\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 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]
=head3 Named or numbered characters and character sequences
-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 or character sequence 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.
+between the curly braces.
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
-ordinal number that Unicode has assigned to the desired character. It is
+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
=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
There are two forms of octal escapes. Each is used to specify a character by
-its ordinal, specified in octal notation.
+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.
"as is".
To summarize, the C<\o{}> form is always safe to use, and the other form is
-safe to use for ordinals up through \077 when you use exactly three digits to
+safe to use for code points through \077 when you use exactly three digits to
specify them.
Mnemonic: I<0>ctal or I<o>ctal.
$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 =~ /\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)
+ /\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>
+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:
$pat .= ")" x 999;
/^($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 the force a backreference interpretation always by using the C<\g{...}>
+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
Like octal escapes, there are two forms of hexadecimal escapes, but both start
-with the same thing, C<\x>. This is followed by either exactly two hexadecimal
+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.
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 provide functionality similar to what
+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 ensures that any character
-between C<\Q> and C<\E> shall be matched literally, not interpreted
-as a metacharacter by the regex 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
=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").
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> 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> 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>
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
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 boundary determinations look for word characters alone, not for
-non-words characters nor for string ends. It may help to understand how
-<\b> and <\B> work by equating them as follows:
+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.
=back
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("|", $s =~ m/ ( .+? \b{wb} ) /xg), "\n";
+ prints
+ He| |said|, "|Is| |pi| |3|.|14|? (|I|'|m| |not| |sure
+ He| |said|,| |"|Is| |pi| |3.14|?| |(|I'm| |not| |sure|)|.|"
+
=head2 Misc
Here we document the backslash sequences that don't fall in one of the
=over 4
-=item \C
-
-C<\C> always matches a single octet, even if the source string is encoded
-in UTF-8 format, and the character to be matched is a multi-octet character.
-C<\C> was introduced in perl 5.6. This is very dangerous, because it violates
-the logical character abstraction and can cause UTF-8 sequences to become malformed.
-
-Mnemonic: oI<C>tet.
-
=item \K
This appeared in perl 5.10.0. Anything matched left of C<\K> is
=item \N
-This is an experimental feature new to perl 5.12.0. It matches any character
+This feature, available starting in v5.12, matches any character
that is B<not> a newline. It is a short-hand for writing C<[^\n]>, and is
identical to the C<.> metasymbol, except under the C</s> flag, which changes
the meaning of C<.>, but not C<\N>.
C<\v> (vertical whitespace), and the multi character sequence C<"\x0D\x0A">
(carriage return followed by a line feed, sometimes called the network
newline; it's the end of line sequence used in Microsoft text files opened
-in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A)|\v) >>. Since
+in binary mode). C<\R> is equivalent to C<< (?>\x0D\x0A|\v) >>. (The
+reason it doesn't backtrack is that the sequence is considered
+inseparable. That means that
+
+ "\x0D\x0A" =~ /^\R\x0A$/ # 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 its notation.
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 (0x256) 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.