The rules determining what it is are quite simple: if the character
following the backslash is an ASCII punctuation (non-word) character (that is,
-anything that is not a letter, digit or underscore), then the backslash just
-takes away the special meaning (if any) of the character following it.
+anything that is not a letter, digit, or underscore), then the backslash just
+takes away any special meaning of the character following it.
If the character following the backslash is an ASCII letter or an ASCII digit,
then the sequence may be special; if so, it's listed below. A few letters have
not been used yet, so escaping them with a backslash doesn't change them to be
special. A future version of Perl may assign a special meaning to them, so if
-you have warnings turned on, Perl will issue a warning if you use such a
+you have warnings turned on, Perl issues a warning if you use such a
sequence. [1].
It is however guaranteed that backslash or escape sequences never have a
=item [1]
-There is one exception. If you use an alphanumerical character as the
+There is one exception. If you use an alphanumeric character as the
delimiter of your pattern (which you probably shouldn't do for readability
-reasons), you will have to escape the delimiter if you want to match
+reasons), you have to escape the delimiter if you want to match
it. Perl won't warn then. See also L<perlop/Gory details of parsing
quoted constructs>.
Those not usable within a bracketed character class (like C<[\da-z]>) are marked
as C<Not in [].>
- \000 Octal escape sequence.
+ \000 Octal escape sequence. See also \o{}.
\1 Absolute backreference. Not in [].
\a Alarm or bell.
\A Beginning of string. Not in [].
- \b Word/non-word boundary. (Backspace in []).
- \B Not a word/non-word boundary. Not in [].
- \cX Control-X
- \C Single octet, even under UTF-8. Not in [].
+ \b{}, \b Boundary. (\b is a backspace in []).
+ \B{}, \B Not a boundary. Not in [].
+ \cX Control-X.
\d Character class for digits.
\D Character class for non-digits.
\e Escape character.
\E Turn off \Q, \L and \U processing. Not in [].
\f Form feed.
- \g{}, \g1 Named, absolute or relative backreference. Not in [].
+ \F Foldcase till \E. Not in [].
+ \g{}, \g1 Named, absolute or relative backreference.
+ Not in [].
\G Pos assertion. Not in [].
\h Character class for horizontal whitespace.
\H Character class for non horizontal whitespace.
\l Lowercase next character. Not in [].
\L Lowercase till \E. Not in [].
\n (Logical) newline character.
- \N Any character but newline. Experimental. Not in [].
- \N{} Named or numbered (Unicode) character.
+ \N Any character but newline. Not in [].
+ \N{} Named or numbered (Unicode) character or sequence.
+ \o{} Octal escape sequence.
\p{}, \pP Character with the given Unicode property.
\P{}, \PP Character without the given Unicode property.
- \Q Quotemeta till \E. Not in [].
+ \Q Quote (disable) pattern metacharacters till \E. Not
+ in [].
\r Return character.
\R Generic new line. Not in [].
\s Character class for whitespace.
=item [1]
-C<\b> is only the backspace character inside a character class. Outside a
-character class, C<\b> is a word/non-word boundary.
+C<\b> is the backspace character only inside a character class. Outside a
+character class, C<\b> alone is a word-character/non-word-character
+boundary, and C<\b{}> is some other type of boundary.
=item [2]
-C<\n> matches a logical newline. Perl will convert between C<\n> and your
-OSses native newline character when reading from or writing to text files.
+C<\n> matches a logical newline. Perl converts between C<\n> and your
+OS's native newline character when reading from or writing to text files.
=back
$str =~ /\cK/; # Matches if $str contains a vertical tab (control-K).
-=head3 Named or numbered characters
+=head3 Named or numbered characters and character sequences
-All Unicode characters have a Unicode name and numeric ordinal value. Use the
+Unicode characters have a Unicode name and numeric code point (ordinal)
+value. Use the
C<\N{}> construct to specify a character by either of these values.
+Certain sequences of characters also have names.
-To specify by name, the name of the character goes between the curly braces.
-In this case, you have to C<use charnames> to load the Unicode names of the
-characters, otherwise Perl will complain.
+To specify by name, the name of the character or character sequence goes
+between the curly braces.
-To specify by Unicode ordinal number, use the form
-C<\N{U+I<wide hex character>}>, where I<wide hex character> is a number in
-hexadecimal that gives the ordinal number that Unicode has assigned to the
-desired character. It is customary (but not required) to use leading zeros to
-pad the number to 4 digits. Thus C<\N{U+0041}> means
-C<Latin Capital Letter A>, and you will rarely see it written without the two
-leading zeros. C<\N{U+0041}> means C<A> even on EBCDIC machines (where the
-ordinal value of C<A> is not 0x41).
+To specify a character by Unicode code point, use the form C<\N{U+I<code
+point>}>, where I<code point> is a number in hexadecimal that gives the
+code point that Unicode has assigned to the desired character. It is
+customary but not required to use leading zeros to pad the number to 4
+digits. Thus C<\N{U+0041}> means C<LATIN CAPITAL LETTER A>, and you will
+rarely see it written without the two leading zeros. C<\N{U+0041}> means
+"A" even on EBCDIC machines (where the ordinal value of "A" is not 0x41).
-It is even possible to give your own names to characters, and even to short
-sequences of characters. For details, see L<charnames>.
+It is even possible to give your own names to characters and character
+sequences. For details, see L<charnames>.
(There is an expanded internal form that you may see in debug output:
-C<\N{U+I<wide hex character>.I<wide hex character>...}>.
-The C<...> means any number of these I<wide hex character>s separated by dots.
+C<\N{U+I<code point>.I<code point>...}>.
+The C<...> means any number of these I<code point>s separated by dots.
This represents the sequence formed by the characters. This is an internal
form only, subject to change, and you should not try to use it yourself.)
Mnemonic: I<N>amed character.
-Note that a character that is expressed as a named or numbered character is
-considered as a character without special meaning by the regex engine, and will
-match "as is".
+Note that a character or character sequence expressed as a named
+or numbered character is considered a character without special
+meaning by the regex engine, and will match "as is".
=head4 Example
- use charnames ':full'; # Loads the Unicode names.
$str =~ /\N{THAI CHARACTER SO SO}/; # Matches the Thai SO SO character
use charnames 'Cyrillic'; # Loads Cyrillic names.
=head3 Octal escapes
-Octal escapes consist of a backslash followed by two or three octal digits
-matching the code point of the character you want to use. This allows for
-512 characters (C<\00> up to C<\777>) that can be expressed this way (but
-anything above C<\377> is deprecated).
-Enough in pre-Unicode days, but most Unicode characters cannot be escaped
-this way.
-
-Note that a character that is expressed as an octal escape is considered
-as a character without special meaning by the regex engine, and will match
+There are two forms of octal escapes. Each is used to specify a character by
+its code point specified in octal notation.
+
+One form, available starting in Perl 5.14 looks like C<\o{...}>, where the dots
+represent one or more octal digits. It can be used for any Unicode character.
+
+It was introduced to avoid the potential problems with the other form,
+available in all Perls. That form consists of a backslash followed by three
+octal digits. One problem with this form is that it can look exactly like an
+old-style backreference (see
+L</Disambiguation rules between old-style octal escapes and backreferences>
+below.) You can avoid this by making the first of the three digits always a
+zero, but that makes \077 the largest code point specifiable.
+
+In some contexts, a backslash followed by two or even one octal digits may be
+interpreted as an octal escape, sometimes with a warning, and because of some
+bugs, sometimes with surprising results. Also, if you are creating a regex
+out of smaller snippets concatenated together, and you use fewer than three
+digits, the beginning of one snippet may be interpreted as adding digits to the
+ending of the snippet before it. See L</Absolute referencing> for more
+discussion and examples of the snippet problem.
+
+Note that a character expressed as an octal escape is considered
+a character without special meaning by the regex engine, and will match
"as is".
-=head4 Examples (assuming an ASCII platform)
+To summarize, the C<\o{}> form is always safe to use, and the other form is
+safe to use for code points through \077 when you use exactly three digits to
+specify them.
- $str = "Perl";
- $str =~ /\120/; # Match, "\120" is "P".
- $str =~ /\120+/; # Match, "\120" is "P", it is repeated at least once.
- $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
+Mnemonic: I<0>ctal or I<o>ctal.
-=head4 Caveat
+=head4 Examples (assuming an ASCII platform)
-Octal escapes potentially clash with backreferences. They both consist
-of a backslash followed by numbers. So Perl has to use heuristics to
-determine whether it is a backreference or an octal escape. Perl uses
-the following rules:
+ $str = "Perl";
+ $str =~ /\o{120}/; # Match, "\120" is "P".
+ $str =~ /\120/; # Same.
+ $str =~ /\o{120}+/; # Match, "\120" is "P",
+ # it's repeated at least once.
+ $str =~ /\120+/; # Same.
+ $str =~ /P\053/; # No match, "\053" is "+" and taken literally.
+ /\o{23073}/ # Black foreground, white background smiling face.
+ /\o{4801234567}/ # Raises a warning, and yields chr(4).
+
+=head4 Disambiguation rules between old-style octal escapes and backreferences
+
+Octal escapes of the C<\000> form outside of bracketed character classes
+potentially clash with old-style backreferences (see L</Absolute referencing>
+below). They both consist of a backslash followed by numbers. So Perl has to
+use heuristics to determine whether it is a backreference or an octal escape.
+Perl uses the following rules to disambiguate:
=over 4
=item 3
-If the number following the backslash is N (decimal), and Perl already has
-seen N capture groups, Perl will consider this to be a backreference.
-Otherwise, it will consider it to be an octal escape. Note that if N > 999,
-Perl only takes the first three digits for the octal escape; the rest is
-matched as is.
+If the number following the backslash is N (in decimal), and Perl already
+has seen N capture groups, Perl considers this a backreference. Otherwise,
+it considers it an octal escape. If N has more than three digits, Perl
+takes only the first three for the octal escape; the rest are matched as is.
my $pat = "(" x 999;
$pat .= "a";
$pat .= ")" x 999;
/^($pat)\1000$/; # Matches 'aa'; there are 1000 capture groups.
/^$pat\1000$/; # Matches 'a@0'; there are 999 capture groups
- # and \1000 is seen as \100 (a '@') and a '0'.
+ # and \1000 is seen as \100 (a '@') and a '0'.
=back
+You can force a backreference interpretation always by using the C<\g{...}>
+form. You can the force an octal interpretation always by using the C<\o{...}>
+form, or for numbers up through \077 (= 63 decimal), by using three digits,
+beginning with a "0".
+
=head3 Hexadecimal escapes
-Hexadecimal escapes start with C<\x> and are then either followed by a
-two digit hexadecimal number, or a hexadecimal number of arbitrary length
-surrounded by curly braces. The hexadecimal number is the code point of
-the character you want to express.
+Like octal escapes, there are two forms of hexadecimal escapes, but both start
+with the sequence C<\x>. This is followed by either exactly two hexadecimal
+digits forming a number, or a hexadecimal number of arbitrary length surrounded
+by curly braces. The hexadecimal number is the code point of the character you
+want to express.
-Note that a character that is expressed as a hexadecimal escape is considered
-as a character without special meaning by the regex engine, and will match
+Note that a character expressed as one of these escapes is considered a
+character without special meaning by the regex engine, and will match
"as is".
Mnemonic: heI<x>adecimal.
$str = "Perl";
$str =~ /\x50/; # Match, "\x50" is "P".
- $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once.
+ $str =~ /\x50+/; # Match, "\x50" is "P", it is repeated at least once
$str =~ /P\x2B/; # No match, "\x2B" is "+" and taken literally.
/\x{2603}\x{2602}/ # Snowman with an umbrella.
A number of backslash sequences have to do with changing the character,
or characters following them. C<\l> will lowercase the character following
it, while C<\u> will uppercase (or, more accurately, titlecase) the
-character following it. (They perform similar functionality as the
-functions C<lcfirst> and C<ucfirst>).
+character following it. They provide functionality similar to the
+functions C<lcfirst> and C<ucfirst>.
To uppercase or lowercase several characters, one might want to use
C<\L> or C<\U>, which will lowercase/uppercase all characters following
-them, until either the end of the pattern, or the next occurrence of
-C<\E>, whatever comes first. They perform similar functionality as the
-functions C<lc> and C<uc> do.
+them, until either the end of the pattern or the next occurrence of
+C<\E>, whichever comes first. They provide functionality similar to what
+the functions C<lc> and C<uc> provide.
-C<\Q> is used to escape all characters following, up to the next C<\E>
-or the end of the pattern. C<\Q> adds a backslash to any character that
-isn't a letter, digit or underscore. This will ensure that any character
-between C<\Q> and C<\E> is matched literally, and will not be interpreted
-by the regexp engine.
+C<\Q> is used to quote (disable) pattern metacharacters, up to the next
+C<\E> or the end of the pattern. C<\Q> adds a backslash to any character
+that could have special meaning to Perl. In the ASCII range, it quotes
+every character that isn't a letter, digit, or underscore. See
+L<perlfunc/quotemeta> for details on what gets quoted for non-ASCII
+code points. Using this ensures that any character between C<\Q> and
+C<\E> will be matched literally, not interpreted as a metacharacter by
+the regex engine.
-Mnemonic: I<L>owercase, I<U>ppercase, I<Q>uotemeta, I<E>nd.
+C<\F> can be used to casefold all characters following, up to the next C<\E>
+or the end of the pattern. It provides the functionality similar to
+the C<fc> function.
+
+Mnemonic: I<L>owercase, I<U>ppercase, I<F>old-case, I<Q>uotemeta, I<E>nd.
=head4 Examples
discuss those here; full details of character classes can be found in
L<perlrecharclass>.
-C<\w> is a character class that matches any single I<word> character (letters,
-digits, underscore). C<\d> is a character class that matches any decimal digit,
-while the character class C<\s> matches any whitespace character.
+C<\w> is a character class that matches any single I<word> character
+(letters, digits, Unicode marks, and connector punctuation (like the
+underscore)). C<\d> is a character class that matches any decimal
+digit, while the character class C<\s> matches any whitespace character.
New in perl 5.10.0 are the classes C<\h> and C<\v> which match horizontal
and vertical whitespace characters.
+The exact set of characters matched by C<\d>, C<\s>, and C<\w> varies
+depending on various pragma and regular expression modifiers. It is
+possible to restrict the match to the ASCII range by using the C</a>
+regular expression modifier. See L<perlrecharclass>.
+
The uppercase variants (C<\W>, C<\D>, C<\S>, C<\H>, and C<\V>) are
-character classes that match any character that isn't a word character,
-digit, whitespace, horizontal whitespace nor vertical whitespace.
+character classes that match, respectively, any character that isn't a
+word character, digit, whitespace, horizontal whitespace, or vertical
+whitespace.
Mnemonics: I<w>ord, I<d>igit, I<s>pace, I<h>orizontal, I<v>ertical.
Mnemonic: I<p>roperty.
-
=head2 Referencing
If capturing parenthesis are used in a regular expression, we can refer
=head3 Absolute referencing
-A backslash sequence that starts with a backslash and is followed by a
-number is an absolute reference (but be aware of the caveat mentioned above).
-If the number is I<N>, it refers to the Nth set of parentheses - whatever
-has been matched by that set of parenthesis has to be matched by the C<\N>
-as well.
+Either C<\gI<N>> (starting in Perl 5.10.0), or C<\I<N>> (old-style) where I<N>
+is a positive (unsigned) decimal number of any length is an absolute reference
+to a capturing group.
+
+I<N> refers to the Nth set of parentheses, so C<\gI<N>> refers to whatever has
+been matched by that set of parentheses. Thus C<\g1> refers to the first
+capture group in the regex.
+
+The C<\gI<N>> form can be equivalently written as C<\g{I<N>}>
+which avoids ambiguity when building a regex by concatenating shorter
+strings. Otherwise if you had a regex C<qr/$a$b/>, and C<$a> contained
+C<"\g1">, and C<$b> contained C<"37">, you would get C</\g137/> which is
+probably not what you intended.
+
+In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
+least I<N> capturing groups, or else I<N> is considered an octal escape
+(but something like C<\18> is the same as C<\0018>; that is, the octal escape
+C<"\001"> followed by a literal digit C<"8">).
+
+Mnemonic: I<g>roup.
=head4 Examples
- /(\w+) \1/; # Finds a duplicated word, (e.g. "cat cat").
- /(.)(.)\2\1/; # Match a four letter palindrome (e.g. "ABBA").
+ /(\w+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
+ /(\w+) \1/; # Same thing; written old-style.
+ /(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
=head3 Relative referencing
-New in perl 5.10.0 is a different way of referring to capture buffers: C<\g>.
-C<\g> takes a number as argument, with the number in curly braces (the
-braces are optional). If the number (N) does not have a sign, it's a reference
-to the Nth capture group (so C<\g{2}> is equivalent to C<\2> - except that
-C<\g> always refers to a capture group and will never be seen as an octal
-escape). If the number is negative, the reference is relative, referring to
-the Nth group before the C<\g{-N}>.
+C<\g-I<N>> (starting in Perl 5.10.0) is used for relative addressing. (It can
+be written as C<\g{-I<N>>.) It refers to the I<N>th group before the
+C<\g{-I<N>}>.
-The big advantage of C<\g{-N}> is that it makes it much easier to write
+The big advantage of this form is that it makes it much easier to write
patterns with references that can be interpolated in larger patterns,
even if the larger pattern also contains capture groups.
-Mnemonic: I<g>roup.
-
=head4 Examples
- /(A) # Buffer 1
- ( # Buffer 2
- (B) # Buffer 3
- \g{-1} # Refers to buffer 3 (B)
- \g{-3} # Refers to buffer 1 (A)
+ /(A) # Group 1
+ ( # Group 2
+ (B) # Group 3
+ \g{-1} # Refers to group 3 (B)
+ \g{-3} # Refers to group 1 (A)
)
/x; # Matches "ABBA".
=head3 Named referencing
-Also new in perl 5.10.0 is the use of named capture buffers, which can be
-referred to by name. This is done with C<\g{name}>, which is a
-backreference to the capture buffer with the name I<name>.
+C<\g{I<name>}> (starting in Perl 5.10.0) can be used to back refer to a
+named capture group, dispensing completely with having to think about capture
+buffer positions.
To be compatible with .Net regular expressions, C<\g{name}> may also be
written as C<\k{name}>, C<< \k<name> >> or C<\k'name'>.
-Note that C<\g{}> has the potential to be ambiguous, as it could be a named
-reference, or an absolute or relative reference (if its argument is numeric).
-However, names are not allowed to start with digits, nor are they allowed to
-contain a hyphen, so there is no ambiguity.
+To prevent any ambiguity, I<name> must not start with a digit nor contain a
+hyphen.
=head4 Examples
=item \A
C<\A> only matches at the beginning of the string. If the C</m> modifier
-isn't used, then C</\A/> is equivalent with C</^/>. However, if the C</m>
+isn't used, then C</\A/> is equivalent to C</^/>. However, if the C</m>
modifier is used, then C</^/> matches internal newlines, but the meaning
of C</\A/> isn't changed by the C</m> modifier. C<\A> matches at the beginning
of the string regardless whether the C</m> modifier is used.
=item \z, \Z
C<\z> and C<\Z> match at the end of the string. If the C</m> modifier isn't
-used, then C</\Z/> is equivalent with C</$/>, that is, it matches at the
-end of the string, or before the newline at the end of the string. If the
+used, then C</\Z/> is equivalent to C</$/>; that is, it matches at the
+end of the string, or one before the newline at the end of the string. If the
C</m> modifier is used, then C</$/> matches at internal newlines, but the
meaning of C</\Z/> isn't changed by the C</m> modifier. C<\Z> matches at
the end of the string (or just before a trailing newline) regardless whether
the C</m> modifier is used.
-C<\z> is just like C<\Z>, except that it will not match before a trailing
-newline. C<\z> will only match at the end of the string - regardless of the
-modifiers used, and not before a newline.
+C<\z> is just like C<\Z>, except that it does not match before a trailing
+newline. C<\z> matches at the end of the string only, regardless of the
+modifiers used, and not just before a newline. It is how to anchor the
+match to the true end of the string under all conditions.
=item \G
-C<\G> is usually only used in combination with the C</g> modifier. If the
-C</g> modifier is used (and the match is done in scalar context), Perl will
-remember where in the source string the last match ended, and the next time,
+C<\G> is usually used only in combination with the C</g> modifier. If the
+C</g> modifier is used and the match is done in scalar context, Perl
+remembers where in the source string the last match ended, and the next time,
it will start the match from where it ended the previous time.
-C<\G> matches the point where the previous match ended, or the beginning
-of the string if there was no previous match.
+C<\G> matches the point where the previous match on that string ended,
+or the beginning of that string if there was no previous match.
=for later add link to perlremodifiers
Mnemonic: I<G>lobal.
-=item \b, \B
+=item \b{}, \b, \B{}, \B
-C<\b> matches at any place between a word and a non-word character; C<\B>
-matches at any place between characters where C<\b> doesn't match. C<\b>
+C<\b{...}>, available starting in v5.22, matches a boundary (between two
+characters, or before the first character of the string, or after the
+final character of the string) based on the Unicode rules for the
+boundary type specified inside the braces. The boundary
+types are given a few paragraphs below. C<\B{...}> matches at any place
+between characters where C<\b{...}> of the same type doesn't match.
+
+C<\b> when not immediately followed by a C<"{"> matches at any place
+between a word (something matched by C<\w>) and a non-word character
+(C<\W>); C<\B> when not immediately followed by a C<"{"> matches at any
+place between characters where C<\b> doesn't match. To get better
+word matching of natural language text, see L<\b{wb}> below.
+
+C<\b>
and C<\B> assume there's a non-word character before the beginning and after
the end of the source string; so C<\b> will match at the beginning (or end)
of the source string if the source string begins (or ends) with a word
-character. Otherwise, C<\B> will match.
+character. Otherwise, C<\B> will match.
+
+Do not use something like C<\b=head\d\b> and expect it to match the
+beginning of a line. It can't, because for there to be a boundary before
+the non-word "=", there must be a word character immediately previous.
+All plain C<\b> and C<\B> boundary determinations look for word
+characters alone, not for
+non-word characters nor for string ends. It may help to understand how
+C<\b> and C<\B> work by equating them as follows:
+
+ \b really means (?:(?<=\w)(?!\w)|(?<!\w)(?=\w))
+ \B really means (?:(?<=\w)(?=\w)|(?<!\w)(?!\w))
+
+In contrast, C<\b{...}> and C<\B{...}> may or may not match at the
+beginning and end of the line, depending on the boundary type. These
+implement the Unicode default boundaries, specified in
+L<http://www.unicode.org/reports/tr14/> and
+L<http://www.unicode.org/reports/tr29/>.
+The boundary types are:
+
+=over
+
+=item C<\b{gcb}> or C<\b{g}>
+
+This matches a Unicode "Grapheme Cluster Boundary". (Actually Perl
+always uses the improved "extended" grapheme cluster"). These are
+explained below under L</C<\X>>. In fact, C<\X> is another way to get
+the same functionality. It is equivalent to C</.+?\b{gcb}/>. Use
+whichever is most convenient for your situation.
+
+=item C<\b{lb}>
+
+This matches according to the default Unicode Line Breaking Algorithm
+(L<http://www.unicode.org/reports/tr14/>), as customized in that
+document
+(L<Example 7 of revision 35|http://www.unicode.org/reports/tr14/tr14-35.html#Example7>)
+for better handling of numeric expressions.
+
+This is suitable for many purposes, but the L<Unicode::LineBreak> module
+is available on CPAN that provides many more features, including
+customization.
+
+=item C<\b{sb}>
+
+This matches a Unicode "Sentence Boundary". This is an aid to parsing
+natural language sentences. It gives good, but imperfect results. For
+example, it thinks that "Mr. Smith" is two sentences. More details are
+at L<http://www.unicode.org/reports/tr29/>. Note also that it thinks
+that anything matching L</\R> (except form feed and vertical tab) is a
+sentence boundary. C<\b{sb}> works with text designed for
+word-processors which wrap lines
+automatically for display, but hard-coded line boundaries are considered
+to be essentially the ends of text blocks (paragraphs really), and hence
+the ends of sententces. C<\b{sb}> doesn't do well with text containing
+embedded newlines, like the source text of the document you are reading.
+Such text needs to be preprocessed to get rid of the line separators
+before looking for sentence boundaries. Some people view this as a bug
+in the Unicode standard, and this behavior is quite subject to change in
+future Perl versions.
+
+=item C<\b{wb}>
+
+This matches a Unicode "Word Boundary", but tailored to Perl
+expectations. This gives better (though not
+perfect) results for natural language processing than plain C<\b>
+(without braces) does. For example, it understands that apostrophes can
+be in the middle of words and that parentheses aren't (see the examples
+below). More details are at L<http://www.unicode.org/reports/tr29/>.
+
+The current Unicode definition of a Word Boundary matches between every
+white space character. Perl tailors this, starting in version 5.24, to
+generally not break up spans of white space, just as plain C<\b> has
+always functioned. This allows C<\b{wb}> to be a drop-in replacement for
+C<\b>, but with generally better results for natural language
+processing. (The exception to this tailoring is when a span of white
+space is immediately followed by something like U+0303, COMBINING TILDE.
+If the final space character in the span is a horizontal white space, it
+is broken out so that it attaches instead to the combining character.
+To be precise, if a span of white space that ends in a horizontal space
+has the character immediately following it have either of the Word
+Boundary property values "Extend" or "Format", the boundary between the
+final horizontal space character and the rest of the span matches
+C<\b{wb}>. In all other cases the boundary between two white space
+characters matches C<\B{wb}>.)
+
+=back
+
+It is important to realize when you use these Unicode boundaries,
+that you are taking a risk that a future version of Perl which contains
+a later version of the Unicode Standard will not work precisely the same
+way as it did when your code was written. These rules are not
+considered stable and have been somewhat more subject to change than the
+rest of the Standard. Unicode reserves the right to change them at
+will, and Perl reserves the right to update its implementation to
+Unicode's new rules. In the past, some changes have been because new
+characters have been added to the Standard which have different
+characteristics than all previous characters, so new rules are
+formulated for handling them. These should not cause any backward
+compatibility issues. But some changes have changed the treatment of
+existing characters because the Unicode Technical Committee has decided
+that the change is warranted for whatever reason. This could be to fix
+a bug, or because they think better results are obtained with the new
+rule.
+
+It is also important to realize that these are default boundary
+definitions, and that implementations may wish to tailor the results for
+particular purposes and locales. For example, some languages, such as
+Japanese and Thai, require dictionary lookup to determine word
+boundaries.
Mnemonic: I<b>oundary.
print $1; # Prints 'cat'
}
+ my $s = "He said, \"Is pi 3.14? (I'm not sure).\"";
+ print join("|", $s =~ m/ ( .+? \b ) /xg), "\n";
+ print join("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
+ prints
+ He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
+ He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"
+
=head2 Misc
Here we document the backslash sequences that don't fall in one of the
-categories above. They are:
+categories above. These are:
=over 4
-=item \C
-
-C<\C> always matches a single octet, even if the source string is encoded
-in UTF-8 format, and the character to be matched is a multi-octet character.
-C<\C> was introduced in perl 5.6.
-
-Mnemonic: oI<C>tet.
-
=item \K
-This is new in perl 5.10.0. Anything that is matched left of C<\K> is
-not included in C<$&> - and will not be replaced if the pattern is
-used in a substitution. This will allow you to write C<s/PAT1 \K PAT2/REPL/x>
+This appeared in perl 5.10.0. Anything matched left of C<\K> is
+not included in C<$&>, and will not be replaced if the pattern is
+used in a substitution. This lets you write C<s/PAT1 \K PAT2/REPL/x>
instead of C<s/(PAT1) PAT2/${1}REPL/x> or C<s/(?<=PAT1) PAT2/REPL/x>.
Mnemonic: I<K>eep.
=item \N
-This is a new experimental feature in perl 5.12.0. It matches any character
-that is not a newline. It is a short-hand for writing C<[^\n]>, and is
+This feature, available starting in v5.12, matches any character
+that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
identical to the C<.> metasymbol, except under the C</s> flag, which changes
the meaning of C<.>, but not C<\N>.
Note that C<\N{...}> can mean a
-L<named or numbered character|/Named or numbered characters>.
+L<named or numbered character
+|/Named or numbered characters and character sequences>.
Mnemonic: Complement of I<\n>.
=item \R
X<\R>
-C<\R> matches a I<generic newline>, that is, anything that is considered
-a newline by Unicode. This includes all characters matched by C<\v>
-(vertical whitespace), and the multi character sequence C<"\x0D\x0A">
-(carriage return followed by a line feed, aka the network newline, or
-the newline used in Windows text files). C<\R> is equivalent to
-C<< (?>\x0D\x0A)|\v) >>. Since C<\R> can match a sequence of more than one
-character, it cannot be put inside a bracketed character class; C</[\R]/> is an
-error; use C<\v> instead. C<\R> was introduced in perl 5.10.0.
+C<\R> matches a I<generic newline>; that is, anything considered a
+linebreak sequence by Unicode. This includes all characters matched by
+C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
+(carriage return followed by a line feed, sometimes called the network
+newline; it's the end of line sequence used in Microsoft text files opened
+in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
+reason it doesn't backtrack is that the sequence is considered
+inseparable. That means that
+
+ "\x0D\x0A" =~ /^\R\x0A$/ # No match
+
+fails, because the C<\R> matches the entire string, and won't backtrack
+to match just the C<"\x0D">.) Since
+C<\R> can match a sequence of more than one character, it cannot be put
+inside a bracketed character class; C</[\R]/> is an error; use C<\v>
+instead. C<\R> was introduced in perl 5.10.0.
+
+Note that this does not respect any locale that might be in effect; it
+matches according to the platform's native character set.
Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
and more importantly because Unicode recommends such a regular expression
-metacharacter, and suggests C<\R> as the notation.
+metacharacter, and suggests C<\R> as its notation.
=item \X
X<\X>
UPWARDS ARROW BELOW", and would be displayed by Unicode-aware software as if it
were a single character.
+The match is greedy and non-backtracking, so that the cluster is never
+broken up into smaller components.
+
+See also L<C<\b{gcb}>|/\b{}, \b, \B{}, \B>.
+
Mnemonic: eI<X>tended Unicode character.
=back
=head4 Examples
- "\x{256}" =~ /^\C\C$/; # Match as chr (256) takes 2 octets in UTF-8.
-
- $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'.
- $str =~ s/(.)\K\1//g; # Delete duplicated characters.
+ $str =~ s/foo\Kbar/baz/g; # Change any 'bar' following a 'foo' to 'baz'
+ $str =~ s/(.)\K\g1//g; # Delete duplicated characters.
"\n" =~ /^\R$/; # Match, \n is a generic newline.
"\r" =~ /^\R$/; # Match, \r is a generic newline.
"\r\n" =~ /^\R$/; # Match, \r\n is a generic newline.
- "P\x{0307}" =~ /^\X$/ # \X matches a P with a dot above.
+ "P\x{307}" =~ /^\X$/ # \X matches a P with a dot above.
=cut
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