Learning PHP 5-B.5 Greed

B.5 Greed

Quantifiers in the PHP interpreter's regular expression engine are greedy. This means they match as much as they can. The pattern .* means "the string , then zero or more characters, then the string ." The "more" in "zero or more" matches as many characters as possible. When the pattern is applied to the string Look Out! Caution! Uh-Oh!, the .* matches Look Out! Caution! Uh-Oh!. The greediness of the quantifier causes it to skip over the first  it sees and gobble up characters to the last  in the string.

To turn a quantifier from greedy to nongreedy, put a question mark after it. The pattern .*? still matches "the string , then zero or more characters, then the string ", but now the "more" in "zero or more" matches as few characters as possible. Example B-1 shows the difference between greedy and nongreedy matching with preg_match_all( ). (Example B-5 details how preg_match_all( ) works, including the meaning of the @ characters at the start and end of the pattern.)

Example B-1. Greedy and nongreedy matching

$meats = "<b>Chicken</b>, <b>Beef</b>, <b>Duck</b>";

// With a non-greedy quantifier, each meat is matched separately
preg_match_all('@<b>.*?</b>@',$meats,$matches);
foreach ($matches[0] as $meat) {
    print "Meat A: $meat\n";
}

// With a greedy quantifier, the whole string is matched just once
preg_match_all('@<b>.*</b>@',$meats,$matches);
foreach ($matches[0] as $meat) {
    print "Meat B: $meat\n";
}

Example B-1 prints:

Meat A: <b>Chicken</b>
Meat A: <b>Beef</b>
Meat A: <b>Duck</b>
Meat B: <b>Chicken</b>, <b>Beef</b>, <b>Duck</b>

The nongreedy quantifier in the first pattern makes the first match by preg_match_all( ) stop short at the first  it sees. This leaves part of $meats to be matched by subsequent applications of the pattern by preg_match_all( ).

But with the greedy quantifier in the second example, the first match by preg_match_all( ) scoops up all of the text, leaving nothing matchable for subsequent applications of the pattern.

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