Programming PHP

Character Classes
Perl-style regular expressions support the POSIX character classes but also define
some of their own, as shown in Table 4-9.
Table 4-9. Perl-style character classes
Character class Meaning Expansion
\s Whitespace [\r\n \t]
\S Non-whitespace [^\r\n \t]
\w Word (identifier) character [0-9A-Za-z_]
\W Non-word (identifier) character [^0-9A-Za-z_]
\d Digit [0-9]
\D Non-digit [^0-9]
Anchors
Perl-style regular expressions also support additional anchors, as listed in Table 4-10.
Table 4-10. Perl-style anchors
Assertion Meaning
\b Word boundary (between \w and \W or at start or end of string)
\B Non-word boundary (between \w and \w, or \W and \W)
\A Beginning of string
\Z End of string or before \n at end
\z End of string
^ Start of line (or after \n if /m flag is enabled)
$ End of line (or before \n if /m flag is enabled)
Quantifiers and Greed
The POSIX quantifiers, which Perl also supports, are always greedy. That is, when
faced with a quantifier, the engine matches as much as it can while still satisfying the
rest of the pattern. For instance:
preg_match('/()/', 'do not press the button', $match);
// $match[1] is 'not'
The regular expression matches from the first less-than sign to the last greater-than
sign. In effect, the .* matches everything after the first less-than sign, and the engine
backtracks to make it match less and less until finally there’s a greater-than sign to be
matched.
This greediness can be a problem. Sometimes you need minimal (non-greedy) matching—that
is, quantifiers that match as few times as possible to satisfy the rest of the
Perl-Compatible Regular Expressions | 105
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