Core_Valid::url

public static function url($url)
{
    // Based on http://www.apps.ietf.org/rfc/rfc1738.html#sec-5
    if ( !preg_match(
    	'~^

        # scheme
        [-a-z0-9+.]++://

        # username:password (optional)
        (?:
                [-a-z0-9$_.+!*\'(),;?&=%]++   # username
            (?::[-a-z0-9$_.+!*\'(),;?&=%]++)? # password (optional)
            @
        )?

        (?:
            # ip address
            \d{1,3}+(?:\.\d{1,3}+){3}+

            | # or

            # hostname (captured)
            (
                     (?!-)[-a-z0-9]{1,63}+(?<!-)
                (?:\.(?!-)[-a-z0-9]{1,63}+(?<!-)){0,126}+
            )
        )

        # port (optional)
        (?::\d{1,5}+)?

        # path (optional)
        (?:/.*)?

        $~iDx', $url, $matches) ) return false;

    // We matched an IP address
    if ( !isset($matches[1]) ) return true;

    // Check maximum length of the whole hostname
    // http://en.wikipedia.org/wiki/Domain_name#cite_note-0
    if (strlen($matches[1]) > 253) return false;

    // An extra check for the top level domain
    // It must start with a letter
    $tld = ltrim(substr($matches[1], (int)strrpos($matches[1], '.')), '.');

    return ctype_alpha($tld[0]);
}