Programming PHP

This code snippet takes several seconds to report failure:
$p = '/(a+|b+)*\.+$/';
$s = 'abababababbabbbabbaaaaaabbbbabbababababababbba..!';
if (preg_match($p, $s)) {
echo "Y";
} else {
echo "N";
}
This is because the regular expression engine tries all the different places to start the
match, but has to backtrack out of each one, which takes time. If you know that
once something is matched it should never be backed out of, you should mark it
with (?>subpattern):
$p = '/(?>a+|b+)*\.+$/';
The cut never changes the outcome of the match; it simply makes it fail faster.
Conditional Expressions
A conditional expression is like an if statement in a regular expression. The general
form is:
(?(condition)yespattern)
(?(condition)yespattern|nopattern)
If the assertion succeeds, the regular expression engine matches the yespattern. With
the second form, if the assertion doesn’t succeed, the regular expression engine skips
the yespattern and tries to match the nopattern.
The assertion can be one of two types: either a backreference, or a lookahead or
lookbehind match. To reference a previously matched substring, the assertion is a
number from 1–99 (the most backreferences available). The condition uses the pattern
in the assertion only if the backreference was matched. If the assertion is not a
backreference, it must be a positive or negative lookahead or lookbehind assertion.
Functions
There are five classes of functions that work with Perl-compatible regular expressions:
matching, replacing, splitting, filtering, and a utility function for quoting text.
Matching
The preg_match( ) function performs Perl-style pattern matching on a string. It’s the
equivalent of the m// operator in Perl. The preg_match( ) function takes the same
arguments and gives the same return value as the ereg( ) function, except that it
takes a Perl-style pattern instead of a standard pattern:
$found = preg_match(pattern, string [, captured ]);
110 | Chapter 4: Strings
This is the Title of the Book, eMatter Edition
Copyright © 2002 O’Reilly & Associates, Inc. All rights reserved.