=head1 NAME
+X<character class>
perlrecharclass - Perl Regular Expression Character Classes
is found in L<perlre>.
This manual page discusses the syntax and use of character
-classes in Perl Regular Expressions.
+classes in Perl regular expressions.
-A character class is a way of denoting a set of characters,
+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
+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, backslashed sequences, and the bracketed form.
+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.
=head2 The dot
The dot (or period), C<.> is probably the most used, and certainly
the most well-known character class. By default, a dot matches any
-character, except for the newline. The default can be changed to
-add matching the newline with the I<single line> modifier: either
-for the entire regular expression using the C</s> modifier, or
-locally using C<(?s)>.
+character, except for the newline. That default can be changed to
+add matching the newline by using the I<single line> modifier: either
+for the entire regular expression with the C</s> modifier, or
+locally with C<(?s)>. (The C<L</\N>> backslash sequence, described
+below, matches any character except newline without regard to the
+I<single line> modifier.)
Here are some examples:
"\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
"ab" =~ /^.$/ # No match (dot matches one character)
+=head2 Backslash sequences
+X<\w> X<\W> X<\s> X<\S> X<\d> X<\D> X<\p> X<\P>
+X<\N> X<\v> X<\V> X<\h> X<\H>
+X<word> X<whitespace>
-=head2 Backslashed sequences
+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.
-Perl regular expressions contain many backslashed sequences that
-constitute a character class. That is, they will match a single
-character, if that character belongs to a specific set of characters
-(defined by the sequence). A backslashed sequence is a sequence of
-characters starting with a backslash. Not all backslashed sequences
-are character class; for a full list, see L<perlrebackslash>.
+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 L<perlrebackslash>.)
-Here's a list of the backslashed sequences, which are discussed in
-more detail below.
-
- \d Match a digit character.
- \D Match a non-digit character.
+ \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 white space character.
- \S Match a non-white space character.
- \h Match a horizontal white space character.
- \H Match a character that isn't horizontal white space.
- \v Match a vertical white space character.
- \V Match a character that isn't vertical white space.
- \pP, \p{Prop} Match a character matching a Unicode property.
- \PP, \P{Prop} Match a character that doesn't match a Unicode property.
+ \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
+
+=head3 \N
+
+C<\N>, available starting in v5.12, like the dot, matches any
+character that is not a newline. The difference is that C<\N> is not influenced
+by the I<single line> regular expression modifier (see L</The dot> above). Note
+that the form C<\N{...}> may mean something completely different. When the
+C<{...}> is a L<quantifier|perlre/Quantifiers>, it means to match a non-newline
+character that many times. For example, C<\N{3}> means to match 3
+non-newlines; C<\N{5,}> means to match 5 or more non-newlines. But if C<{...}>
+is not a legal quantifier, it is presumed to be a named character. See
+L<charnames> for those. For example, none of C<\N{COLON}>, C<\N{4F}>, and
+C<\N{F4}> contain legal quantifiers, so Perl will try to find characters whose
+names are respectively C<COLON>, C<4F>, and C<F4>.
=head3 Digits
-C<\d> matches a single character that is considered to be a I<digit>.
-What is considered a digit depends on the internal encoding of
-the source string. If the source string is in UTF-8 format, C<\d>
-not only matches the digits '0' - '9', but also Arabic, Devanagari and
-digits from other languages. Otherwise, if there is a locale in effect,
-it will match whatever characters the locale considers digits. Without
-a locale, C<\d> matches the digits '0' to '9'.
-See L</Locale, Unicode and UTF-8>.
-
-Any character that isn't matched by C<\d> will be matched by C<\D>.
+C<\d> matches a single character considered to be a decimal I<digit>.
+If the C</a> regular expression modifier is in effect, it matches [0-9].
+Otherwise, it
+matches anything that is matched by C<\p{Digit}>, which includes [0-9].
+(An unlikely possible exception is that under locale matching rules, the
+current locale might not have C<[0-9]> matched by C<\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 C<[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 C</a> modifier is in effect C<\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 C<\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
+C<\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. L<Unicode::UCD/num()> can
+be used to safely
+calculate the value, returning C<undef> if the input string contains
+such a mixture.
+
+What C<\p{Digit}> means (and hence C<\d> except under the C</a>
+modifier) is C<\p{General_Category=Decimal_Number}>, or synonymously,
+C<\p{General_Category=Digit}>. Starting with Unicode version 4.1, this
+is the same set of characters matched by C<\p{Numeric_Type=Decimal}>.
+But Unicode also has a different property with a similar name,
+C<\p{Numeric_Type=Digit}>, which matches a completely different set of
+characters. These characters are things such as C<CIRCLED DIGIT ONE>
+or subscripts, or are from writing systems that lack all ten digits.
+
+The design intent is for C<\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",
+C<\p{Numeric_Type=Digit}>, and so C<\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 L<http://www.unicode.org/notes/tn21>.)
+
+Any character not matched by C<\d> is matched by C<\D>.
=head3 Word characters
-C<\w> matches a single I<word> character: an alphanumeric character
-(that is, an alphabetic character, or a digit), or the underscore (C<_>).
-What is considered a word character depends on the internal encoding
-of the string. If it's in UTF-8 format, C<\w> matches those characters
-that are considered word characters in the Unicode database. That is, it
-not only matches ASCII letters, but also Thai letters, Greek letters, etc.
-If the source string isn't in UTF-8 format, C<\w> matches those characters
-that are considered word characters by the current locale. Without
-a locale in effect, C<\w> matches the ASCII letters, digits and the
-underscore.
-
-Any character that isn't matched by C<\w> will be matched by C<\W>.
-
-=head3 White space
-
-C<\s> matches any single character that is consider white space. In the
-ASCII range, C<\s> matches the horizontal tab (C<\t>), the new line
-(C<\n>), the form feed (C<\f>), the carriage return (C<\r>), and the
-space (the vertical tab, C<\cK> is not matched by C<\s>). The exact set
-of characters matched by C<\s> depends on whether the source string is
-in UTF-8 format. If it is, C<\s> matches what is considered white space
-in the Unicode database. Otherwise, if there is a locale in effect, C<\s>
-matches whatever is considered white space by the current locale. Without
-a locale, C<\s> matches the five characters mentioned in the beginning
-of this paragraph. Perhaps the most notable difference is that C<\s>
-matches a non-breaking space only if the non-breaking space is in a
-UTF-8 encoded string.
-
-Any character that isn't matched by C<\s> will be matched by C<\S>.
-
-C<\h> will match any character that is considered horizontal white space;
-this includes the space and the tab characters. C<\H> will match any character
-that is not considered horizontal white space.
-
-C<\v> will match any character that is considered vertical white space;
-this includes the carriage return and line feed characters (newline).
-C<\V> will match any character that is not considered vertical white space.
+A C<\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 C<\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.
+
+=over
+
+=item If the C</a> modifier is in effect ...
+
+C<\w> matches the 63 characters [a-zA-Z0-9_].
+
+=item otherwise ...
+
+=over
+
+=item For code points above 255 ...
+
+C<\w> matches the same as C<\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 C<COMBINING TILDE> and the modifier letters, which
+are generally used to add auxiliary markings to letters.
+
+=item For code points below 256 ...
+
+=over
+
+=item if locale rules are in effect ...
+
+C<\w> matches the platform's native underscore character plus whatever
+the locale considers to be alphanumeric.
+
+=item if Unicode rules are in effect ...
+
+C<\w> matches exactly what C<\p{Word}> matches.
+
+=item otherwise ...
+
+C<\w> matches [a-zA-Z0-9_].
+
+=back
+
+=back
+
+=back
+
+Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.
+
+There are a number of security issues with the full Unicode list of word
+characters. See L<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 L</Unicode Properties>, C<\p{ID_Start}>,
+C<\p{ID_Continue}>, C<\p{XID_Start}>, and C<\p{XID_Continue}>. See
+L<http://unicode.org/reports/tr31>.
+
+Any character not matched by C<\w> is matched by C<\W>.
+
+=head3 Whitespace
+
+C<\s> matches any single character considered whitespace.
+
+=over
+
+=item If the C</a> modifier is in effect ...
+
+In all Perl versions, C<\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, experimentally, it also matches the vertical tab, C<\cK>.
+See note C<[1]> below for a discussion of this.
+
+=item otherwise ...
+
+=over
+
+=item For code points above 255 ...
+
+C<\s> matches exactly the code points above 255 shown with an "s" column
+in the table below.
+
+=item For code points below 256 ...
+
+=over
+
+=item if locale rules are in effect ...
+
+C<\s> matches whatever the locale considers to be whitespace.
+
+=item if Unicode rules are in effect ...
+
+C<\s> matches exactly the characters shown with an "s" column in the
+table below.
+
+=item otherwise ...
+
+C<\s> matches [\t\n\f\r ] and, starting, experimentally in Perl
+v5.18, the vertical tab, C<\cK>.
+(See note C<[1]> below for a discussion of this.)
+Note that this list doesn't include the non-breaking space.
+
+=back
+
+=back
+
+=back
+
+Which rules apply are determined as described in L<perlre/Which character set modifier is in effect?>.
+
+Any character not matched by C<\s> is matched by C<\S>.
+
+C<\h> matches any character considered horizontal whitespace;
+this includes the platform's space and tab characters and several others
+listed in the table below. C<\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.
+
+C<\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.
+C<\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.
C<\R> matches anything that can be considered a newline under Unicode
rules. It's not a character class, as it can match a multi-character
sequence. Therefore, it cannot be used inside a bracketed character
-class. Details are discussed in L<perlrebackslash>.
-
-C<\h>, C<\H>, C<\v>, C<\V>, and C<\R> are new in perl 5.10.0.
+class; use C<\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 L<perlrebackslash>.
-Note that unlike C<\s>, C<\d> and C<\w>, C<\h> and C<\v> always match
-the same characters, regardless whether the source string is in UTF-8
-format or not. The set of characters they match is also not influenced
-by locale.
+Note that unlike C<\s> (and C<\d> and C<\w>), C<\h> and C<\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 C<\s> is equivalent with C<[\h\v]>. This is not true.
-The vertical tab (C<"\x0b">) is not matched by C<\s>, it is however
-considered vertical white space. Furthermore, if the source string is
-not in UTF-8 format, the next line (C<"\x85">) and the no-break space
-(C<"\xA0">) are not matched by C<\s>, but are by C<\v> and C<\h> respectively.
-If the source string is in UTF-8 format, both the next line and the
-no-break space are matched by C<\s>.
+One might think that C<\s> is equivalent to C<[\h\v]>. This is indeed true
+starting in Perl v5.18, but prior to that, the sole difference was that the
+vertical tab (C<"\cK">) was not matched by C<\s>.
The following table is a complete listing of characters matched by
-C<\s>, C<\h> and C<\v>.
+C<\s>, C<\h> and C<\v> as of Unicode 6.3.
-The first column gives the code point of the character (in hex format),
+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.
-
- 0x00009 CHARACTER TABULATION h s
- 0x0000a LINE FEED (LF) vs
- 0x0000b LINE TABULATION v
- 0x0000c FORM FEED (FF) vs
- 0x0000d CARRIAGE RETURN (CR) vs
- 0x00020 SPACE h s
- 0x00085 NEXT LINE (NEL) vs [1]
- 0x000a0 NO-BREAK SPACE h s [1]
- 0x01680 OGHAM SPACE MARK h s
- 0x0180e MONGOLIAN VOWEL SEPARATOR h s
- 0x02000 EN QUAD h s
- 0x02001 EM QUAD h s
- 0x02002 EN SPACE h s
- 0x02003 EM SPACE h s
- 0x02004 THREE-PER-EM SPACE h s
- 0x02005 FOUR-PER-EM SPACE h s
- 0x02006 SIX-PER-EM SPACE h s
- 0x02007 FIGURE SPACE h s
- 0x02008 PUNCTUATION SPACE h s
- 0x02009 THIN SPACE h s
- 0x0200a HAIR SPACE h s
- 0x02028 LINE SEPARATOR vs
- 0x02029 PARAGRAPH SEPARATOR vs
- 0x0202f NARROW NO-BREAK SPACE h s
- 0x0205f MEDIUM MATHEMATICAL SPACE h s
- 0x03000 IDEOGRAPHIC SPACE h s
+by which class(es) the character is matched (assuming no locale is in
+effect that changes the C<\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
=over 4
=item [1]
-NEXT LINE and NO-BREAK SPACE only match C<\s> if the source string is in
-UTF-8 format.
+Prior to Perl v5.18, C<\s> did not match the vertical tab. The change
+in v5.18 is considered an experiment, which means it could be backed out
+in v5.22 if experience indicates that it breaks too much
+existing code. If this change adversely affects you, send email to
+C<perlbug@perl.org>; if it affects you positively, email
+C<perlthanks@perl.org>. In the meantime, C<[^\S\cK]> (obscurely)
+matches what C<\s> traditionally did.
-=back
+=item [2]
-It is worth noting that C<\d>, C<\w>, etc, match single characters, not
-complete numbers or words. To match a number (that consists of integers),
-use C<\d+>; to match a word, use C<\w+>.
+NEXT LINE and NO-BREAK SPACE may or may not match C<\s> depending
+on the rules in effect. See
+L<the beginning of this section|/Whitespace>.
+=back
=head3 Unicode Properties
-C<\pP> and C<\p{Prop}> are character classes to match characters that
-fit given Unicode classes. One letter classes can be used in the C<\pP>
-form, with the class name following the C<\p>, otherwise, the property
-name is enclosed in braces, and follows the C<\p>. For instance, a
-match for a number can be written as C</\pN/> or as C</\p{Number}/>.
-Lowercase letters are matched by the property I<LowercaseLetter> which
-has as short form I<Ll>. They have to be written as C</\p{Ll}/> or
-C</\p{LowercaseLetter}/>. C</\pLl/> is valid, but means something different.
+C<\pP> and C<\p{Prop}> are character classes to match characters that fit given
+Unicode properties. One letter property names can be used in the C<\pP> form,
+with the property name following the C<\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 C<\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 C</\pN/> or as
+C</\p{Number}/>, or as C</\p{Number=True}/>.
+Lowercase letters are matched by the property I<Lowercase_Letter> which
+has the short form I<Ll>. They need the braces, so are written as C</\p{Ll}/> or
+C</\p{Lowercase_Letter}/>, or C</\p{General_Category=Lowercase_Letter}/>
+(the underscores are optional).
+C</\pLl/> is valid, but means something different.
It matches a two character string: a letter (Unicode property C<\pL>),
followed by a lowercase C<l>.
-For a list of possible properties, see
-L<perlunicode/Unicode Character Properties>. It is also possible to
-defined your own properties. This is discussed in
+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 C</i> regular expression modifier does not change what
+they match. There are two sets that are affected. The first set is
+C<Uppercase_Letter>,
+C<Lowercase_Letter>,
+and C<Titlecase_Letter>,
+all of which match C<Cased_Letter> under C</i> matching.
+The second set is
+C<Uppercase>,
+C<Lowercase>,
+and C<Titlecase>,
+all of which match C<Cased> under C</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 C<Cased>, but
+aren't considered to be letters, so they aren't C<Cased_Letter>s. They're
+actually C<Letter_Number>s.)
+This set also includes its subsets C<PosixUpper> and C<PosixLower>, both
+of which under C</i> match C<PosixAlpha>.
+
+For more details on Unicode properties, see L<perlunicode/Unicode
+Character Properties>; for a
+complete list of possible properties, see
+L<perluniprops/Properties accessible through \p{} and \P{}>,
+which notes all forms that have C</i> differences.
+It is also possible to define your own properties. This is discussed in
L<perlunicode/User-Defined Character Properties>.
+Unicode properties are defined (surprise!) only on Unicode code points.
+Starting in v5.20, when matching against C<\p> and C<\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:
+
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails on Perls < v5.20.
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Also fails on Perls
+ # < v5.20
+
+Even though these two matches might be thought of as complements, until
+v5.20 they were so only on Unicode code points.
=head4 Examples
"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 white space.
+ " " =~ /\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-white space.
+ " " =~ /\S/ # No match, a space is not non-whitespace.
- " " =~ /\h/ # Match, space is horizontal white space.
- " " =~ /\v/ # No match, space is not vertical white space.
- "\r" =~ /\v/ # Match, a return is vertical white space.
+ " " =~ /\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 Laoian character.
+ "a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character.
+It is worth emphasizing that C<\d>, C<\w>, etc, match single characters, not
+complete numbers or words. To match a number (that consists of digits),
+use C<\d+>; to match a word, use C<\w+>. But be aware of the security
+considerations in doing so, as mentioned above.
=head2 Bracketed Character Classes
The third form of character class you can use in Perl regular expressions
-is the bracketed form. In its simplest form, it lists the characters
-that may be matched inside square brackets, like this: C<[aeiou]>.
-This matches one of C<a>, C<e>, C<i>, C<o> or C<u>. Just as the other
-character classes, exactly one character will be matched. To match
-a longer string consisting of characters mentioned in the characters
-class, follow the character class with a quantifier. For instance,
-C<[aeiou]+> matches a string of one or more lowercase ASCII vowels.
+is the bracketed character class. In its simplest form, it lists the characters
+that may be matched, surrounded by square brackets, like this: C<[aeiou]>.
+This matches one of C<a>, C<e>, C<i>, C<o> or C<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 L<quantifier|perlre/Quantifiers>. For
+instance, C<[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.
# 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:
+
+=over
+
+=item *
+
+When the class is to match caselessly under C</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 C<LATIN SMALL LETTER SHARP S> should match the sequence C<ss>
+under C</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 L</Negation>)
+and the character must be explicitly specified, and not be part of a
+multi-character range (not even as one of its endpoints). (L</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.
+
+=item *
+
+Some names known to C<\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 C<\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 L<C<(?[...])>|/Extended Bracketed Character Classes>
+class; and only the first code point is used (with
+a C<regexp>-type warning raised) otherwise.
+
+=back
+
=head3 Special Characters Inside a Bracketed Character Class
Most characters that are meta characters in regular expressions (that
-is, characters that carry a special meaning like C<*> or C<(>) lose
+is, characters that carry a special meaning like C<.>, C<*>, or C<(>) lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, C<[()]> matches either an opening
parenthesis, or a closing parenthesis, and the parens inside the character
case the backslash may be omitted.
The sequence C<\b> is special inside a bracketed character class. While
-outside the character class C<\b> is an assertion indicating a point
+outside the character class, C<\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, C<\b> matches a
backspace character.
-A C<[> is not special inside a character class, unless it's the start
-of a POSIX character class (see below). It normally does not need escaping.
+The sequences
+C<\a>,
+C<\c>,
+C<\e>,
+C<\f>,
+C<\n>,
+C<\N{I<NAME>}>,
+C<\N{U+I<hex char>}>,
+C<\r>,
+C<\t>,
+and
+C<\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 C<[> is not special inside a character class, unless it's the start of a
+POSIX character class (see L</POSIX Character Classes> below). It normally does
+not need escaping.
+
+A C<]> is normally either the end of a POSIX character class (see
+L</POSIX Character Classes> below), or it signals the end of the bracketed
+character class. If you want to include a C<]> in the set of characters, you
+must generally escape it.
-A C<]> is either the end of a POSIX character class (see below), or it
-signals the end of the bracketed character class. Normally it needs
-escaping if you want to include a C<]> in the set of characters.
However, if the C<]> is the I<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)
Examples:
"+" =~ /[+?*]/ # Match, "+" in a character class is not special.
- "\cH" =~ /[\b]/ # Match, \b inside in a character class
- # is equivalent with a backspace.
+ "\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
=head3 Character Ranges
It is not uncommon to want to match a range of characters. Luckily, instead
-of listing all the characters in the range, one may use the hyphen (C<->).
+of listing all characters in the range, one may use the hyphen (C<->).
If inside a bracketed character class you have two characters separated
-by a hyphen, it's treated as if all the characters between the two are in
+by a hyphen, it's treated as if all characters between the two were in
the class. For instance, C<[0-9]> matches any ASCII digit, and C<[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
-necessary both letters or both digits. Any character is possible,
+necessarily both letters or both digits. Any character is possible,
although not advisable. C<['-?]> contains a range of characters, but
-most people will not know which characters that will be. Furthermore,
+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 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 will be
-considered a character that may be matched. You have to escape the hyphen
-with a backslash if you want to have a hyphen in your set of characters to
-be matched, and its position in the class is such that it can be considered
-part of a range.
+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'.
+ [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 C<\N{...}> form for the range
+endpoints. These indicate that the specified range is to be interpreted
+using Unicode values, so C<[\N{U+27}-\N{U+3F}]> means to match
+C<\N{U+27}>, C<\N{U+28}>, C<\N{U+29}>, ..., C<\N{U+3D}>, C<\N{U+3E}>,
+and C<\N{U+3F}>, whatever the native code point versions for those are.
+
+Perl also guarantees that the ranges C<A-Z>, C<a-z>, C<0-9>, and any
+subranges of these match what an English-only speaker would expect them
+to match on any platform. That is, C<[A-Z]> matches the 26 ASCII
+uppercase letters;
+C<[a-z]> matches the 26 lowercase letters; and C<[0-9]> matches the 10
+digits. Subranges, like C<[h-k]>, match correspondingly, in this case
+just the four letters C<"h">, C<"i">, C<"j">, and C<"k">. This is the
+natural behavior on ASCII platforms where the code points (ordinal
+values) for C<"h"> through C<"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 C<"h"> is 0x88, C<"i"> is 0x89, C<"j"> is
+0x91, and C<"k"> is 0x92. Perl specially treats C<[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 C<"A">, or as a named character (C<\N{...}>,
+including the C<\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
=head3 Negation
It is also possible to instead list the characters you do not want to
match. You can do so by using a caret (C<^>) as the first character in the
-character class. For instance, C<[^a-z]> matches a character that is not a
-lowercase ASCII letter.
+character class. For instance, C<[^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
-to have the caret as one of the characters you want to match, you either
-have to escape the caret, or not list it first.
+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 C</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 C<\xDF> nor
+what C<\xDF> matches under C</i>. C<"s"> isn't C<\xDF>, but Unicode
+says that C<"ss"> is what C<\xDF> matches under C</i>. So which one
+"wins"? Do you fail the match because the string has C<ss> or accept it
+because it has an C<s> followed by another C<s>? Perl has chosen the
+latter. (See note in L</Bracketed Character Classes> above.)
Examples:
=head3 Backslash Sequences
-You can put a backslash sequence character class inside a bracketed character
-class, and it will act just as if you put all the characters matched by
-the backslash sequence inside the character class. For instance,
-C<[a-f\d]> will match any digit, or any of the lowercase letters between
-'a' and 'f' inclusive.
+You can put any backslash sequence character class (with the exception of
+C<\N> and C<\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, C<[a-f\d]> matches any decimal digit, or any
+of the lowercase letters between 'a' and 'f' inclusive.
+
+C<\N> within a bracketed character class must be of the forms C<\N{I<name>}>
+or C<\N{U+I<hex char>}>, and NOT be the form that matches non-newlines,
+for the same reason that a dot C<.> inside a bracketed character class loses
+its special meaning: it matches nearly anything, which generally isn't what you
+want to happen.
+
Examples:
# character, nor a parenthesis.
Backslash sequence character classes cannot form one of the endpoints
-of a range.
+of a range. Thus, you can't say:
-=head3 Posix Character Classes
+ /[\p{Thai}-\d]/ # Wrong!
-Posix character classes have the form C<[:class:]>, where I<class> is
-name, and the C<[:> and C<:]> delimiters. Posix character classes appear
+=head3 POSIX Character Classes
+X<character class> X<\p> X<\p{}>
+X<alpha> X<alnum> X<ascii> X<blank> X<cntrl> X<digit> X<graph>
+X<lower> X<print> X<punct> X<space> X<upper> X<word> X<xdigit>
+
+POSIX character classes have the form C<[:class:]>, where I<class> is the
+name, and the C<[:> and C<:]> delimiters. POSIX character classes only appear
I<inside> bracketed character classes, and are a convenient and descriptive
-way of listing a group of characters. Be careful about the syntax,
+way of listing a group of characters.
+
+Be careful about the syntax,
# Correct:
$string =~ /[[:alpha:]]/
The latter pattern would be a character class consisting of a colon,
and the letters C<a>, C<l>, C<p> and C<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.
- alnum Any alphanumerical character.
- ascii Any ASCII character.
- blank A GNU extension, equal to a space or a horizontal tab (C<\t>).
- cntrl Any control character.
- digit Any digit, equivalent to C<\d>.
- graph Any printable character, excluding a space.
- lower Any lowercase character.
- print Any printable character, including a space.
- punct Any punctuation character.
- space Any white space character. C<\s> plus the vertical tab (C<\cK>).
- upper Any uppercase character.
- word Any "word" character, equivalent to C<\w>.
- xdigit Any hexadecimal digit, '0' - '9', 'a' - 'f', 'A' - 'F'.
-
-The exact set of characters matched depends on whether the source string
-is internally in UTF-8 format or not. See L</Locale, Unicode and UTF-8>.
-
-Most POSIX character classes have C<\p> counterparts. The difference
-is that the C<\p> classes will always match according to the Unicode
-properties, regardless whether the string is in UTF-8 format or not.
-
-The following table shows the relation between POSIX character classes
-and the Unicode properties:
-
- [[:...:]] \p{...} backslash
-
- alpha IsAlpha
- alnum IsAlnum
- ascii IsASCII
- blank
- cntrl IsCntrl
- digit IsDigit \d
- graph IsGraph
- lower IsLower
- print IsPrint
- punct IsPunct
- space IsSpace
- IsSpacePerl \s
- upper IsUpper
- word IsWord
- xdigit IsXDigit
-
-Some character classes may have a non-obvious name:
+ 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]").
+
+Most POSIX character classes have two Unicode-style C<\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,
+C<\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}
=over 4
-=item cntrl
+=item [1]
-Any control character. Usually, control characters don't produce output
-as such, but instead control the terminal somehow: for example newline
-and backspace are control characters. All characters with C<ord()> less
-than 32 are usually classified as control characters (in ASCII, the ISO
-Latin character sets, and Unicode), as is the character C<ord()> value
-of 127 (C<DEL>).
+C<\p{Blank}> and C<\p{HorizSpace}> are synonyms.
-=item graph
+=item [2]
+
+Control characters don't produce output as such, but instead usually control
+the terminal somehow: for example, newline and backspace are control characters.
+In the ASCII range, characters whose code points are between 0 and 31 inclusive,
+plus 127 (C<DEL>) are control characters.
+
+=item [3]
Any character that is I<graphical>, that is, visible. This class consists
-of all the alphanumerical characters and all punctuation characters.
+of all alphanumeric characters and all punctuation characters.
+
+=item [4]
+
+All printable characters, which is the set of all graphical characters
+plus those whitespace characters which are not also controls.
+
+=item [5]
-=item print
+C<\p{PosixPunct}> and C<[[:punct:]]> in the ASCII range match all
+non-controls, non-alphanumeric, non-space characters:
+C<[-!"#$%&'()*+,./:;<=E<gt>?@[\\\]^_`{|}~]> (although if a locale is in effect,
+it could alter the behavior of C<[[:punct:]]>).
-All printable characters, which is the set of all the graphical characters
-plus the space.
+The similarly named property, C<\p{Punct}>, matches a somewhat different
+set in the ASCII range, namely
+C<[-!"#%&'()*,./:;?@[\\\]_{}]>. That is, it is missing the nine
+characters C<[$+E<lt>=E<gt>^`|~]>.
+This is because Unicode splits what POSIX considers to be punctuation into two
+categories, Punctuation and Symbols.
-=item punct
+C<\p{XPosixPunct}> and (under Unicode rules) C<[[:punct:]]>, match what
+C<\p{PosixPunct}> matches in the ASCII range, plus what C<\p{Punct}>
+matches. This is different than strictly matching according to
+C<\p{Punct}>. Another way to say it is that
+if Unicode rules are in effect, C<[[:punct:]]> matches all characters
+that Unicode considers punctuation, plus all ASCII-range characters that
+Unicode considers symbols.
-Any punctuation (special) character.
+=item [6]
+
+C<\p{XPerlSpace}> and C<\p{Space}> match identically starting with Perl
+v5.18. In earlier versions, these differ only in that in non-locale
+matching, C<\p{XPerlSpace}> does not match the vertical tab, C<\cK>.
+Same for the two ASCII-only range forms.
+
+=back
+
+There are various other synonyms that can be used besides the names
+listed in the table. For example, C<\p{PosixAlpha}> can be written as
+C<\p{Alpha}>. All are listed in
+L<perluniprops/Properties accessible through \p{} and \P{}>.
+
+Both the C<\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:
+
+=over
+
+=item If the C</a> modifier, is in effect ...
+
+Each of the POSIX classes matches exactly the same as their ASCII-range
+counterparts.
+
+=item otherwise ...
+
+=over
+
+=item For code points above 255 ...
+
+The POSIX class matches the same as its Full-range counterpart.
+
+=item For code points below 256 ...
+
+=over
+
+=item if locale rules are in effect ...
+
+The POSIX class matches according to the locale, except:
+
+=over
+
+=item C<word>
+
+also includes the platform's native underscore character, no matter what
+the locale is.
+
+=item C<ascii>
+
+on platforms that don't have the POSIX C<ascii> extension, this matches
+just the platform's native ASCII-range characters.
+
+=item C<blank>
+
+on platforms that don't have the POSIX C<blank> extension, this matches
+just the platform's native tab and space characters.
=back
-=head4 Negation
+=item if Unicode rules are in effect ...
+
+The POSIX class matches the same as the Full-range counterpart.
+
+=item otherwise ...
+
+The POSIX class matches the same as the ASCII range counterpart.
+
+=back
+
+=back
+
+=back
+
+Which rules apply are determined as described in
+L<perlre/Which character set modifier is in effect?>.
+
+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 C<perl5-porters@perl.org>.
+
+=head4 Negation of POSIX character classes
+X<character class, negation>
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 (C<^>).
Some examples:
- POSIX Unicode Backslash
- [[:^digit:]] \P{IsDigit} \D
- [[:^space:]] \P{IsSpace} \S
- [[:^word:]] \P{IsWord} \W
+ 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
-=head4 [= =] and [. .]
+The backslash sequence can mean either ASCII- or Full-range Unicode,
+depending on various factors as described in L<perlre/Which character set modifier is in effect?>.
-Perl will recognize the POSIX character classes C<[=class=]>, and
-C<[.class.]>, but does not (yet?) support this construct. Use of
-such a constructs will lead to an error.
+=head4 [= =] and [. .]
+Perl recognizes the POSIX character classes C<[=class=]> and
+C<[.class.]>, but does not (yet?) support them. Any attempt to use
+either construct raises an exception.
=head4 Examples
/[[: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,
- # but the letters 'a' to 'f' in either case.
- # This is because the character class contains
- # all digits, and anything that isn't a
- # hex digit, resulting in a class containing
- # all characters, but the letters 'a' to 'f'
- # and 'A' to 'F'.
+ /[[: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.
+=head3 Extended Bracketed Character Classes
+X<character class>
+X<set operations>
-=head2 Locale, Unicode and UTF-8
+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
-Some of the character classes have a somewhat different behaviour depending
-on the internal encoding of the source string, and the locale that is
-in effect.
+ /(?[ \p{Thai} & \p{Digit} ])/
-C<\w>, C<\d>, C<\s> and the POSIX character classes (and their negations,
-including C<\W>, C<\D>, C<\S>) suffer from this behaviour.
+This will match all the digit characters that are in the Thai script.
-The rule is that if the source string is in UTF-8 format, the character
-classes match according to the Unicode properties. If the source string
-isn't, then the character classes match according to whatever locale is
-in effect. If there is no locale, they match the ASCII defaults
-(52 letters, 10 digits and underscore for C<\w>, 0 to 9 for C<\d>, etc).
+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
-This usually means that if you are matching against characters whose C<ord()>
-values are between 128 and 255 inclusive, your character class may match
-or not depending on the current locale, and whether the source string is
-in UTF-8 format. The string will be in UTF-8 format if it contains
-characters whose C<ord()> value exceeds 255. But a string may be in UTF-8
-format without it having such characters.
+ no warnings "experimental::regex_sets";
-For portability reasons, it may be better to not use C<\w>, C<\d>, C<\s>
-or the POSIX character classes, and use the Unicode properties instead.
+Comments on this feature are welcome; send email to
+C<perl5-porters@perl.org>.
-=head4 Examples
+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 C<E<sol>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, and are of equal precedence.
+The unary operator right associates, and has higher precedence. Use
+parentheses to override the default associations. Some feedback we've
+received indicates a desire for intersection to have higher precedence
+than union. This is something that feedback from the field may cause us
+to change in future releases; you may want to parenthesize copiously to
+avoid such changes affecting your code, until this feature is no longer
+considered experimental.
+
+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 C<[ab]>.) You could also have said the
+equivalent:
+
+ /(?[[ a b ]])/
+
+(You can, of course, specify single characters by using, C<\x{...}>,
+C<\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; C<E<sol>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 C<\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 C<\p{}> or C<\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 C<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 C<\w> and C<\D>
+respect the C<E<sol>a> (and C<E<sol>aa>) modifiers.
+
+C<< (?[ ]) >> 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:
+
+=over 4
+
+=item 1
+
+This construct cannot be used within the scope of
+C<use locale> (or the C<E<sol>l> regex modifier).
+
+=item 2
+
+Any
+L<user-defined property|perlunicode/"User-Defined Character Properties">
+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).
+
+=item 3
+
+A regular expression that otherwise would compile
+using C<E<sol>d> rules, and which uses this construct will instead
+use C<E<sol>u>. Thus this construct tells Perl that you don't want
+C<E<sol>d> rules for the entire regular expression containing it.
+
+=back
+
+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 C<(?[>. Nor may there be space between the
+closing C<])> 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 C<\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:
+
+ my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
+ my $lower = qr/(?[ \p{Lower} + \p{Digit} ])/;
+
+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.
- $str = "\xDF"; # $str is not in UTF-8 format.
- $str =~ /^\w/; # No match, as $str isn't in UTF-8 format.
- $str .= "\x{0e0b}"; # Now $str is in UTF-8 format.
- $str =~ /^\w/; # Match! $str is now in UTF-8 format.
- chop $str;
- $str =~ /^\w/; # Still a match! $str remains in UTF-8 format.
+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 C<perlbug@perl.org>, so that we can have a
+concrete example for this man page.
-=cut
+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 C<(?[ [aa] ])>. The motivation for such a change
+is that this usage is likely a typo, as the second "a" adds nothing.
https://perl5.git.perl.org / perl5.git / blobdiff