Replace _words_ in a string with <em>words</em> but not when they are inside an URL

replace.php

<?php
// Lookbehinds require fixed length, but lookaheads does not, so just reverse the string
$string = 'some string with _foo_ and _bar_ and http://example.com/some_foo_bar';
$replaced = strrev(preg_replace('#_([a-zæøå -]+)_(?![^ ]+//:ptth)#i', '>me/<\1>me<', strrev($string)));
echo $replaced; // some string with <em>foo</em> and <em>bar</em> and http://example.com/some_foo_bar