camel case to snake case

camelToSnake.js

String.prototype.toSnakeCase = function() {
    var upperChars = this.match(/([A-Z])/g);
    if (! upperChars) {
        return this;
    }

    var str = this.toString();
    for (var i = 0, n = upperChars.length; i < n; i++) {
        str = str.replace(new RegExp(upperChars[i]), '_' + upperChars[i].toLowerCase());
    }

    if (str.slice(0, 1) === '_') {
        str = str.slice(1);
    }

    return str;
};

camelToSnake.test.js

test('camelToSnake', function() {
    var testCtoS = function(str, expect) {
        equal(str.toSnakeCase(), expect);
    };

    testCtoS('aaaa', 'aaaa');
    testCtoS('Aaaa', 'aaaa');
    testCtoS('AAaa', 'a_aaa');
    testCtoS('aAaA', 'a_aa_a');
    testCtoS('toLocaleDateString', 'to_locale_date_string');
    testCtoS('ABCDE', 'a_b_c_d_e');
});