purpose of this document is to have a quick reference guide describing all
backslash and escape sequences.
-
=head2 The backslash
In a regular expression, the backslash can perform one of two tasks:
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, 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].
+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
Those not usable within a bracketed character class (like C<[\da-z]>) are marked
as C<Not in [].>
- \000 Octal escape sequence.
+ \000 Octal escape sequence. See also \o{}.
\1 Absolute backreference. Not in [].
\a Alarm or bell.
\A Beginning of string. Not in [].
\b Word/non-word boundary. (Backspace in []).
\B Not a word/non-word boundary. Not in [].
- \cX Control-X (X can be any ASCII character).
+ \cX Control-X
\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{}, \g1 Named, absolute or relative backreference. Not in []
\G Pos assertion. Not in [].
- \h Character class for horizontal white space.
- \H Character class for non horizontal white space.
+ \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 [].
\n (Logical) newline character.
\N Any character but newline. Experimental. Not in [].
- \N{} Named or numbered (Unicode) character.
+ \N{} Named or numbered (Unicode) character or sequence.
+ \o{} Octal escape sequence.
\p{}, \pP Character with the given Unicode property.
\P{}, \PP Character without the given Unicode property.
\Q Quotemeta till \E. Not in [].
\r Return character.
\R Generic new line. Not in [].
- \s Character class for white space.
- \S Character class for non white space.
+ \s Character class for whitespace.
+ \S Character class for non whitespace.
\t Tab character.
\u Titlecase next character. Not in [].
\U Uppercase till \E. Not in [].
- \v Character class for vertical white space.
- \V Character class for non vertical white space.
+ \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.
A handful of characters have a dedicated I<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.
+their ASCII name, the control escape on ASCII platforms and a short
+description. (For EBCDIC platforms, see L<perlebcdic/OPERATOR DIFFERENCES>.)
- Seq. Code Point ASCII Cntr Description.
+ Seq. Code Point ASCII Cntrl Description.
Dec Hex
\a 7 07 BEL \cG alarm or bell
\b 8 08 BS \cH backspace [1]
=item [1]
-C<\b> is only the backspace character inside a character class. Outside a
+C<\b> is the backspace character only inside a character class. Outside a
character class, C<\b> is a word/non-word boundary.
=item [2]
C<\n> matches a logical newline. Perl will convert between C<\n> and your
-OSses native newline character when reading from or writing to text files.
+OS's native newline character when reading from or writing to text files.
=back
=head3 Control characters
C<\c> is used to denote a control character; the character following C<\c>
-is the name of the control character. For instance, C</\cM/> matches the
-character I<control-M> (a carriage return, code point 13). The case of the
-character following C<\c> doesn't matter: C<\cM> and C<\cm> match the same
-character.
+determines the value of the construct. For example the value of C<\cA> is
+C<chr(1)>, and the value of C<\cb> is C<chr(2)>, etc.
+The gory details are in L<perlop/"Regexp Quote-Like Operators">. A complete
+list of what C<chr(1)>, etc. means for ASCII and EBCDIC platforms is in
+L<perlebcdic/OPERATOR DIFFERENCES>.
+
+Note that C<\c\> alone at the end of a regular expression (or doubled-quoted
+string) is not valid. The backslash must be followed by another character.
+That is, C<\c\I<X>> means C<chr(28) . 'I<X>'> for all characters I<X>.
+
+To write platform-independent code, you must use C<\N{I<NAME>}> instead, like
+C<\N{ESCAPE}> or C<\N{U+001B}>, see L<charnames>.
Mnemonic: I<c>ontrol character.
$str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
-=head3 Named or numbered characters
+=head3 Named or numbered characters and character sequences
-All Unicode characters have a Unicode name and numeric ordinal value. Use the
+Unicode characters have a Unicode name and numeric ordinal value. Use the
C<\N{}> construct to specify a character by either of these values.
+Certain sequences of characters also have names.
-To specify by name, the name of the character goes between the curly braces.
-In this case, you have to C<use charnames> to load the Unicode names of the
-characters, otherwise Perl will complain.
+To specify by name, the name of the character or character sequence goes
+between the curly braces. In this case, you have to C<use charnames> to
+load the Unicode names of the characters, otherwise Perl will complain.
-To specify by Unicode ordinal number, use the form
+To specify a character by Unicode code point, use the form
C<\N{U+I<wide hex character>}>, where I<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 C<\N{U+0041}> means
-C<Latin Capital Letter A>, and you will rarely see it written without the two
-leading zeros. C<\N{U+0041}> means C<A> even on EBCDIC machines (where the
-ordinal value of C<A> is not 0x41).
+C<LATIN CAPITAL LETTER A>, and you will rarely see it written without the two
+leading zeros. C<\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 L<charnames>.
+It is even possible to give your own names to characters and character
+sequences. For details, see L<charnames>.
(There is an expanded internal form that you may see in debug output:
C<\N{U+I<wide hex character>.I<wide hex character>...}>.
Mnemonic: I<N>amed 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".
+Note that a character or character sequence 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".
=head4 Example
=head3 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 (C<\00> up to C<\777>) that can be expressed this way (but
-anything above C<\377> is deprecated).
-Enough in pre-Unicode days, but most Unicode characters cannot be escaped
-this way.
+There are two forms of octal escapes. Each is used to specify a character by
+its ordinal, specified in octal notation.
+
+One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
+represent one or more octal digits. It can be used for any Unicode character.
+
+It was introduced to avoid the potential problems with the other form,
+available in all Perls. That form consists of a backslash followed by three
+octal digits. One problem with this form is that it can look exactly like an
+old-style backreference (see
+L</Disambiguation rules between old-style octal escapes and backreferences>
+below.) You can avoid this by making the first of the three digits always a
+zero, but that makes \077 the largest ordinal unambiguously specifiable by this
+form.
+
+In some contexts, a backslash followed by two or even one octal digits may be
+interpreted as an octal escape, sometimes with a warning, and because of some
+bugs, sometimes with surprising results. Also, if you are creating a regex
+out of smaller snippets concatentated together, and you use fewer than three
+digits, the beginning of one snippet may be interpreted as adding digits to the
+ending of the snippet before it. See L</Absolute referencing> for more
+discussion and examples of the snippet problem.
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".
-=head4 Examples (assuming an ASCII platform)
+To summarize, the C<\o{}> form is always safe to use, and the other form is
+safe to use for ordinals up through \077 when you use exactly three digits to
+specify them.
- $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.
+Mnemonic: I<0>ctal or I<o>ctal.
-=head4 Caveat
+=head4 Examples (assuming an ASCII platform)
-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:
+ $str = "Perl";
+ $str =~ /\o{120}/; # Match, "\120" is "P".
+ $str =~ /\120/; # Same.
+ $str =~ /\o{120}+/; # Match, "\120" is "P", it's repeated at least once
+ $str =~ /\120+/; # Same.
+ $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
+ /\o{23073}/ # Black foreground, white background smiling face.
+ /\o{4801234567}/ # Raises a warning, and yields chr(4)
+
+=head4 Disambiguation rules between old-style octal escapes and backreferences
+
+Octal escapes of the C<\000> form outside of bracketed character classes
+potentially clash with old-style backreferences. (see L</Absolute referencing>
+below). 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 to disambiguate:
=over 4
=item 3
-If the number following the backslash is N (decimal), and Perl already has
+If the number following the backslash is N (in 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.
+Otherwise, it will consider it to be an octal escape. Note that if N has more
+than three digits, Perl only takes the first three for the octal escape;
+the rest are 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'.
+ # and \1000 is seen as \100 (a '@') and a '0'
=back
+You can the force a backreference interpretation always by using the C<\g{...}>
+form. You can the force an octal interpretation always by using the C<\o{...}>
+form, or for numbers up through \077 (= 63 decimal), by using three digits,
+beginning with a "0".
+
=head3 Hexadecimal escapes
-Hexadecimal escapes start with C<\x> and are then either followed by a
-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.
+Like octal escapes, there are two forms of hexadecimal escapes, but both start
+with the same thing, C<\x>. This is followed by either exactly two hexadecimal
+digits forming a 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
+Note that a character that is expressed as one of these escapes is considered
as a character without special meaning by the regex engine, and will match
"as is".
Mnemonic: heI<x>adecimal.
-=head4 Examples
+=head4 Examples (assuming an ASCII platform)
$str = "Perl";
$str =~ /\x50/; # Match, "\x50" is "P".
- $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
+ $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.
discuss those here; full details of character classes can be found in
L<perlrecharclass>.
-C<\w> is a character class that matches any single I<word> character (letters,
-digits, underscore). C<\d> is a character class that matches any decimal digit,
-while the character class C<\s> matches any white space character.
+C<\w> is a character class that matches any single I<word> character
+(letters, digits, Unicode marks, and connector punctuation (like the
+underscore)). C<\d> is a character class that matches any decimal
+digit, while the character class C<\s> matches any whitespace character.
New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
-and vertical white space characters.
+and vertical whitespace characters.
The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
-character classes that match any character that isn't a word character,
-digit, white space, horizontal white space nor vertical white space.
+character classes that match, respectively, any character that isn't a
+word character, digit, whitespace, horizontal whitespace, or vertical
+whitespace.
Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
include things like "letter", or "thai character". Capitalizing the
sequence to C<\PP> and C<\P{Property}> make the sequence match a character
that doesn't match the given Unicode property. For more details, see
-L<perlrecharclass/Backslashed sequences> and
+L<perlrecharclass/Backslash sequences> and
L<perlunicode/Unicode Character Properties>.
Mnemonic: I<p>roperty.
-
=head2 Referencing
If capturing parenthesis are used in a regular expression, we can refer
=head3 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 I<N>, it refers to the Nth set of parentheses - whatever
-has been matched by that set of parenthesis has to be matched by the C<\N>
-as well.
+Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
+is a positive (unsigned) decimal number of any length is an absolute reference
+to a capturing group.
+
+I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
+been matched by that set of parentheses. Thus C<\g1> refers to the first
+capture group in the regex.
+
+The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
+which avoids ambiguity when building a regex by concatenating shorter
+strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
+C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
+probably not what you intended.
+
+In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
+least I<N> capturing groups, or else I<N> will be considered an octal escape
+(but something like C<\18> is the same as C<\0018>, that is the octal escape
+C<"\001"> followed by a literal digit C<"8">).
+
+Mnemonic: I<g>roup.
=head4 Examples
- /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
- /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
+ /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
+ /(\w+) \1/; # Same thing; written old-style
+ /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
=head3 Relative referencing
-New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>.
-C<\g> takes a number as argument, with the number in curly braces (the
-braces are optional). If the number (N) does not have a sign, it's a reference
-to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
-C<\g> always 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 C<\g{-N}>.
+C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
+be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
+C<\g{-I<N>}>.
-The big advantage of C<\g{-N}> is that it makes it much easier to write
+The big advantage of this form 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: I<g>roup.
-
=head4 Examples
- /(A) # Buffer 1
- ( # Buffer 2
- (B) # Buffer 3
- \g{-1} # Refers to buffer 3 (B)
- \g{-3} # Refers to buffer 1 (A)
+ /(A) # Group 1
+ ( # Group 2
+ (B) # Group 3
+ \g{-1} # Refers to group 3 (B)
+ \g{-3} # Refers to group 1 (A)
)
/x; # Matches "ABBA".
=head3 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 C<\g{name}>, which is a
-backreference to the capture buffer with the name I<name>.
+C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
+named capture group, dispensing completely with having to think about capture
+buffer positions.
To be compatible with .Net regular expressions, C<\g{name}> may also be
written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
-Note that C<\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 they allowed to
-contain a hyphen, so there is no ambiguity.
+To prevent any ambiguity, I<name> must not start with a digit nor contain a
+hyphen.
=head4 Examples
=item \A
C<\A> only matches at the beginning of the string. If the C</m> modifier
-isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
+isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
modifier is used, then C</^/> matches internal newlines, but the meaning
of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
of the string regardless whether the C</m> modifier is used.
=item \z, \Z
C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
-used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
+used, then C</\Z/> is equivalent to C</$/>, that is, it matches at the
end of the string, or before the newline at the end of the string. If the
C</m> modifier is used, then C</$/> matches at internal newlines, but the
meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
the meaning of C<.>, but not C<\N>.
Note that C<\N{...}> can mean a
-L<named or numbered character|/Named or numbered characters>.
+L<named or numbered character
+|/Named or numbered characters and character sequences>.
Mnemonic: Complement of I<\n>.
=item \R
+X<\R>
C<\R> matches a I<generic newline>, that is, anything that is considered
a newline by Unicode. This includes all characters matched by C<\v>
-(vertical white space), and the multi character sequence C<"\x0D\x0A">
+(vertical whitespace), and the multi character sequence C<"\x0D\x0A">
(carriage return followed by a line feed, aka the network newline, or
the newline used in Windows text files). C<\R> is equivalent to
C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
metacharacter, and suggests C<\R> as the notation.
=item \X
+X<\X>
This matches a Unicode I<extended grapheme cluster>.
"\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.
+ $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
+ $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
"\n" =~ /^\R$/; # Match, \n is a generic newline.
"\r" =~ /^\R$/; # Match, \r is a generic newline.
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