Understanding PHP preg_match() Function with Detailed Examples

The preg_match() function searches a subject string for a match to a given regular expression pattern. It takes the pattern, the subject, an optional matches array (filled with the full match and captured sub‑patterns), optional flags (e.g., PREG_OFFSET_CAPTURE ), and an optional offset to start the search.

It returns the number of matches (0 or 1) because it stops after the first match; preg_match_all() continues searching. On error, it returns FALSE .

Parameters :

pattern – the regular expression pattern (string).

subject – the input string to search.

matches – optional array that receives the results; $matches[0] contains the full match, $matches[1] the first captured group, and so on.

flags – optional flags such as PREG_OFFSET_CAPTURE to include the offset of each match.

offset – optional byte offset to start the search.

Example 1 – Simple case‑insensitive match

<?php
if (preg_match("/php/i", "PHP is the web scripting language of choice.")) {
    echo "A match was found.";
} else {
    echo "A match was not found.";
}
?>

Example 2 – Word‑boundary match

<?php
if (preg_match("/\bweb\b/i", "PHP is the web scripting language of choice.")) {
    echo "A match was found.";
} else {
    echo "A match was not found.";
}
?>

Example 3 – Extract host name from a URL

<?php
$subject = "http://www.php.net/index.html";
if (preg_match('@^(?:http://)?([^/]+)@i', $subject, $matches)) {
    $host = $matches[1];
    // Get the last two parts of the host
    if (preg_match('/[^.]+\.[^.]+$/', $host, $matches)) {
        echo "domain name is: {$matches[0]}\n";
    }
}
?>

Example 4 – Named capture groups

<?php
$str = 'foobar: 2008';
preg_match('/(?P
\w+): (?P
\d+)/', $str, $matches);
print_r($matches);
?>