Return of the regex mystery

The little one-off function that I wrote to extract the file name from a URL still continues to baffle. Rekha, in the course of reviewing some other code and functionality, threw a random string of letters (no punctuation) at it, which caused the browser's script engine to time out. I rewrote the code somewhat, but even this version takes about 30 seconds to complete, when given a string like "asdfghjklasdfghjklasdfghjkl":

var URL_REGEX = /^(http:\/\/)?(\w+\.?)*client\.org(.*)$/i;

stripUrlPrefix = function (url) {
    var regex = new RegExp(URL_REGEX);
    var result = jQuery.trim(url);
    var matches = regex.exec(result);
    if (matches) {
        return matches[3];
    } else {
        return result;
    }
}