Creates links for urls on text

auto_link_text.php

function auto_link_text($text) {
    $regexp = '/
    (                                           # leading text
    <\w+.*?>|                                   # leading HTML tag, or
    [^=!:\'"\/]|                                # leading punctuation, or
    ^                                           # beginning of line
    )
    (
    (?:https?:\/\/)|                            # protocol spec, or
    (?:www\.)                                   # www.*
    )
    (
    [-\w]+                                      # subdomain or domain
    (?:\.[-\w]+)*                               # remaining subdomains or domain
    (?::\d+)?                                   # port
    (?:\/(?:(?:[~\w\+%-]|(?:[,.;:][^\s$]))+)?)* # path
    (?:\?[\w\+%&=.;-]+)?                        # query string
    (?:\#[\w\-]*)?                              # trailing anchor
    )
    ([[:punct:]]|\s|<|$)                        # trailing text
    /x';

    return preg_replace_callback($regexp, function($matched) {
        list($all, $before, $protocol, $address, $after) = $matched;

        // already linked
        if (preg_match('/<a\s/i', $before)) {
            return $all;
        }

        $text = $protocol . $address;
        $protocol = $protocol == 'www.' ? 'http://www.' : $protocol;

        return "$before<a href=\"$protocol$address\">$text</a>$after";
    }, $text);
}