Programming PHP

pattern. Perl provides a parallel set of quantifiers that match minimally. They’re easy
to remember, because they’re the same as the greedy quantifiers, but with a question
mark (?) appended. Table 4-11 shows the corresponding greedy and non-greedy
quantifiers supported by Perl-style regular expressions.
Table 4-11. Greedy and non-greedy quantifiers in Perl-compatible regular expressions
Greedy quantifier Non-greedy quantifier
? ??
* *?
+ +?
{m} {m}?
{m,} {m,}?
{m,n} {m,n}?
Here’s how to match a tag using a non-greedy quantifier:
preg_match('/()/', 'do not press the button', $match);
// $match[1] is ''
Another, faster way is to use a character class to match every non-greater-than character
up to the next greater-than sign:
preg_match('/(]*>)/', 'do not press the button', $match);
// $match[1] is ''
Non-Capturing Groups
If you enclose a part of a pattern in parentheses, the text that matches that subpattern
is captured and can be accessed later. Sometimes, though, you want to create a
subpattern without capturing the matching text. In Perl-compatible regular expressions,
you can do this using the (?:subpattern) construct:
preg_match('/(?:ello)(.*)/', 'jello biafra', $match);
// $match[1] is ' biafra'
Backreferences
You can refer to text captured earlier in a pattern with a backreference: \1 refers to
the contents of the first subpattern, \2 refers to the second, and so on. If you nest
subpatterns, the first begins with the first opening parenthesis, the second begins
with the second opening parenthesis, and so on.
For instance, this identifies doubled words:
preg_match('/([[:alpha:]]+)\s+\1/', 'Paris in the the spring', $m);
// returns true and $m[1] is 'the'
You can’t capture more than 99 subpatterns.
106 | Chapter 4: Strings
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