PERLREBACKSLASH(1) Perl Programmers Reference Guide PERLREBACKSLASH(1)
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, it's 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 an ASCII 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 an ASCII letter or an ASCII digit, then the sequence may be special; if so, it's listed below.
A few letters have not been used yet, so escaping them with a backslash doesn't change them to be special. A future version of Perl may
assign a special meaning to them, so 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 shouldn't do for
readability reasons), you will have to escape the delimiter if you want to match it. Perl won't warn then. See also "Gory details of
parsing quoted constructs" in perlop.
All the sequences and escapes
Those not usable within a bracketed character class (like "[da-z]") are marked as "Not in []."
00 Octal escape sequence.
1 Absolute backreference. Not in [].
a Alarm or bell.
A Beginning of string. Not in [].
Word/non-word boundary. (Backspace in []).
B Not a word/non-word boundary. Not in [].
cX Control-X (X can be any ASCII character).
C Single octet, even under UTF-8. Not in [].
d Character class for digits.
D Character class for non-digits.
e Escape character.
E Turn off Q, L and U processing. Not in [].
f Form feed.
g{}, g1 Named, absolute or relative backreference. Not in [].
G Pos assertion. Not in [].
h Character class for horizontal whitespace.
H Character class for non horizontal whitespace.
k{}, k<>, k'' Named backreference. Not in [].
K Keep the stuff left of K. Not in [].
l Lowercase next character. Not in [].
L Lowercase till E. Not in [].
(Logical) newline character.
N Any character but newline. Experimental. Not in [].
N{} Named or numbered (Unicode) character.
p{}, pP Character with the given Unicode property.
P{}, PP Character without the given Unicode property.
Q Quotemeta till E. Not in [].
Return character.
R Generic new line. Not in [].
s Character class for whitespace.
S Character class for non whitespace.
Tab character.
u Titlecase next character. Not in [].
U Uppercase till E. Not in [].
v Character class for vertical whitespace.
V Character class for non vertical whitespace.
w Character class for word characters.
W Character class for non-word characters.
x{}, x00 Hexadecimal escape sequence.
X Unicode "extended grapheme cluster". Not in [].
z End of string. Not in [].
End of string. Not in [].
Character Escapes
Fixed characters
A handful of characters have a dedicated character escape. The following table shows them, along with their ASCII 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
8 08 BS cH backspace [1]
e 27 1B ESC c[ escape character
f 12 0C FF cL form feed
10 0A LF cJ line feed [2]
13 0D CR cM carriage return
9 09 TAB cI tab
[1] "" is only the backspace character inside a character class. Outside a character class, "" is a word/non-word boundary.
[2] "
" matches a logical newline. Perl will convert between "
" and your OSses native newline character when reading from or writing to
text files.
Example
$str =~ / /; # Matches if $str contains a (horizontal) tab.
Control characters
"c" is used to denote a control character; the character following "c" is 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 "c" doesn't matter: "cM" and
"cm" match the same character.
Mnemonic: control character.
Example
$str =~ /cK/; # Matches if $str contains a vertical tab (control-K).
Named or numbered characters
All Unicode characters have a Unicode name and numeric ordinal value. Use the "N{}" construct to specify a character by either of these
values.
To specify by name, the name of the character goes between the curly braces. In this case, you have to "use charnames" to load the Unicode
names of the characters, otherwise Perl will complain.
To specify by Unicode ordinal number, use the form "N{U+wide hex character}", where wide hex character is a number in hexadecimal that
gives the ordinal number that Unicode has assigned to the desired character. It is customary (but not required) to use leading zeros to
pad the number to 4 digits. Thus "N{U+0041}" means "Latin Capital Letter A", and you will rarely see it written without the two leading
zeros. "N{U+0041}" means "A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
It is even possible to give your own names to characters, and even to short sequences of characters. For details, see charnames.
(There is an expanded internal form that you may see in debug output: "N{U+wide hex character.wide hex character...}". The "..." means
any number of these wide hex characters separated by dots. This represents the sequence formed by the characters. This is an internal
form only, subject to change, and you should not try to use it yourself.)
Mnemonic: Named character.
Note that a character that is expressed as a named or numbered character is considered as a character without special meaning by the regex
engine, and will match "as is".
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 ("