Regex Non-greedy (or Lazy)
Summary: in this tutorial, you’ll learn about the regex non-greedy (or lazy) quantifiers that match their preceding elements as few times as possible.
Introduction to the regex non-greedy (or lazy) quantifiers
In regular expressions, the quantifiers have two versions: greedy and non-greedy (or lazy). In the previous tutorial, you learned how greedy quantifiers work.
To turn a greedy quantifier into a non-greedy quantifier, you can append a question mark (?) to it. The following table shows the greedy and non-greedy quantifiers:
| Greedy quantifier | Lazy quantifier | Meaning |
|---|---|---|
* | *? | Match its preceding element zero or more times. |
+ | +? | Match its preceding element one or more times. |
? | ?? | Match its preceding element zero or one time. |
{ n } | { n }? | Match its preceding element exactly n times. |
{ n ,} | { n ,}? | Match its preceding element at least n times. |
{ n , m } | { n , m }? | Match its preceding element from n to m times. |
The following example uses the non-greedy quantifier (+?) to match the text within the quotes (""):
$str = '<a href="/" title="Go to homepage">Home</a>';
$pattern = '/".+?"/';
if (preg_match_all($pattern, $str, $matches)) {
print_r($matches[0]);
}
Code language: HTML, XML (xml)
Output:
Array
(
[0] => "/"
[1] => "Go to homepage"
)
Code language: PHP (php)
How the non-greedy quantifier (+? ) works.
First, the regex engine searches for a match starting from the first position in the string. Because the regex engine cannot find a match, it continues searching until it finds the first quote ("):
Second, the regex engine finds a match for the rule .+? . However, the non-greedy quantifier +? just matches one time, not multiple times to the end of the string like the greedy quantifier (+). And the regex engine immediately looks at the next rule in the pattern, which matches a quote (“):
Third, the regex engine repeats the first and second steps starting from the position after the first match and returns the next result:
Finally, the regex engine continues to search till the end of the string:
Summary
- Append a question mark (?) to a quantifier to turn it into a non-greedy quantifier.
- A non-greedy quantifer tries to match its preceding element as few times as possible.