\K> is
520 not included in C<$&> - and will not be replaced if the pattern is
521 used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
522 instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
528 This is a new experimental feature in perl 5.12.0. It matches any character
529 that is not a newline. It is a short-hand for writing C<[^\n]>, and is
530 identical to the C<.> metasymbol, except under the C</s> flag, which changes
531 the meaning of C<.>, but not C<\N>.
533 Note that C<\N{...}> can mean a
534 L<named or numbered character|/Named or numbered characters>.
536 Mnemonic: Complement of I<\n>.
541 C<\R> matches a I<generic newline>, that is, anything that is considered
542 a newline by Unicode. This includes all characters matched by C<\v>
543 (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
544 (carriage return followed by a line feed, aka the network newline, or
545 the newline used in Windows text files). C<\R> is equivalent to
546 C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
547 character, it cannot be put inside a bracketed character class; C</[\R]/> is an
548 error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
550 Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
551 and more importantly because Unicode recommends such a regular expression
552 metacharacter, and suggests C<\R> as the notation.
557 This matches a Unicode I<extended grapheme cluster>.
559 C<\X> matches quite well what normal (non-Unicode-programmer) usage
560 would consider a single character. As an example, consider a G with some sort
561 of diacritic mark, such as an arrow. There is no such single character in
562 Unicode, but one can be composed by using a G followed by a Unicode "COMBINING
563 UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
564 were a single character.
566 Mnemonic: eI<X>tended Unicode character.
572 "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
574 $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
575 $str =~ s/(.)\K\1//g; # Delete duplicated characters.
577 "\n" =~ /^\R$/; # Match, \n is a generic newline.
578 "\r" =~ /^\R$/; # Match, \r is a generic newline.
579 "\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
581 "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.