Get query string argument (using regexp)

getqueryarg-regexp.js

/**
 * Returns specified argument from query string.
 *
 * @params  {string} key - Argument to be returned.
 * @returns {string} Value of key ('' for ?arg=, null for ?arg, undefined if not present).
 */
function getQueryArg(key) {
    // look for key prefixed by ?/&/;, (optionally) suffixed
    // by =val (using lazy match), followed by &/;/# or EOS
    var re = new RegExp('[?&;]'+key+'(=(.*?))?([&;#]|$)');
    var results = re.exec(location.search);
    if (results == null) return undefined; // not found
    if (results[2] == undefined) return null; // ?key without '='
    return decodeURIComponent(results[2].replace(/\+/g, ' '));
}