perlrebackslash
(1)
名前
perlrebackslash - Perl Regular Expression Backslash
Sequences and Escapes
形式
Please see following description for synopsis
説明
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
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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 []."
\000 Octal escape sequence.
\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
\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 [].
\n (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 [].
\r Return character.
\R Generic new line. Not in [].
\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 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 [].
\Z End of string. Not in [].
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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 on ASCII platforms and a short description. (For
EBCDIC platforms, see "OPERATOR DIFFERENCES" in perlebcdic.)
Seq. Code Point ASCII Cntrl 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] "\b" is the backspace character only inside a character
class. Outside a character class, "\b" is a
word/non-word boundary.
[2] "\n" matches a logical newline. Perl will convert
between "\n" and your OS's native newline character when
reading from or writing to text files.
Example
$str =~ /\t/; # Matches if $str contains a (horizontal) tab.
Control characters
"\c" is used to denote a control character; the character
following "\c" determines the value of the construct. For
example the value of "\cA" is chr(1), and the value of "\cb"
is chr(2), etc. The gory details are in "Regexp Quote-Like
Operators" in perlop. A complete list of what chr(1), etc.
means for ASCII and EBCDIC platforms is in "OPERATOR
DIFFERENCES" in perlebcdic.
Note that "\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\X" means
"chr(28) . 'X'" for all characters X.
To write platform-independent code, you must use "\N{NAME}"
instead, like "\N{ESCAPE}" or "\N{U+001B}", see charnames.
Mnemonic: control character.
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Example
$str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
Named or numbered characters
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".
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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 ("\00" up to
"\777") that can be expressed this way (but anything above
"\377" is deprecated). 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 (assuming an ASCII platform)
$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, it's a
backreference.
2. If the first digit following the backslash is a 0, it's
an octal escape.
3. 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 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'.
Hexadecimal escapes
Hexadecimal escapes start with "\x" and are then either
followed by a two digit hexadecimal number, or a hexadecimal
number of arbitrary length surrounded by curly braces. The
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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 (assuming an ASCII platform)
$str = "Perl";
$str =~ /\x50/; # Match, "\x50" is "P".
$str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
$str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
/\x{2603}\x{2602}/ # Snowman with an umbrella.
# The Unicode character 2603 is a snowman,
# the Unicode character 2602 is an umbrella.
/\x{263B}/ # Black smiling face.
/\x{263b}/ # Same, the hex digits A - F are case insensitive.
Modifiers
A number of backslash sequences have to do with changing the
character, or characters following them. "\l" will lowercase
the character following it, while "\u" will uppercase (or,
more accurately, titlecase) the character following it.
(They perform similar functionality as the functions
"lcfirst" and "ucfirst").
To uppercase or lowercase several characters, one might want
to use "\L" or "\U", which will lowercase/uppercase all
characters following them, until either the end of the
pattern, or the next occurrence of "\E", whatever comes
first. They perform similar functionality as the functions
"lc" and "uc" do.
"\Q" is used to escape all characters following, up to the
next "\E" or the end of the pattern. "\Q" adds a backslash
to any character that isn't a letter, digit or underscore.
This will ensure that any character between "\Q" and "\E" is
matched literally, and will not be interpreted by the regexp
engine.
Mnemonic: Lowercase, Uppercase, Quotemeta, End.
Examples
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$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.
"\w" is a character class that matches any single word
character (letters, digits, underscore). "\d" is a character
class that matches any decimal digit, while the character
class "\s" matches any whitespace character. New in perl
5.10.0 are the classes "\h" and "\v" which match horizontal
and vertical whitespace characters.
The uppercase variants ("\W", "\D", "\S", "\H", and "\V")
are character classes that match any character that isn't a
word character, digit, whitespace, horizontal whitespace nor
vertical whitespace.
Mnemonics: word, digit, space, horizontal, vertical.
Unicode classes
"\pP" (where "P" is 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 "\PP" and
"\P{Property}" make the sequence match a character that
doesn't match the given Unicode property. For more details,
see "Backslash 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
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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 parentheses - whatever has been
matched by that set of parenthesis has to be matched by the
"\N" as 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". "\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 "\g{2}" is equivalent to "\2" -
except that "\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 "\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$qr/ # Matches 'ababcdcd'.
Named referencing
Also new in perl 5.10.0 is the use of named capture buffers,
which can be referred to by name. This is done with
"\g{name}", which is a backreference to the capture buffer
with the name name.
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To be compatible with .Net regular expressions, "\g{name}"
may also be written as "\k{name}", "\k<name>" or "\k'name'".
Note that "\g{}" has the potential to be ambiguous, as it
could be a named reference, or an absolute or relative
reference (if its argument is numeric). However, names are
not allowed to start with digits, nor are they allowed to
contain a hyphen, so there is no ambiguity.
Examples
/(?<word>\w+) \g{word}/ # Finds duplicated word, (e.g. "cat cat")
/(?<word>\w+) \k{word}/ # Same.
/(?<word>\w+) \k<word>/ # Same.
/(?<letter1>.)(?<letter2>.)\g{letter2}\g{letter1}/
# Match a four letter palindrome (e.g. "ABBA")
Assertions
Assertions are conditions that have to be true; they don't
actually match parts of the substring. There are six
assertions that are written as backslash sequences.
\A "\A" only matches at the beginning of the string. If the
"/m" modifier isn't used, then "/\A/" is equivalent with
"/^/". However, if the "/m" modifier is used, then "/^/"
matches internal newlines, but the meaning of "/\A/"
isn't changed by the "/m" modifier. "\A" matches at the
beginning of the string regardless whether the "/m"
modifier is used.
\z, \Z
"\z" and "\Z" match at the end of the string. If the
"/m" modifier isn't used, then "/\Z/" is equivalent with
"/$/", that is, it matches at the end of the string, or
before the newline at the end of the string. If the "/m"
modifier is used, then "/$/" matches at internal
newlines, but the meaning of "/\Z/" isn't changed by the
"/m" modifier. "\Z" matches at the end of the string (or
just before a trailing newline) regardless whether the
"/m" modifier is used.
"\z" is just like "\Z", except that it will not match
before a trailing newline. "\z" will only match at the
end of the string - regardless of the modifiers used,
and not before a newline.
\G "\G" is usually only used in combination with the "/g"
modifier. If the "/g" modifier 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.
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"\G" matches the point where the previous match ended,
or the beginning of the string if there was no previous
match.
Mnemonic: Global.
\b, \B
"\b" matches at any place between a word and a non-word
character; "\B" matches at any place between characters
where "\b" doesn't match. "\b" and "\B" assume there's a
non-word character before the beginning and after the
end of the source string; so "\b" will match at the
beginning (or end) of the source string if the source
string begins (or ends) with a word character.
Otherwise, "\B" will match.
Mnemonic: boundary.
Examples
"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 don't fall in
one of the categories above. They are:
\C "\C" always 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. "\C" was
introduced in perl 5.6.
Mnemonic: oCtet.
\K This is new in perl 5.10.0. Anything that is matched
left of "\K" is not included in $& - and will not be
replaced if the pattern is used in a substitution. This
will allow you to write "s/PAT1 \K PAT2/REPL/x" instead
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of "s/(PAT1) PAT2/${1}REPL/x" or "s/(?<=PAT1)
PAT2/REPL/x".
Mnemonic: Keep.
\N This is a new experimental feature in perl 5.12.0. It
matches any character that is not a newline. It is a
short-hand for writing "[^\n]", and is identical to the
"." metasymbol, except under the "/s" flag, which
changes the meaning of ".", but not "\N".
Note that "\N{...}" can mean a named or numbered
character.
Mnemonic: Complement of \n.
\R "\R" matches a generic newline, that is, anything that
is considered a newline by Unicode. This includes all
characters matched by "\v" (vertical whitespace), 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). "\R" is equivalent
to "(?>\x0D\x0A)|\v)". Since "\R" can match a sequence
of more than one character, it cannot be put inside a
bracketed character class; "/[\R]/" is an error; use
"\v" instead. "\R" was introduced in perl 5.10.0.
Mnemonic: none really. "\R" was picked because PCRE
already uses "\R", and more importantly because Unicode
recommends such a regular expression metacharacter, and
suggests "\R" as the notation.
\X This matches a Unicode extended grapheme cluster.
"\X" matches quite well what normal (non-Unicode-
programmer) usage would consider a single character. As
an example, consider a G with some sort of diacritic
mark, such as an arrow. There is no such single
character in Unicode, but one can be composed by using a
G followed by a Unicode "COMBINING UPWARDS ARROW BELOW",
and would be displayed by Unicode-aware software as if
it were a single character.
Mnemonic: eXtended Unicode character.
Examples
"\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'.
$str =~ s/(.)\K\1//g; # Delete duplicated characters.
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"\n" =~ /^\R$/; # 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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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