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 the backspace character only inside a character class. Outside a
-character class, C<\b> is a word/non-word boundary.
+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 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:
+ $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.
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.
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 parenthesis. Thus C<\g1> refers to the first
+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>}>
probably not what you intended.
In the C<\I<N>> form, I<N> must not begin with a "0", and there must be at
-least I<N> capturing groups, or else I<N> will be considered an octal escape
-(but something like C<\18> is the same as C<\0018>, that is the octal escape
+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+) \g1/; # Finds a duplicated word, (e.g. "cat cat").
- /(\w+) \1/; # Same thing; written old-style
+ /(\w+) \1/; # Same thing; written old-style.
/(.)(.)\g2\g1/; # Match a four letter palindrome (e.g. "ABBA").
=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{...}>, 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> 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> 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", "Format" or "ZWJ", 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'
$str =~ s/(.)\K\g1//g; # Delete duplicated characters.
"\r" =~ /^\R$/; # 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