PHP Cookbook

13.4. Choosing Greedy or Nongreedy Matches

Problem

You want your pattern to match the smallest possible string instead of the largest.

Solution

Place a ? after a quantifier to alter that portion of the pattern:

// find all bolded sections
preg_match_all('#<b>.+?</b>#', $html, $matches);

Or, use the U pattern modifier ending to invert all quantifiers from greedy to nongreedy:

// find all bolded sections
preg_match_all('#<b>.+</b>#U', $html, $matches);

Discussion

By default, all regular expressions in PHP are what’s known as greedy. This means a quantifier always tries to match as many characters as possible.

For example, take the pattern p.*, which matches a p and then 0 or more characters, and match it against the string php. A greedy regular expression finds one match, because after it grabs the opening p, it continues on and also matches the hp. A nongreedy regular expression, on the other hand, finds a pair of matches. As before, it matches the p and also the h, but then instead of continuing on, it backs off and leaves the final p uncaptured. A second match then goes ahead and takes the closing letter.

The following code shows that the greedy match finds only one hit; the nongreedy ones find two:

print preg_match_all('/p.*/', "php");  // greedy
print preg_match_all('/p.*?/', "php"); // nongreedy
print preg_match_all('/p.*/U', "php"); // nongreedy
1
               2
               2

Greedy matching is also known as maximal matching and nongreedy matching can be called minimal matching, because these options ...