regular experssion help

I hope someone has time to help with a regular expression? I need a regex that will replace get variables, regardless of there location in a URL, and if the get var does not exist I hope that it will create the get var in the URL. Some examples $REQUEST_URI can look like this "value1": filename.phtml?test=value1 filename.phtml?another=value&test=value1 filename.phtml filename.phtml?another=value The variable $test will either exist preceded by "?" "&" or not at all. I would like to take a value pair, put it into a function with $REQUEST_URI and the have a new $REQUEST_URI returned with either the value replaced or added. Replacing/adding "value2" filename.phtml?test=value2 filename.phtml?another=value&test=value2 filename.phtml?test=value2 filename.phtml?another=value&test=value2 The variable located by matching ?test &test or adding it if it does not exist, with a "?" or a "&" I currently have a function: replace_get_var_value($string, $var, $new_value){ return eregi_replace("&$var=([a-z0-9\+]+)", "&$var=$new_value", $string); } But it won't work if the value does not exist, or if the value is the first variable in the set within $REQUEST_URI. Example ($REQUEST_URI that will not work with this function) filename.phml?another=value filename.phml?var=old_value filename.phml The only $REQUEST_URI that this will work with is filename.phtml?another=value&var=old_value I hope my description makes sense. Thanks for any help! -Rob Z. http://www.zwink.net