How do I find the index of a regex match in a string?

Question

In php, it's fairly simple to find and capture all substrings that match a given regex, but is there a simple way to find the index of the first regex match in a string?

i.e. I'd like something that operates like this:

$str = "123456789abcdefgh";
$index = preg_index("#abcd#", $str);
// $index == 9

Have you tried [`strpos()`](http://php.net/manual/en/function.strpos.php)? — Jared Farrish, Sep 18 '11 at 23:02
`strpos()` only does string matching, not regular expressions, though you could do it on the match, but that seems silly. — Daniel Beardsley, Sep 28 '11 at 18:48

Jon · Accepted Answer · 2011-09-18T23:16:16.740

24

Use the PREG_OFFSET_CAPTURE flag with preg_match:

$str = "123456789abcdefgh";
$index = -1;
if(preg_match("#abcd#", $str, $matches, PREG_OFFSET_CAPTURE)) {
    $index = $matches[0][1];
}

The [0] above corresponds to the capture group inside the regex whose index you want (0 for the whole expression, 1 for the first capturing group if it exists, etc) while the [1] is a fixed index, as documented.

Edit: Added an if to make the code more presentable, it now doesn't take for granted that the pattern will definitely match.

See it in action.

edited Sep 18 '11 at 23:16

answered Sep 18 '11 at 23:10

Jon

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1

Great answer, just what I was looking for. – Daniel Beardsley Sep 19 '11 at 04:57

How do I find the index of a regex match in a string?

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