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