Parameters

pattern

The pattern to search for, as a string.

subject

The input string.

matches

If matches is provided, then it is filled with the results of search. $matches[0] will contain the text that matched the full pattern, $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on.

flags

flags can be a combination of the following flags:

PREG_OFFSET_CAPTURE

If this flag is passed, for every occurring match the appendant string offset (in bytes) will also be returned. Note that this changes the value of matches into an array where every element is an array consisting of the matched string at offset 0 and its string offset into subject at offset 1.

<?php
preg_match('/(foo)(bar)(baz)/', 'foobarbaz', $matches, PREG_OFFSET_CAPTURE);
print_r($matches);
?>

The above example will output:

Array
(
    [0] => Array
        (
            [0] => foobarbaz
            [1] => 0
        )

    [1] => Array
        (
            [0] => foo
            [1] => 0
        )

    [2] => Array
        (
            [0] => bar
            [1] => 3
        )

    [3] => Array
        (
            [0] => baz
            [1] => 6
        )

)

PREG_UNMATCHED_AS_NULL

If this flag is passed, unmatched subpatterns are reported as null; otherwise they are reported as an empty string.

<?php
preg_match('/(a)(b)*(c)/', 'ac', $matches);
var_dump($matches);
preg_match('/(a)(b)*(c)/', 'ac', $matches, PREG_UNMATCHED_AS_NULL);
var_dump($matches);
?>

The above example will output:

array(4) {
  [0]=>
  string(2) "ac"
  [1]=>
  string(1) "a"
  [2]=>
  string(0) ""
  [3]=>
  string(1) "c"
}
array(4) {
  [0]=>
  string(2) "ac"
  [1]=>
  string(1) "a"
  [2]=>
  NULL
  [3]=>
  string(1) "c"
}

offset

Normally, the search starts from the beginning of the subject string. The optional parameter offset can be used to specify the alternate place from which to start the search (in bytes).

Note:

Using offset is not equivalent to passing substr($subject, $offset) to preg_match() in place of the subject string, because pattern can contain assertions such as ^, $ or (?<=x). Compare:

<?php
$subject = "abcdef";
$pattern = '/^def/';
preg_match($pattern, $subject, $matches, PREG_OFFSET_CAPTURE, 3);
print_r($matches);
?>

The above example will output:

Array
(
)

while this example

<?php
$subject = "abcdef";
$pattern = '/^def/';
preg_match($pattern, substr($subject,3), $matches, PREG_OFFSET_CAPTURE);
print_r($matches);
?>

will produce

Array
(
    [0] => Array
        (
            [0] => def
            [1] => 0
        )

)

Alternatively, to avoid using substr(), use the \G assertion rather than the ^ anchor, or the A modifier instead, both of which work with the offset parameter.