Javascript String subst(replacements)

/**//from   w ww  .j av  a2 s.  c  o m
 * Extend String class with a placeholder replacement function
 *
 * Placeholder strings must be surrounded by %, start with an uppercase character, followed by on ore more
 * uppercase characters, numbers or _
 *
 * @param {Array} replacements
 * 
 * @returns {String}
 */
String.prototype.subst = function(replacements) {
    return this.replace(/%[A-Z][A-Z0-9_]+%/g, function (placeholder) {
        return placeholder in replacements ? replacements[placeholder] : placeholder;
    });
};