RegEx whitelist of words in any order

I came across the need to limit the words in a string to a certain list of words. This turns out to be a little more complicated than blacklisting a list of words, such as profanity, from a string.

Parameters

• Assume the whitelist is foo bar baz.
• Match whitelisted words in any order
  Pass: foo bar baz
  Pass: bar foo baz
• Do not match on any word not in the whitelist
  Fail: foo boom baz
• Do not match on extra inner, trailing or leading whitespace

Solution

Ruby

regex = %r{\A(?! )(?:(?:\A| )(?:foo|bar|baz)){1,3}\z}

JavaScript

const regex = /^(?! )(?:(?:^| )(?:foo|bar|baz)){1,3}$/

Shortcomings

The RegEx above does allow duplicate approved words up to the quantified limit. So 'foo bar bar' will pass, but 'foo bar bar bar' will not.

While I like to keep things succinct, this duplicate word problem can easily be avoided using other means.