'use strict'; module.exports = function (str, sep) {

if (typeof str !== 'string') {
        throw new TypeError('Expected a string');
}

sep = typeof sep === 'undefined' ? '_' : sep;

return str
        .replace(/([a-z\d])([A-Z])/g, '$1' + sep + '$2')
        .replace(/([A-Z]+)([A-Z][a-z\d]+)/g, '$1' + sep + '$2')
        .toLowerCase();

};