$re = '/(?:\b([a-z]++)\B|\G(?<!^))([A-Z][a-z]+|[a-z]+|[A-Z]+)/';
$str = 'parsingHTTPorSomeURLrequestToday enhanceThisGold thisIsCOOL xP anotherACRONYMiTest loadedTHISupLIKEaMaDmAnS NoReplacement NONEok None none n ';
$subst = "\L$1_$2";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php