Remove special symbols and extra spaces and replace with underscore using the replace method

Your regular expression [^a-zA-Z0-9]\s/g says match any character that is not a number or letter followed by a space.

Remove the \s and you should get what you are after if you want a _ for every special character.

var newString = str.replace(/[^A-Z0-9]/ig, "_");

That will result in hello_world___hello_universe

If you want it to be single underscores use a + to match multiple

var newString = str.replace(/[^A-Z0-9]+/ig, "_");

That will result in hello_world_hello_universe