Nice and crappy string formatting in Javascript

stringFormat.js

String.prototype.format = function(args) {
    var paramsRegex = /\(([A-Za-z_0-9:.,\'\"]*)\)/;
    var replaceRegex = /{([A-Za-z_0-9:.,\'\"\(\)]+)}/g;
    var string = this;
    return string.replace(replaceRegex, function(match, key) { 
        if(/:/.test(key)) {
            var filters = key.split(':');
            var value = args[filters.shift()];
            for(var i = 0; i < filters.length; i++) {
                var filterName = filters[i];
                var params = [];
                if(paramsRegex.test(filterName)) {
                    var params = paramsRegex.exec(filterName)[1];
                    if(params.length > 0) {
                        params = params.split(',').map(function(s) {
                            return eval(s.trim());
                        });
                        filterName = filterName.substring(0, filterName.indexOf("("));
                    }
                }
                var filter = value[filterName] || String.prototype[filterName] || args._filters[filterName];
                if(typeof filter == 'function') {
                    var value = filter.apply(value, params);
                } else {
                    var value = filter;
                }
            }
            return value;
        } 

        return typeof args[key] != 'undefined'
            ? args[key] 
            : match;
    });
};

try {
    "{firstName} {lastName:slice(0,5):toUpperCase} {initials:join('.')} (provided {initials:length} initials)".format({
        firstName: 'Jesse',
        lastName: 'Dhillon',
        initials: ['Ph', 'D']
    });
} catch(e) {
    debugger;
}