perlrecharclass - Perl Regular Expression Character Classes
The top level documentation about Perl regular expressions is found in perlre.
This manual page discusses the syntax and use of character classes in Perl regular expressions.
A character class is a way of denoting a set of characters in such a way that one character of the set is matched. It's important to remember that: matching a character class consumes exactly one character in the source string. (The source string is the string the regular expression is matched against.)
There are three types of character classes in Perl regular expressions: the dot, backslash sequences, and the form enclosed in square brackets. Keep in mind, though, that often the term "character class" is used to mean just the bracketed form. Certainly, most Perl documentation does that.
The dot (or period), . is probably the most used, and certainly
the most well-known character class. By default, a dot matches any
character, except for the newline. That default can be changed to
add matching the newline by using the single line modifier: either
for the entire regular expression with the /s modifier, or
locally with (?s). (The \N backslash sequence, described
below, matches any character except newline without regard to the
single line modifier.)
Here are some examples:
- "a" =~ /./ # Match
- "." =~ /./ # Match
- "" =~ /./ # No match (dot has to match a character)
- "\n" =~ /./ # No match (dot does not match a newline)
- "\n" =~ /./s # Match (global 'single line' modifier)
- "\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
- "ab" =~ /^.$/ # No match (dot matches one character)
A backslash sequence is a sequence of characters, the first one of which is a backslash. Perl ascribes special meaning to many such sequences, and some of these are character classes. That is, they match a single character each, provided that the character belongs to the specific set of characters defined by the sequence.
Here's a list of the backslash sequences that are character classes. They are discussed in more detail below. (For the backslash sequences that aren't character classes, see perlrebackslash.)
- \d Match a decimal digit character.
- \D Match a non-decimal-digit character.
- \w Match a "word" character.
- \W Match a non-"word" character.
- \s Match a whitespace character.
- \S Match a non-whitespace character.
- \h Match a horizontal whitespace character.
- \H Match a character that isn't horizontal whitespace.
- \v Match a vertical whitespace character.
- \V Match a character that isn't vertical whitespace.
- \N Match a character that isn't a newline.
- \pP, \p{Prop} Match a character that has the given Unicode property.
- \PP, \P{Prop} Match a character that doesn't have the Unicode property
\N
, available starting in v5.12, like the dot, matches any
character that is not a newline. The difference is that \N
is not influenced
by the single line regular expression modifier (see The dot above). Note
that the form \N{...}
may mean something completely different. When the
{...}
is a quantifier, it means to match a non-newline
character that many times. For example, \N{3}
means to match 3
non-newlines; \N{5,}
means to match 5 or more non-newlines. But if {...}
is not a legal quantifier, it is presumed to be a named character. See
charnames for those. For example, none of \N{COLON}
, \N{4F}, and
\N{F4}
contain legal quantifiers, so Perl will try to find characters whose
names are respectively COLON
, 4F, and F4
.
\d
matches a single character considered to be a decimal digit.
If the /a
regular expression modifier is in effect, it matches [0-9].
Otherwise, it
matches anything that is matched by \p{Digit}
, which includes [0-9].
(An unlikely possible exception is that under locale matching rules, the
current locale might not have [0-9]
matched by \d
, and/or might match
other characters whose code point is less than 256. The only such locale
definitions that are legal would be to match [0-9]
plus another set of
10 consecutive digit characters; anything else would be in violation of
the C language standard, but Perl doesn't currently assume anything in
regard to this.)
What this means is that unless the /a
modifier is in effect \d
not
only matches the digits '0' - '9', but also Arabic, Devanagari, and
digits from other languages. This may cause some confusion, and some
security issues.
Some digits that \d
matches look like some of the [0-9] ones, but
have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks
very much like an ASCII DIGIT EIGHT (U+0038). An application that
is expecting only the ASCII digits might be misled, or if the match is
\d+
, the matched string might contain a mixture of digits from
different writing systems that look like they signify a number different
than they actually do. num() in Unicode::UCD can
be used to safely
calculate the value, returning undef if the input string contains
such a mixture.
What \p{Digit}
means (and hence \d
except under the /a
modifier) is \p{General_Category=Decimal_Number}
, or synonymously,
\p{General_Category=Digit}
. Starting with Unicode version 4.1, this
is the same set of characters matched by \p{Numeric_Type=Decimal}
.
But Unicode also has a different property with a similar name,
\p{Numeric_Type=Digit}
, which matches a completely different set of
characters. These characters are things such as CIRCLED DIGIT ONE
or subscripts, or are from writing systems that lack all ten digits.
The design intent is for \d
to exactly match the set of characters
that can safely be used with "normal" big-endian positional decimal
syntax, where, for example 123 means one 'hundred', plus two 'tens',
plus three 'ones'. This positional notation does not necessarily apply
to characters that match the other type of "digit",
\p{Numeric_Type=Digit}
, and so \d
doesn't match them.
The Tamil digits (U+0BE6 - U+0BEF) can also legally be used in old-style Tamil numbers in which they would appear no more than one in a row, separated by characters that mean "times 10", "times 100", etc. (See http://www.unicode.org/notes/tn21.)
Any character not matched by \d
is matched by \D
.
A \w
matches a single alphanumeric character (an alphabetic character, or a
decimal digit); or a connecting punctuation character, such as an
underscore ("_"); or a "mark" character (like some sort of accent) that
attaches to one of those. It does not match a whole word. To match a
whole word, use \w+
. This isn't the same thing as matching an
English word, but in the ASCII range it is the same as a string of
Perl-identifier characters.
/a
modifier is in effect ...
\w
matches the 63 characters [a-zA-Z0-9_].
\w
matches the same as \p{Word}
matches in this range. That is,
it matches Thai letters, Greek letters, etc. This includes connector
punctuation (like the underscore) which connect two words together, or
diacritics, such as a COMBINING TILDE
and the modifier letters, which
are generally used to add auxiliary markings to letters.
Which rules apply are determined as described in Which character set modifier is in effect? in perlre.
There are a number of security issues with the full Unicode list of word characters. See http://unicode.org/reports/tr36.
Also, for a somewhat finer-grained set of characters that are in programming
language identifiers beyond the ASCII range, you may wish to instead use the
more customized Unicode Properties, \p{ID_Start}
,
\p{ID_Continue}
, \p{XID_Start}
, and \p{XID_Continue}
. See
http://unicode.org/reports/tr31.
Any character not matched by \w
is matched by \W
.
\s matches any single character considered whitespace.
/a
modifier is in effect ...
In all Perl versions, \s matches the 5 characters [\t\n\f\r ]; that
is, the horizontal tab,
the newline, the form feed, the carriage return, and the space.
Starting in Perl v5.18, it also matches the vertical tab, \cK
.
See note [1]
below for a discussion of this.
\s matches exactly the code points above 255 shown with an "s" column
in the table below.
\s matches whatever the locale considers to be whitespace.
\s matches exactly the characters shown with an "s" column in the
table below.
\s matches [\t\n\f\r ] and, starting in Perl
v5.18, the vertical tab, \cK
.
(See note [1]
below for a discussion of this.)
Note that this list doesn't include the non-breaking space.
Which rules apply are determined as described in Which character set modifier is in effect? in perlre.
Any character not matched by \s is matched by \S
.
\h
matches any character considered horizontal whitespace;
this includes the platform's space and tab characters and several others
listed in the table below. \H
matches any character
not considered horizontal whitespace. They use the platform's native
character set, and do not consider any locale that may otherwise be in
use.
\v
matches any character considered vertical whitespace;
this includes the platform's carriage return and line feed characters (newline)
plus several other characters, all listed in the table below.
\V
matches any character not considered vertical whitespace.
They use the platform's native character set, and do not consider any
locale that may otherwise be in use.
\R
matches anything that can be considered a newline under Unicode
rules. It can match a multi-character sequence. It cannot be used inside
a bracketed character class; use \v
instead (vertical whitespace).
It uses the platform's
native character set, and does not consider any locale that may
otherwise be in use.
Details are discussed in perlrebackslash.
Note that unlike \s (and \d
and \w
), \h
and \v
always match
the same characters, without regard to other factors, such as the active
locale or whether the source string is in UTF-8 format.
One might think that \s is equivalent to [\h\v]
. This is indeed true
starting in Perl v5.18, but prior to that, the sole difference was that the
vertical tab ("\cK"
) was not matched by \s.
The following table is a complete listing of characters matched by
\s, \h
and \v
as of Unicode 6.3.
The first column gives the Unicode code point of the character (in hex format),
the second column gives the (Unicode) name. The third column indicates
by which class(es) the character is matched (assuming no locale is in
effect that changes the \s matching).
- 0x0009 CHARACTER TABULATION h s
- 0x000a LINE FEED (LF) vs
- 0x000b LINE TABULATION vs [1]
- 0x000c FORM FEED (FF) vs
- 0x000d CARRIAGE RETURN (CR) vs
- 0x0020 SPACE h s
- 0x0085 NEXT LINE (NEL) vs [2]
- 0x00a0 NO-BREAK SPACE h s [2]
- 0x1680 OGHAM SPACE MARK h s
- 0x2000 EN QUAD h s
- 0x2001 EM QUAD h s
- 0x2002 EN SPACE h s
- 0x2003 EM SPACE h s
- 0x2004 THREE-PER-EM SPACE h s
- 0x2005 FOUR-PER-EM SPACE h s
- 0x2006 SIX-PER-EM SPACE h s
- 0x2007 FIGURE SPACE h s
- 0x2008 PUNCTUATION SPACE h s
- 0x2009 THIN SPACE h s
- 0x200a HAIR SPACE h s
- 0x2028 LINE SEPARATOR vs
- 0x2029 PARAGRAPH SEPARATOR vs
- 0x202f NARROW NO-BREAK SPACE h s
- 0x205f MEDIUM MATHEMATICAL SPACE h s
- 0x3000 IDEOGRAPHIC SPACE h s
Prior to Perl v5.18, \s did not match the vertical tab.
[^\S\cK]
(obscurely) matches what \s traditionally did.
NEXT LINE and NO-BREAK SPACE may or may not match \s depending
on the rules in effect. See
the beginning of this section.
\pP
and \p{Prop}
are character classes to match characters that fit given
Unicode properties. One letter property names can be used in the \pP
form,
with the property name following the \p
, otherwise, braces are required.
When using braces, there is a single form, which is just the property name
enclosed in the braces, and a compound form which looks like \p{name=value}
,
which means to match if the property "name" for the character has that particular
"value".
For instance, a match for a number can be written as /\pN/
or as
/\p{Number}/
, or as /\p{Number=True}/
.
Lowercase letters are matched by the property Lowercase_Letter which
has the short form Ll. They need the braces, so are written as /\p{Ll}/
or
/\p{Lowercase_Letter}/
, or /\p{General_Category=Lowercase_Letter}/
(the underscores are optional).
/\pLl/
is valid, but means something different.
It matches a two character string: a letter (Unicode property \pL
),
followed by a lowercase l
.
If locale rules are not in effect, the use of a Unicode property will force the regular expression into using Unicode rules, if it isn't already.
Note that almost all properties are immune to case-insensitive matching.
That is, adding a /i regular expression modifier does not change what
they match. There are two sets that are affected. The first set is
Uppercase_Letter
,
Lowercase_Letter
,
and Titlecase_Letter
,
all of which match Cased_Letter
under /i matching.
The second set is
Uppercase
,
Lowercase
,
and Titlecase
,
all of which match Cased
under /i matching.
(The difference between these sets is that some things, such as Roman
numerals, come in both upper and lower case, so they are Cased
, but
aren't considered to be letters, so they aren't Cased_Letter
s. They're
actually Letter_Number
s.)
This set also includes its subsets PosixUpper
and PosixLower
, both
of which under /i match PosixAlpha
.
For more details on Unicode properties, see Unicode Character Properties in perlunicode; for a
complete list of possible properties, see
Properties accessible through \p{} and \P{} in perluniprops,
which notes all forms that have /i differences.
It is also possible to define your own properties. This is discussed in
User-Defined Character Properties in perlunicode.
Unicode properties are defined (surprise!) only on Unicode code points.
Starting in v5.20, when matching against \p
and \P
, Perl treats
non-Unicode code points (those above the legal Unicode maximum of
0x10FFFF) as if they were typical unassigned Unicode code points.
Prior to v5.20, Perl raised a warning and made all matches fail on non-Unicode code points. This could be somewhat surprising:
Even though these two matches might be thought of as complements, until v5.20 they were so only on Unicode code points.
- "a" =~ /\w/ # Match, "a" is a 'word' character.
- "7" =~ /\w/ # Match, "7" is a 'word' character as well.
- "a" =~ /\d/ # No match, "a" isn't a digit.
- "7" =~ /\d/ # Match, "7" is a digit.
- " " =~ /\s/ # Match, a space is whitespace.
- "a" =~ /\D/ # Match, "a" is a non-digit.
- "7" =~ /\D/ # No match, "7" is not a non-digit.
- " " =~ /\S/ # No match, a space is not non-whitespace.
- " " =~ /\h/ # Match, space is horizontal whitespace.
- " " =~ /\v/ # No match, space is not vertical whitespace.
- "\r" =~ /\v/ # Match, a return is vertical whitespace.
- "a" =~ /\pL/ # Match, "a" is a letter.
- "a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters.
- "\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character
- # 'THAI CHARACTER SO SO', and that's in
- # Thai Unicode class.
- "a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character.
It is worth emphasizing that \d
, \w
, etc, match single characters, not
complete numbers or words. To match a number (that consists of digits),
use \d+
; to match a word, use \w+
. But be aware of the security
considerations in doing so, as mentioned above.
The third form of character class you can use in Perl regular expressions
is the bracketed character class. In its simplest form, it lists the characters
that may be matched, surrounded by square brackets, like this: [aeiou]
.
This matches one of a
, e
, i
, o
or u
. Like the other
character classes, exactly one character is matched.* To match
a longer string consisting of characters mentioned in the character
class, follow the character class with a quantifier. For
instance, [aeiou]+
matches one or more lowercase English vowels.
Repeating a character in a character class has no effect; it's considered to be in the set only once.
Examples:
- "e" =~ /[aeiou]/ # Match, as "e" is listed in the class.
- "p" =~ /[aeiou]/ # No match, "p" is not listed in the class.
- "ae" =~ /^[aeiou]$/ # No match, a character class only matches
- # a single character.
- "ae" =~ /^[aeiou]+$/ # Match, due to the quantifier.
- -------
* There are two exceptions to a bracketed character class matching a single character only. Each requires special handling by Perl to make things work:
When the class is to match caselessly under /i matching rules, and a
character that is explicitly mentioned inside the class matches a
multiple-character sequence caselessly under Unicode rules, the class
will also match that sequence. For example, Unicode says that the
letter LATIN SMALL LETTER SHARP S
should match the sequence ss
under /i rules. Thus,
- 'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i # Matches
- 'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i # Matches
For this to happen, the class must not be inverted (see Negation) and the character must be explicitly specified, and not be part of a multi-character range (not even as one of its endpoints). (Character Ranges will be explained shortly.) Therefore,
- 'ss' =~ /\A[\0-\x{ff}]\z/ui # Doesn't match
- 'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/ui # No match
- 'ss' =~ /\A[\xDF-\xDF]\z/ui # Matches on ASCII platforms, since
- # \xDF is LATIN SMALL LETTER SHARP S,
- # and the range is just a single
- # element
Note that it isn't a good idea to specify these types of ranges anyway.
Some names known to \N{...}
refer to a sequence of multiple characters,
instead of the usual single character. When one of these is included in
the class, the entire sequence is matched. For example,
- "\N{TAMIL LETTER KA}\N{TAMIL VOWEL SIGN AU}"
- =~ / ^ [\N{TAMIL SYLLABLE KAU}] $ /x;
matches, because \N{TAMIL SYLLABLE KAU}
is a named sequence
consisting of the two characters matched against. Like the other
instance where a bracketed class can match multiple characters, and for
similar reasons, the class must not be inverted, and the named sequence
may not appear in a range, even one where it is both endpoints. If
these happen, it is a fatal error if the character class is within an
extended (?[...])
class; and only the first code point is used (with
a regexp
-type warning raised) otherwise.
Most characters that are meta characters in regular expressions (that
is, characters that carry a special meaning like ., *
, or () lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, [()]
matches either an opening
parenthesis, or a closing parenthesis, and the parens inside the character
class don't group or capture.
Characters that may carry a special meaning inside a character class are:
\
, ^, -
, [ and ], and are discussed below. They can be
escaped with a backslash, although this is sometimes not needed, in which
case the backslash may be omitted.
The sequence \b
is special inside a bracketed character class. While
outside the character class, \b
is an assertion indicating a point
that does not have either two word characters or two non-word characters
on either side, inside a bracketed character class, \b
matches a
backspace character.
The sequences
\a
,
\c
,
\e
,
\f
,
\n
,
\N{NAME},
\N{U+hex char},
\r
,
\t
,
and
\x
are also special and have the same meanings as they do outside a
bracketed character class.
Also, a backslash followed by two or three octal digits is considered an octal number.
A [ is not special inside a character class, unless it's the start of a
POSIX character class (see POSIX Character Classes below). It normally does
not need escaping.
A ] is normally either the end of a POSIX character class (see
POSIX Character Classes below), or it signals the end of the bracketed
character class. If you want to include a ] in the set of characters, you
must generally escape it.
However, if the ] is the first (or the second if the first
character is a caret) character of a bracketed character class, it
does not denote the end of the class (as you cannot have an empty class)
and is considered part of the set of characters that can be matched without
escaping.
Examples:
- "+" =~ /[+?*]/ # Match, "+" in a character class is not special.
- "\cH" =~ /[\b]/ # Match, \b inside in a character class
- # is equivalent to a backspace.
- "]" =~ /[][]/ # Match, as the character class contains
- # both [ and ].
- "[]" =~ /[[]]/ # Match, the pattern contains a character class
- # containing just [, and the character class is
- # followed by a ].
It is not uncommon to want to match a range of characters. Luckily, instead
of listing all characters in the range, one may use the hyphen (-
).
If inside a bracketed character class you have two characters separated
by a hyphen, it's treated as if all characters between the two were in
the class. For instance, [0-9]
matches any ASCII digit, and [a-m]
matches any lowercase letter from the first half of the ASCII alphabet.
Note that the two characters on either side of the hyphen are not
necessarily both letters or both digits. Any character is possible,
although not advisable. ['-?] contains a range of characters, but
most people will not know which characters that means. Furthermore,
such ranges may lead to portability problems if the code has to run on
a platform that uses a different character set, such as EBCDIC.
If a hyphen in a character class cannot syntactically be part of a range, for instance because it is the first or the last character of the character class, or if it immediately follows a range, the hyphen isn't special, and so is considered a character to be matched literally. If you want a hyphen in your set of characters to be matched and its position in the class is such that it could be considered part of a range, you must escape that hyphen with a backslash.
Examples:
- [a-z] # Matches a character that is a lower case ASCII letter.
- [a-fz] # Matches any letter between 'a' and 'f' (inclusive) or
- # the letter 'z'.
- [-z] # Matches either a hyphen ('-') or the letter 'z'.
- [a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
- # hyphen ('-'), or the letter 'm'.
- ['-?] # Matches any of the characters '()*+,-./0123456789:;<=>?
- # (But not on an EBCDIC platform).
- [\N{APOSTROPHE}-\N{QUESTION MARK}]
- # Matches any of the characters '()*+,-./0123456789:;<=>?
- # even on an EBCDIC platform.
- [\N{U+27}-\N{U+3F}] # Same. (U+27 is "'", and U+3F is "?")
As the final two examples above show, you can achieve portablity to
non-ASCII platforms by using the \N{...}
form for the range
endpoints. These indicate that the specified range is to be interpreted
using Unicode values, so [\N{U+27}-\N{U+3F}]
means to match
\N{U+27}
, \N{U+28}
, \N{U+29}
, ..., \N{U+3D}
, \N{U+3E}
,
and \N{U+3F}
, whatever the native code point versions for those are.
These are called "Unicode" ranges. If either end is of the \N{...}
form, the range is considered Unicode. A regexp
warning is raised
under "use re 'strict'"
if the other endpoint is specified
non-portably:
- [\N{U+00}-\x09] # Warning under re 'strict'; \x09 is non-portable
- [\N{U+00}-\t] # No warning;
Both of the above match the characters \N{U+00}
\N{U+01}
, ...
\N{U+08}
, \N{U+09}
, but the \x09
looks like it could be a
mistake so the warning is raised (under re 'strict'
) for it.
Perl also guarantees that the ranges A-Z
, a-z
, 0-9
, and any
subranges of these match what an English-only speaker would expect them
to match on any platform. That is, [A-Z]
matches the 26 ASCII
uppercase letters;
[a-z]
matches the 26 lowercase letters; and [0-9]
matches the 10
digits. Subranges, like [h-k]
, match correspondingly, in this case
just the four letters "h"
, "i"
, "j"
, and "k"
. This is the
natural behavior on ASCII platforms where the code points (ordinal
values) for "h"
through "k"
are consecutive integers (0x68 through
0x6B). But special handling to achieve this may be needed on platforms
with a non-ASCII native character set. For example, on EBCDIC
platforms, the code point for "h"
is 0x88, "i"
is 0x89, "j"
is
0x91, and "k"
is 0x92. Perl specially treats [h-k]
to exclude the
seven code points in the gap: 0x8A through 0x90. This special handling is
only invoked when the range is a subrange of one of the ASCII uppercase,
lowercase, and digit ranges, AND each end of the range is expressed
either as a literal, like "A"
, or as a named character (\N{...}
,
including the \N{U+...
form).
EBCDIC Examples:
- [i-j] # Matches either "i" or "j"
- [i-\N{LATIN SMALL LETTER J}] # Same
- [i-\N{U+6A}] # Same
- [\N{U+69}-\N{U+6A}] # Same
- [\x{89}-\x{91}] # Matches 0x89 ("i"), 0x8A .. 0x90, 0x91 ("j")
- [i-\x{91}] # Same
- [\x{89}-j] # Same
- [i-J] # Matches, 0x89 ("i") .. 0xC1 ("J"); special
- # handling doesn't apply because range is mixed
- # case
It is also possible to instead list the characters you do not want to
match. You can do so by using a caret (^) as the first character in the
character class. For instance, [^a-z] matches any character that is not a
lowercase ASCII letter, which therefore includes more than a million
Unicode code points. The class is said to be "negated" or "inverted".
This syntax make the caret a special character inside a bracketed character class, but only if it is the first character of the class. So if you want the caret as one of the characters to match, either escape the caret or else don't list it first.
In inverted bracketed character classes, Perl ignores the Unicode rules
that normally say that named sequence, and certain characters should
match a sequence of multiple characters use under caseless /i
matching. Following those rules could lead to highly confusing
situations:
- "ss" =~ /^[^\xDF]+$/ui; # Matches!
This should match any sequences of characters that aren't \xDF
nor
what \xDF
matches under /i. "s"
isn't \xDF
, but Unicode
says that "ss"
is what \xDF
matches under /i. So which one
"wins"? Do you fail the match because the string has ss
or accept it
because it has an s followed by another s? Perl has chosen the
latter. (See note in Bracketed Character Classes above.)
Examples:
- "e" =~ /[^aeiou]/ # No match, the 'e' is listed.
- "x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel.
- "^" =~ /[^^]/ # No match, matches anything that isn't a caret.
- "^" =~ /[x^]/ # Match, caret is not special here.
You can put any backslash sequence character class (with the exception of
\N
and \R
) inside a bracketed character class, and it will act just
as if you had put all characters matched by the backslash sequence inside the
character class. For instance, [a-f\d]
matches any decimal digit, or any
of the lowercase letters between 'a' and 'f' inclusive.
\N
within a bracketed character class must be of the forms \N{name}
or \N{U+hex char}, and NOT be the form that matches non-newlines,
for the same reason that a dot . inside a bracketed character class loses
its special meaning: it matches nearly anything, which generally isn't what you
want to happen.
Examples:
- /[\p{Thai}\d]/ # Matches a character that is either a Thai
- # character, or a digit.
- /[^\p{Arabic}()]/ # Matches a character that is neither an Arabic
- # character, nor a parenthesis.
Backslash sequence character classes cannot form one of the endpoints of a range. Thus, you can't say:
- /[\p{Thai}-\d]/ # Wrong!
POSIX character classes have the form [:class:], where class is the
name, and the [: and :] delimiters. POSIX character classes only appear
inside bracketed character classes, and are a convenient and descriptive
way of listing a group of characters.
Be careful about the syntax,
- # Correct:
- $string =~ /[[:alpha:]]/
- # Incorrect (will warn):
- $string =~ /[:alpha:]/
The latter pattern would be a character class consisting of a colon,
and the letters a
, l
, p
and h
.
POSIX character classes can be part of a larger bracketed character class. For example,
- [01[:alpha:]%]
is valid and matches '0', '1', any alphabetic character, and the percent sign.
Perl recognizes the following POSIX character classes:
- alpha Any alphabetical character ("[A-Za-z]").
- alnum Any alphanumeric character ("[A-Za-z0-9]").
- ascii Any character in the ASCII character set.
- blank A GNU extension, equal to a space or a horizontal tab ("\t").
- cntrl Any control character. See Note [2] below.
- digit Any decimal digit ("[0-9]"), equivalent to "\d".
- graph Any printable character, excluding a space. See Note [3] below.
- lower Any lowercase character ("[a-z]").
- print Any printable character, including a space. See Note [4] below.
- punct Any graphical character excluding "word" characters. Note [5].
- space Any whitespace character. "\s" including the vertical tab
- ("\cK").
- upper Any uppercase character ("[A-Z]").
- word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
- xdigit Any hexadecimal digit ("[0-9a-fA-F]").
Like the Unicode properties, most of the POSIX
properties match the same regardless of whether case-insensitive (/i)
matching is in effect or not. The two exceptions are [:upper:] and
[:lower:]. Under /i, they each match the union of [:upper:] and
[:lower:].
Most POSIX character classes have two Unicode-style \p
property
counterparts. (They are not official Unicode properties, but Perl extensions
derived from official Unicode properties.) The table below shows the relation
between POSIX character classes and these counterparts.
One counterpart, in the column labelled "ASCII-range Unicode" in the table, matches only characters in the ASCII character set.
The other counterpart, in the column labelled "Full-range Unicode", matches any
appropriate characters in the full Unicode character set. For example,
\p{Alpha}
matches not just the ASCII alphabetic characters, but any
character in the entire Unicode character set considered alphabetic.
An entry in the column labelled "backslash sequence" is a (short)
equivalent.
- [[:...:]] ASCII-range Full-range backslash Note
- Unicode Unicode sequence
- -----------------------------------------------------
- alpha \p{PosixAlpha} \p{XPosixAlpha}
- alnum \p{PosixAlnum} \p{XPosixAlnum}
- ascii \p{ASCII}
- blank \p{PosixBlank} \p{XPosixBlank} \h [1]
- or \p{HorizSpace} [1]
- cntrl \p{PosixCntrl} \p{XPosixCntrl} [2]
- digit \p{PosixDigit} \p{XPosixDigit} \d
- graph \p{PosixGraph} \p{XPosixGraph} [3]
- lower \p{PosixLower} \p{XPosixLower}
- print \p{PosixPrint} \p{XPosixPrint} [4]
- punct \p{PosixPunct} \p{XPosixPunct} [5]
- \p{PerlSpace} \p{XPerlSpace} \s [6]
- space \p{PosixSpace} \p{XPosixSpace} [6]
- upper \p{PosixUpper} \p{XPosixUpper}
- word \p{PosixWord} \p{XPosixWord} \w
- xdigit \p{PosixXDigit} \p{XPosixXDigit}
\p{Blank}
and \p{HorizSpace}
are synonyms.
Control characters don't produce output as such, but instead usually control
the terminal somehow: for example, newline and backspace are control characters.
On ASCII platforms, in the ASCII range, characters whose code points are
between 0 and 31 inclusive, plus 127 (DEL
) are control characters; on
EBCDIC platforms, their counterparts are control characters.
Any character that is graphical, that is, visible. This class consists of all alphanumeric characters and all punctuation characters.
All printable characters, which is the set of all graphical characters plus those whitespace characters which are not also controls.
\p{PosixPunct}
and [[:punct:]] in the ASCII range match all
non-controls, non-alphanumeric, non-space characters:
[-!"#$%&'()*+,./:;<=>?@[\\\]^_`{|}~] (although if a locale is in effect,
it could alter the behavior of [[:punct:]]).
The similarly named property, \p{Punct}
, matches a somewhat different
set in the ASCII range, namely
[-!"#%&'()*,./:;?@[\\\]_{}]
. That is, it is missing the nine
characters [$+<=>^`|~]
.
This is because Unicode splits what POSIX considers to be punctuation into two
categories, Punctuation and Symbols.
\p{XPosixPunct}
and (under Unicode rules) [[:punct:]], match what
\p{PosixPunct}
matches in the ASCII range, plus what \p{Punct}
matches. This is different than strictly matching according to
\p{Punct}
. Another way to say it is that
if Unicode rules are in effect, [[:punct:]] matches all characters
that Unicode considers punctuation, plus all ASCII-range characters that
Unicode considers symbols.
\p{XPerlSpace}
and \p{Space}
match identically starting with Perl
v5.18. In earlier versions, these differ only in that in non-locale
matching, \p{XPerlSpace}
did not match the vertical tab, \cK
.
Same for the two ASCII-only range forms.
There are various other synonyms that can be used besides the names
listed in the table. For example, \p{PosixAlpha}
can be written as
\p{Alpha}
. All are listed in
Properties accessible through \p{} and \P{} in perluniprops.
Both the \p
counterparts always assume Unicode rules are in effect.
On ASCII platforms, this means they assume that the code points from 128
to 255 are Latin-1, and that means that using them under locale rules is
unwise unless the locale is guaranteed to be Latin-1 or UTF-8. In contrast, the
POSIX character classes are useful under locale rules. They are
affected by the actual rules in effect, as follows:
/a
modifier, is in effect ...
Each of the POSIX classes matches exactly the same as their ASCII-range counterparts.
The POSIX class matches the same as its Full-range counterpart.
The POSIX class matches according to the locale, except:
word
also includes the platform's native underscore character, no matter what the locale is.
ascii
on platforms that don't have the POSIX ascii
extension, this matches
just the platform's native ASCII-range characters.
blank
on platforms that don't have the POSIX blank
extension, this matches
just the platform's native tab and space characters.
The POSIX class matches the same as the Full-range counterpart.
The POSIX class matches the same as the ASCII range counterpart.
Which rules apply are determined as described in Which character set modifier is in effect? in perlre.
It is proposed to change this behavior in a future release of Perl so that
whether or not Unicode rules are in effect would not change the
behavior: Outside of locale, the POSIX classes
would behave like their ASCII-range counterparts. If you wish to
comment on this proposal, send email to perl5-porters@perl.org
.
A Perl extension to the POSIX character class is the ability to
negate it. This is done by prefixing the class name with a caret (^).
Some examples:
- POSIX ASCII-range Full-range backslash
- Unicode Unicode sequence
- -----------------------------------------------------
- [[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D
- [[:^space:]] \P{PosixSpace} \P{XPosixSpace}
- \P{PerlSpace} \P{XPerlSpace} \S
- [[:^word:]] \P{PerlWord} \P{XPosixWord} \W
The backslash sequence can mean either ASCII- or Full-range Unicode, depending on various factors as described in Which character set modifier is in effect? in perlre.
Perl recognizes the POSIX character classes [=class=]
and
[.class.], but does not (yet?) support them. Any attempt to use
either construct raises an exception.
- /[[:digit:]]/ # Matches a character that is a digit.
- /[01[:lower:]]/ # Matches a character that is either a
- # lowercase letter, or '0' or '1'.
- /[[:digit:][:^xdigit:]]/ # Matches a character that can be anything
- # except the letters 'a' to 'f' and 'A' to
- # 'F'. This is because the main character
- # class is composed of two POSIX character
- # classes that are ORed together, one that
- # matches any digit, and the other that
- # matches anything that isn't a hex digit.
- # The OR adds the digits, leaving only the
- # letters 'a' to 'f' and 'A' to 'F' excluded.
This is a fancy bracketed character class that can be used for more readable and less error-prone classes, and to perform set operations, such as intersection. An example is
- /(?[ \p{Thai} & \p{Digit} ])/
This will match all the digit characters that are in the Thai script.
This is an experimental feature available starting in 5.18, and is subject to change as we gain field experience with it. Any attempt to use it will raise a warning, unless disabled via
- no warnings "experimental::regex_sets";
Comments on this feature are welcome; send email to
perl5-porters@perl.org
.
We can extend the example above:
- /(?[ ( \p{Thai} + \p{Lao} ) & \p{Digit} ])/
This matches digits that are in either the Thai or Laotian scripts.
Notice the white space in these examples. This construct always has
the /x modifier turned on within it.
The available binary operators are:
- & intersection
- + union
- | another name for '+', hence means union
- - subtraction (the result matches the set consisting of those
- code points matched by the first operand, excluding any that
- are also matched by the second operand)
- ^ symmetric difference (the union minus the intersection). This
- is like an exclusive or, in that the result is the set of code
- points that are matched by either, but not both, of the
- operands.
There is one unary operator:
- ! complement
All the binary operators left associate; "&"
is higher precedence
than the others, which all have equal precedence. The unary operator
right associates, and has highest precedence. Thus this follows the
normal Perl precedence rules for logical operators. Use parentheses to
override the default precedence and associativity.
The main restriction is that everything is a metacharacter. Thus, you cannot refer to single characters by doing something like this:
- /(?[ a + b ])/ # Syntax error!
The easiest way to specify an individual typable character is to enclose it in brackets:
- /(?[ [a] + [b] ])/
(This is the same thing as [ab]
.) You could also have said the
equivalent:
- /(?[[ a b ]])/
(You can, of course, specify single characters by using, \x{...}
,
\N{...}
, etc.)
This last example shows the use of this construct to specify an ordinary
bracketed character class without additional set operations. Note the
white space within it; /x is turned on even within bracketed
character classes, except you can't have comments inside them. Hence,
- (?[ [#] ])
matches the literal character "#". To specify a literal white space character, you can escape it with a backslash, like:
- /(?[ [ a e i o u \ ] ])/
This matches the English vowels plus the SPACE character. All the other escapes accepted by normal bracketed character classes are accepted here as well; but unrecognized escapes that generate warnings in normal classes are fatal errors here.
All warnings from these class elements are fatal, as well as some practices that don't currently warn. For example you cannot say
- /(?[ [ \xF ] ])/ # Syntax error!
You have to have two hex digits after a braceless \x
(use a leading
zero to make two). These restrictions are to lower the incidence of
typos causing the class to not match what you thought it would.
If a regular bracketed character class contains a \p{}
or \P{}
and
is matched against a non-Unicode code point, a warning may be
raised, as the result is not Unicode-defined. No such warning will come
when using this extended form.
The final difference between regular bracketed character classes and these, is that it is not possible to get these to match a multi-character fold. Thus,
- /(?[ [\xDF] ])/iu
does not match the string ss
.
You don't have to enclose POSIX class names inside double brackets, hence both of the following work:
- /(?[ [:word:] - [:lower:] ])/
- /(?[ [[:word:]] - [[:lower:]] ])/
Any contained POSIX character classes, including things like \w
and \D
respect the /a
(and /aa
) modifiers.
(?[ ])
is a regex-compile-time construct. Any attempt to use
something which isn't knowable at the time the containing regular
expression is compiled is a fatal error. In practice, this means
just three limitations:
This construct cannot be used within the scope of
use locale
(or the /l
regex modifier).
Any user-defined property used must be already defined by the time the regular expression is compiled (but note that this construct can be used instead of such properties).
A regular expression that otherwise would compile
using /d rules, and which uses this construct will instead
use /u
. Thus this construct tells Perl that you don't want
/d rules for the entire regular expression containing it.
Note that skipping white space applies only to the interior of this
construct. There must not be any space between any of the characters
that form the initial (?[
. Nor may there be space between the
closing ])
characters.
Just as in all regular expressions, the pattern can be built up by including variables that are interpolated at regex compilation time. Care must be taken to ensure that you are getting what you expect. For example:
- my $thai_or_lao = '\p{Thai} + \p{Lao}';
- ...
- qr/(?[ \p{Digit} & $thai_or_lao ])/;
compiles to
- qr/(?[ \p{Digit} & \p{Thai} + \p{Lao} ])/;
But this does not have the effect that someone reading the code would
likely expect, as the intersection applies just to \p{Thai}
,
excluding the Laotian. Pitfalls like this can be avoided by
parenthesizing the component pieces:
- my $thai_or_lao = '( \p{Thai} + \p{Lao} )';
But any modifiers will still apply to all the components:
- my $lower = '\p{Lower} + \p{Digit}';
- qr/(?[ \p{Greek} & $lower ])/i;
matches upper case things. You can avoid surprises by making the components into instances of this construct by compiling them:
When these are embedded in another pattern, what they match does not change, regardless of parenthesization or what modifiers are in effect in that outer pattern.
Due to the way that Perl parses things, your parentheses and brackets
may need to be balanced, even including comments. If you run into any
examples, please send them to perlbug@perl.org
, so that we can have a
concrete example for this man page.
We may change it so that things that remain legal uses in normal bracketed
character classes might become illegal within this experimental
construct. One proposal, for example, is to forbid adjacent uses of the
same character, as in (?[ [aa] ])
. The motivation for such a change
is that this usage is likely a typo, as the second "a" adds nothing.