The PCRE library is a set of functions that implement regular
expression pattern matching using the same syntax and semantics
as Perl 5, with just a few differences (see below). The current
implementation corresponds to Perl 5.005.
The differences described here are with respect to Perl
5.005.
1. By default, a whitespace character is any character that
the C library function isspace() recognizes, though it is
possible to compile PCRE with alternative character type
tables. Normally isspace() matches space, formfeed, newline,
carriage return, horizontal tab, and vertical tab. Perl 5 no
longer includes vertical tab in its set of whitespace char-
acters. The \v escape that was in the Perl documentation for
a long time was never in fact recognized. However, the char-
acter itself was treated as whitespace at least up to 5.002.
In 5.004 and 5.005 it does not match \s.
2. PCRE does not allow repeat quantifiers on lookahead
assertions. Perl permits them, but they do not mean what you
might think. For example, (?!a){3} does not assert t