codewars #78 Convert PascalCase string into snake_case (5kyu)

 #78 Convert PascalCase string into snake_case

Complete the function/method so that it takes CamelCase string and returns the string in snake_case notation. Lowercase characters can be numbers. If method gets number, it should return string.

Examples:

//  returns test_controller

toUnderscore('TestController');

// returns movies_and_books

toUnderscore('MoviesAndBooks');

// returns app7_test

toUnderscore('App7Test');

// returns "1"

toUnderscore(1);

function toUnderscore(string) {
  return string.toString().replace(/\.?([A-Z])/g, function (x,y){return "_" + y.toLowerCase()}).replace(/^_/, "");
}