var str = "This string has too many		 spaces.";
var result = str.replace(/(\s| |&\#160;)+/gi, "_");
document.write(result);

I'll never learn how to use this Markdown system. Anyway, here is the fixed code based on my comment on the following page:

http://snipplr.com/view/14206/replace-spaces-with-underscores/

I'm sorry, I don't mean to hijack the code. :(

Replace space with _

replace

JavaScript