m:  # use : as delims so / in code won't hurt
  ^ # Anchor it at start, don't need to backtrack
  (? # Start a conditional test
    (?{/this/ && !/that/}) # What to test-
    # By itself, a code assertion always succeeds,
    # That's why we wrap the code inside a conditional
    |(?!) # Simplest "T|F" - match succeed, nonmatch fails
  ) # End the condition
:x