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
=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.
https://perl5.git.perl.org / perl5.git / blobdiff