--- /dev/null
+'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();
+};