A better underscorize method?

gistfile1.php

<?php

$tests = [
	'FavoriteURL',
	'URLHelper',
	'FavoriteURLs',
	'AmazingURLsFromSpace',
	'FavoriteURLHelper',
	'FBIManBob',
	'removeXMLTags',
	'fuckItTillItFBIs'
];

function us($string) {


	$rules = [
		'/[a-z]([A-Z][a-z])/',      // Matches words following words
		'/[A-Z]+s([A-Z][a-z])/',    // Matches words following pluralized acronyms
		'/[a-z]([A-Z]+)/',          // Matches acronyms following words
		'/[A-Z]+([A-Z][a-z][^_])/', // Matches word following acronyms 
	];

	foreach ($rules as $rule) {
		if (preg_match_all($rule, $string, $matches)) {
			foreach ($matches[0] as $i => $match) {
				$fragment    = $matches[1][$i];
				$replacement = str_replace($fragment, '_' . $fragment, $match);
				$string      = str_replace($match, $replacement, $string);
			}
		}
	}

	return strtolower($string);
}

foreach ($tests as $test) {
		echo $test . ' => ' . us($test) . "\n";
}