A regex to match a substring that isn't followed by a certain other substring

Try:

/(?!.*bar)(?=.*foo)^(\w+)$/

Tests:

blahfooblah            # pass
blahfooblahbarfail     # fail
somethingfoo           # pass
shouldbarfooshouldfail # fail
barfoofail             # fail

Regular expression explanation

NODE                     EXPLANATION
--------------------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
    bar                      'bar'
--------------------------------------------------------------------------------
  )                        end of look-ahead
--------------------------------------------------------------------------------
  (?=                      look ahead to see if there is:
--------------------------------------------------------------------------------
    .*                       any character except \n (0 or more times
                             (matching the most amount possible))
--------------------------------------------------------------------------------
    foo                      'foo'
--------------------------------------------------------------------------------
  )                        end of look-ahead
--------------------------------------------------------------------------------
  ^                        the beginning of the string
--------------------------------------------------------------------------------
  (                        group and capture to \1:
--------------------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
  )                        end of \1
--------------------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string

Other regex

If you only want to exclude bar when it is directly after foo, you can use

/(?!.*foobar)(?=.*foo)^(\w+)$/

Edit

You made an update to your question to make it specific.

/(?=.*foo(?!bar))^(\w+)$/

New tests

fooshouldbarpass               # pass
butnotfoobarfail               # fail
fooshouldpassevenwithfoobar    # pass
nofuuhere                      # fail

New explanation

(?=.*foo(?!bar)) ensures a foo is found but is not followed directly bar

To match a foo following by something that doesn't start with bar, try

foo(?!bar)

Your version with negative lookbehind is effectively "match a foo followed by something that doesn't end in bar". The .* matches all of barblah, and the (?<!bar) looks back at lah and checks that it doesn't match bar, which it doesn't, so the whole pattern matches.

Use a negative look ahead instead:

\s*(?!\w*(bar)\w*)\w*(foo)\w*\s*

This worked for me, hope it helps. Good luck!