Matching URLs with Regular Expressions

Regular expressions can be daunting initially, but they offer powerful pattern-matching capabilities for diverse data types. In the context of extracting URLs, a flexible pattern is necessary to accommodate variations in URL formats.

One robust regular expression that can capture URLs with or without leading protocols (e.g., "http://www" or "www") is:

((https?|ftp)://)? // Optional SCHEME
([a-z0-9 !*(),;?&=$_.-] (:[a-z0-9 !*(),;?&=$_.-] )?@)? // Optional User and Pass
([a-z0-9\-\.]*)\.(([a-z]{2,4})|([0-9]{1,3}\.([0-9]{1,3})\.([0-9]{1,3}))) // Host or IP address
(:[0-9]{2,5})? // Optional Port
(/([a-z0-9 $_%-]\.?) )*/? // Path
(\?[a-z &\$_.-][a-z0-9;:@&%= /$_.-]*)? // Optional GET Query
(#[a-z_.-][a-z0-9 $%_.-]*)? // Optional Anchor

To use this expression in PHP, enclose it in double quotes and pass it to the preg_match function along with the URL you want to evaluate. For example:

$url = 'www.example.com/etcetc';
if (preg_match("~^$regex$~i", $url)) {
    echo 'Matched URL without protocol';
}

Similarly, for URLs with protocols:

$url = 'http://www.example.com/etcetc';
if (preg_match("~^$regex$~i", $url)) {
    echo 'Matched URL with protocol';
}

This pattern should cover a wide range of URL formats while also protecting against potential malicious input containing characters such as "/".