JavaScript & regex : How do I check if the string is ASCII only?

I know I can validate against string with words ( 0-9 A-Z a-z and underscore ) by applying W in regex like this:

function isValid(str) { return /^\w+$/.test(str); }

But how do I check whether the string contains ASCII characters only? ( I think I'm close, but what did I miss? )

Reference: https://stackoverflow.com/a/8253200/188331

UPDATE : Standard character set is enough for my case.