PHP preg_match funtion

The PHP function preg_match compares a given string $subject with a given regular expression $pattern.

preg_match(
string $pattern,
string $subject,
array &$matches = null,
int $flags = 0,
int $offset = 0
): int|false

Parameters

$pattern – the pattern of the regular expression that will be compared with the given string.

$subject – is the string with which we will compare the regular expression.

$matches – an array that we will provide if we want to obtain matches of patterns in $pattern compared to the string.

$matches[0] will contain the text that matched the complete pattern given in $pattern.

$matches[1] will have the text that matched the first captured parenthesized subpattern, and so on.

$flags – we can pass the following flags as parameters:

First, let’s consider the case of having no flags: The flag value is 0 by default.

preg_match('/(foo)(bar)(baz)/', 'foobarbaz', $matches);
print_r($matches);

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

PREG_OFFSET_CAPTURE

If this constant is used as a flag parameter, the index where the match was found will also be returned in $matches.

preg_match('/(foo)(bar)(baz)/', 'foobarbaz', $matches, PREG_OFFSET_CAPTURE);
print_r($matches);

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

When we pass this constant as a flag, non-matching sub-patterns will be reported as null in $matches; if not used, they will be 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);
?>

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 – This parameter is optional and can be used to specify the starting point for the search for matches (in bytes).

Returns

1 – If a pattern matching the given pattern in $pattern is found in the string.

0 – If no matches are found with the pattern $pattern.

false in case of a failure.