Question

I have a string A and want to test if another string B is not part of it. This is a very
simple regex whose result can be inverted afterwards.

I could do:

/foobar/.test('[e-mail]')

and invert it afterwards, like this:

!(/foobar/).test('[e-mail]') 

The problem I have is, that I need to do it within the regular expression and not with their result. Something like:

/!foobar/.test('[e-mail]')

(which does not work)

In other words: the regular expression should test for a non-existence and return true in that case.

Is this possible with JavaScript?

Answer 1

Try:

/^(?!.*foobar)/.test('[e-mail]')

A (short) explanation:

^          # start of the string 
(?!        # start negative look-ahead
  .*       # zero or more characters of any kind (except line terminators)
  foobar   # foobar
)          # end negative look-ahead

So, in plain English, that regex will look from the start of the string if the string ‘foobar’ can be "seen". If it can be "seen" there is no* match.

* no match because it’s negative look-ahead!

More about this look-ahead stuff: http://www.regular-expressions.info/lookaround.html