\b{}, \b Boundary. (\b is a backspace in []).
\B{}, \B Not a boundary. Not in [].
\cX Control-X.
- \C Single octet, even under UTF-8. Not in [].
- (Deprecated)
\d Character class for digits.
\D Character class for non-digits.
\e Escape character.
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 currently known boundary
+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.
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
-<\b> and <\B> work by equating them as follows:
+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 currently available are:
+The boundary types are:
=over
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
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.
+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". This gives better (though not
+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/>.
+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 also important to realize that these are default boundary
definitions, and that implementations may wish to tailor the results for
-particular purposes and locales.
-
-Unicode defines a fourth boundary type, accessible through the
-L<Unicode::LineBreak> module.
+particular purposes and locales. For example, some languages, such as
+Japanese and Thai, require dictionary lookup to determine word
+boundaries.
Mnemonic: I<b>oundary.
=over 4
-=item \C
-
-(Deprecated.) 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.
-This is very dangerous, because it violates
-the logical character abstraction and can cause UTF-8 sequences to become malformed.
-
-Use C<utf8::encode()> instead.
-
-Mnemonic: oI<C>tet.
-
=item \K
This appeared in perl 5.10.0. Anything matched left of C<\K> is
https://perl5.git.perl.org / perl5.git / blobdiff