NAME
perlrebackslash - Perl Regular Expression Backslash Sequences and Escapes
DESCRIPTION
The top level documentation about Perl regular expressions is found in perlre.
This document describes all backslash and escape sequences. After explaining the role of the backslash, it lists all the sequences that have a special meaning in Perl regular expressions (in alphabetical order), then describes each of them.
Most sequences are described in detail in different documents; the primary purpose of this document is to have a quick reference guide describing all backslash and escape sequences.
The backslash
In a regular expression, the backslash can perform one of two tasks: it either takes away the special meaning of the character following it (for instance,
\|matches a vertical bar, its not an alternation), or it is the start of a backslash or escape sequence.
The rules determining what it is are quite simple: if the character following the backslash is a punctuation (non-word) character (that is, anything that is not a letter, digit or underscore), then the backslash just takes away the special meaning (if any) of the character following it.
If the character following the backslash is a letter or a digit, then the sequence may be special; if so, its listed below. A few letters have not been used yet, and escaping them with a backslash is safe for now, but a future version of Perl may assign a special meaning to it. However, if you have warnings turned on, Perl will issue a warning if you use such a sequence. [1].
It is however guaranteed that backslash or escape sequences never have a punctuation character following the backslash, not now, and not in a future version of Perl 5. So it is safe to put a backslash in front of a non-word character.
Note that the backslash itself is special; if you want to match a backslash, you have to escape the backslash with a backslash:
/\\/matches a single backslash.
[1] | There is one exception. If you use an alphanumerical character as the delimiter of your pattern (which you probably shouldnt do for readability reasons), you will have to escape the delimiter if you want to match it. Perl wont warn then. See also Gory details of parsing quoted constructs in perlop. |
All the sequences and escapes
\000 Octal escape sequence.
\1 Absolute backreference.
\a Alarm or bell.
\A Beginning of string.
\b Word/non-word boundary. (Backspace in a char class).
\B Not a word/non-word boundary.
\cX Control-X (X can be any ASCII character).
\C Single octet, even under UTF-8.
\d Character class for digits.
\D Character class for non-digits.
\e Escape character.
\E Turn off \Q, \L and \U processing.
\f Form feed.
\g{}, \g1 Named, absolute or relative backreference.
\G Pos assertion.
\h Character class for horizontal white space.
\H Character class for non horizontal white space.
\k{}, \k<>, \k Named backreference.
\K Keep the stuff left of \K.
\l Lowercase next character.
\L Lowercase till \E.
\n (Logical) newline character.
\N{} Named (Unicode) character.
\p{}, \pP Character with a Unicode property.
\P{}, \PP Character without a Unicode property.
\Q Quotemeta till \E.
\r Return character.
\R Generic new line.
\s Character class for white space.
\S Character class for non white space.
\t Tab character.
\u Titlecase next character.
\U Uppercase till \E.
\v Character class for vertical white space.
\V Character class for non vertical white space.
\w Character class for word characters.
\W Character class for non-word characters.
\x{}, \x00 Hexadecimal escape sequence.
\X Extended Unicode "combining character sequence".
\z End of string.
\Z End of string.
Character Escapes
Fixed characters
A handful of characters have a dedicated character escape. The following table shows them, along with their code points (in decimal and hex), their ASCII name, the control escape (see below) and a short description.
Seq. Code Point ASCII Cntr Description.
Dec Hex
\a 7 07 BEL \cG alarm or bell
\b 8 08 BS \cH backspace [1]
\e 27 1B ESC \c[ escape character
\f 12 0C FF \cL form feed
\n 10 0A LF \cJ line feed [2]
\r 13 0D CR \cM carriage return
\t 9 09 TAB \cI tab
[1] |
\bis only the backspace character inside a character class. Outside a character class, \bis a word/non-word boundary. |
[2] |
\nmatches a logical newline. Perl will convert between \nand your OSses native newline character when reading from or writing to text files. |
$str =~ /\t/; # Matches if $str contains a (horizontal) tab.
Control characters
\cis used to denote a control character; the character following
\cis the name of the control character. For instance,
/\cM/matches the character control-M (a carriage return, code point 13). The case of the character following
\cdoesnt matter:
\cMand
\cmmatch the same character.
Mnemonic: control character.
Example
$str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
Named characters
All Unicode characters have a Unicode name, and characters in various scripts have names as well. It is even possible to give your own names to characters. You can use a character by name by using the
\N{}construct; the name of the character goes between the curly braces. You do have to
use charnamesto load the names of the characters, otherwise Perl will complain you use a name it doesnt know about. For more details, see charnames.
Mnemonic: Named character.
Example
use charnames :full; # Loads the Unicode names.
$str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
use charnames Cyrillic; # Loads Cyrillic names.
$str =~ /\N{ZHE}\N{KA}/; # Match "ZHE" followed by "KA".
Octal escapes
Octal escapes consist of a backslash followed by two or three octal digits matching the code point of the character you want to use. This allows for 512 characters (
\00up to
\777) that can be expressed this way. Enough in pre-Unicode days, but most Unicode characters cannot be escaped this way.
Note that a character that is expressed as an octal escape is considered as a character without special meaning by the regex engine, and will match as is.
Examples
$str = "Perl";
$str =~ /\120/; # Match, "\120" is "P".
$str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
$str =~ /P\053/; # No match, "\053" is "+" and taken literally.
Caveat
Octal escapes potentially clash with backreferences. They both consist of a backslash followed by numbers. So Perl has to use heuristics to determine whether it is a backreference or an octal escape. Perl uses the following rules:
1. | If the backslash is followed by a single digit, its a backreference. |
2. | If the first digit following the backslash is a 0, its an octal escape. |
3. |
If the number following the backslash is N (decimal), and Perl already has
seen N capture groups, Perl will consider this to be a backreference.
Otherwise, it will consider it to be an octal escape. Note that if N > 999,
Perl only takes the first three digits for the octal escape; the rest is
matched as is.
my $pat = "(" x 999;
$pat .= "a";
$pat .= ")" x 999;
/^($pat)\1000$/; # Matches aa; there are 1000 capture groups.
/^$pat\1000$/; # Matches a@0; there are 999 capture groups
# and \1000 is seen as \100 (a @) and a 0.
|
Hexadecimal escapes start with
\xand are then either followed by two digit hexadecimal number, or a hexadecimal number of arbitrary length surrounded by curly braces. The hexadecimal number is the code point of the character you want to express.
Note that a character that is expressed as a hexadecimal escape is considered as a character without special meaning by the regex engine, and will match as is.
Mnemonic: hexadecimal.
Examples
$str = "Perl";
$str =~ /\x50/; # Match, "\x50" is "P".
$str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
$str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
/\x{2603}\x{2602}/ # Snowman with an umbrella.
# The Unicode character 2603 is a snowman,
# the Unicode character 2602 is an umbrella.
/\x{263B}/ # Black smiling face.
/\x{263b}/ # Same, the hex digits A - F are case insensitive.
Modifiers
A number of backslash sequences have to do with changing the character, or characters following them.
\lwill lowercase the character following it, while
\uwill uppercase (or, more accurately, titlecase) the character following it. (They perform similar functionality as the functions
lcfirstand
ucfirst).
To uppercase or lowercase several characters, one might want to use
\Lor
\U, which will lowercase/uppercase all characters following them, until either the end of the pattern, or the next occurrence of
\E, whatever comes first. They perform similar functionality as the functions
lcand
ucdo.
\Qis used to escape all characters following, up to the next
\Eor the end of the pattern.
\Qadds a backslash to any character that isnt a letter, digit or underscore. This will ensure that any character between
\Qand
\Eis matched literally, and will not be interpreted by the regexp engine.
Mnemonic: Lowercase, Uppercase, Quotemeta, End.
Examples
$sid = "sid";
$greg = "GrEg";
$miranda = "(Miranda)";
$str =~ /\u$sid/; # Matches Sid
$str =~ /\L$greg/; # Matches greg
$str =~ /\Q$miranda\E/; # Matches (Miranda), as if the pattern
# had been written as /\(Miranda\)/
Character classes
Perl regular expressions have a large range of character classes. Some of the character classes are written as a backslash sequence. We will briefly discuss those here; full details of character classes can be found in perlrecharclass.
\wis a character class that matches any word character (letters, digits, underscore).
\dis a character class that matches any digit, while the character class
\smatches any white space character. New in perl 5.10.0 are the classes
\hand
\vwhich match horizontal and vertical white space characters.
The uppercase variants (
\W,
\D,
\S,
\H, and
\V) are character classes that match any character that isnt a word character, digit, white space, horizontal white space or vertical white space.
Mnemonics: word, digit, space, horizontal, vertical.
Unicode classes
\pP(where
Pis a single letter) and
\p{Property}are used to match a character that matches the given Unicode property; properties include things like letter, or thai character. Capitalizing the sequence to
\PPand
\P{Property}make the sequence match a character that doesnt match the given Unicode property. For more details, see Backslashed sequences in perlrecharclass and Unicode Character Properties in perlunicode.
Mnemonic: property.
Referencing
If capturing parenthesis are used in a regular expression, we can refer to the part of the source string that was matched, and match exactly the same thing. There are three ways of referring to such backreference: absolutely, relatively, and by name.
Absolute referencing
A backslash sequence that starts with a backslash and is followed by a number is an absolute reference (but be aware of the caveat mentioned above). If the number is N, it refers to the Nth set of parenthesis - whatever has been matched by that set of parenthesis has to be matched by the
\Nas well.
Examples
/(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
/(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
Relative referencing
New in perl 5.10.0 is a different way of referring to capture buffers:
\g.
\gtakes a number as argument, with the number in curly braces (the braces are optional). If the number (N) does not have a sign, its a reference to the Nth capture group (so
\g{2}is equivalent to
\2- except that
\galways refers to a capture group and will never be seen as an octal escape). If the number is negative, the reference is relative, referring to the Nth group before the
\g{-N}.
The big advantage of
\g{-N}is that it makes it much easier to write patterns with references that can be interpolated in larger patterns, even if the larger pattern also contains capture groups.
Mnemonic: group.
Examples
/(A) # Buffer 1
( # Buffer 2
(B) # Buffer 3
\g{-1} # Refers to buffer 3 (B)
\g{-3} # Refers to buffer 1 (A)
)
/x; # Matches "ABBA".
my $qr = qr /(.)(.)\g{-2}\g{-1}/; # Matches abab, cdcd, etc.
/$qr$qr/ # Matches ababcdcd.
Named referencing
Also new in perl 5.10.0 is the use of named capture buffers, which can be referred to by name. This is done with
\g{name}, which is a backreference to the capture buffer with the name name.
To be compatible with .Net regular expressions,
\g{name}may also be written as
\k{name},
\k<name>or
\kname.
Note that
\g{}has the potential to be ambiguous, as it could be a named reference, or an absolute or relative reference (if its argument is numeric). However, names are not allowed to start with digits, nor are allowed to contain a hyphen, so there is no ambiguity.
Examples
/(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
/(?<word>\w+) \k{word}/ # Same.
/(?<word>\w+) \k<word>/ # Same.
/(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
# Match a four letter palindrome (e.g. "ABBA")
Assertions
Assertions are conditions that have to be true they dont actually match parts of the substring. There are six assertions that are written as backslash sequences.
\A |
\Aonly matches at the beginning of the string. If the /mmodifier isnt used, then /\A/is equivalent with /^/. However, if the /mmodifier is used, then /^/matches internal newlines, but the meaning of /\A/isnt changed by the /mmodifier. \Amatches at the beginning of the string regardless whether the /mmodifier is used. |
\z, \Z |
\zand \Zmatch at the end of the string. If the /mmodifier isnt used, then /\Z/is equivalent with /$/, that is, it matches at the end of the string, or before the newline at the end of the string. If the /mmodifier is used, then /$/matches at internal newlines, but the meaning of /\Z/isnt changed by the /mmodifier. \Zmatches at the end of the string (or just before a trailing newline) regardless whether the /mmodifier is used.
\zis just like \Z, except that it will not match before a trailing newline. \zwill only match at the end of the string - regardless of the modifiers used, and not before a newline. |
\G |
\Gis usually only used in combination with the /gmodifier. If the /gmodifier is used (and the match is done in scalar context), Perl will remember where in the source string the last match ended, and the next time, it will start the match from where it ended the previous time.
\Gmatches the point where the previous match ended, or the beginning of the string if there was no previous match. Mnemonic: Global. |
\b, \B |
\bmatches at any place between a word and a non-word character; \Bmatches at any place between characters where \bdoesnt match. \band \Bassume theres a non-word character before the beginning and after the end of the source string; so \bwill match at the beginning (or end) of the source string if the source string begins (or ends) with a word character. Otherwise, \Bwill match. Mnemonic: boundary. |
"cat" =~ /\Acat/; # Match.
"cat" =~ /cat\Z/; # Match.
"cat\n" =~ /cat\Z/; # Match.
"cat\n" =~ /cat\z/; # No match.
"cat" =~ /\bcat\b/; # Matches.
"cats" =~ /\bcat\b/; # No match.
"cat" =~ /\bcat\B/; # No match.
"cats" =~ /\bcat\B/; # Match.
while ("cat dog" =~ /(\w+)/g) {
print $1; # Prints catdog
}
while ("cat dog" =~ /\G(\w+)/g) {
print $1; # Prints cat
}
Misc
Here we document the backslash sequences that dont fall in one of the categories above. They are:
\C |
\Calways matches a single octet, even if the source string is encoded in UTF-8 format, and the character to be matched is a multi-octet character. \Cwas introduced in perl 5.6. Mnemonic: oCtet. |
\K |
This is new in perl 5.10.0. Anything that is matched left of \Kis not included in $&- and will not be replaced if the pattern is used in a substitution. This will allow you to write s/PAT1 \K PAT2/REPL/xinstead of s/(PAT1) PAT2/${1}REPL/xor s/(?<=PAT1) PAT2/REPL/x. Mnemonic: Keep. |
\R |
\Rmatches a generic newline, that is, anything that is considered a newline by Unicode. This includes all characters matched by \v(vertical white space), and the multi character sequence "\x0D\x0A"(carriage return followed by a line feed, aka the network newline, or the newline used in Windows text files). \Ris equivalent with (?>\x0D\x0A)|\v). Since \Rcan match a more than one character, it cannot be put inside a bracketed character class; /[\R]/is an error. \Rwas introduced in perl 5.10.0. Mnemonic: none really. \Rwas picked because PCRE already uses \R, and more importantly because Unicode recommends such a regular expression metacharacter, and suggests \Ras the notation. |
\X |
This matches an extended Unicode combining character sequence, and
is equivalent to (?>\PM\pM*). \PMmatches any character that is not considered a Unicode mark character, while \pMmatches any character that is considered a Unicode mark character; so \Xmatches any non mark character followed by zero or more mark characters. Mark characters include (but are not restricted to) combining characters and vowel signs.
\Xmatches quite well what normal (non-Unicode-programmer) usage would consider a single character: for example a base character (the \PMabove), for example a letter, followed by zero or more diacritics, which are combining characters (the \pM*above). Mnemonic: eXtended Unicode character. |
"\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
$str =~ s/foo\Kbar/baz/g; # Change any bar following a foo to baz.
$str =~ s/(.)\K\1//g; # Delete duplicated characters.
"\n" =~ /^\R$/; # Match, \n is a generic newline.
"\r" =~ /^\R$/; # Match, \r is a generic newline.
"\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
"P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.