except vs if not

except

conj
  • With the exception (that); used to introduce a clause, phrase or adverb forming an exception or qualification to something previously stated. 

prep
  • with the exception of; but. 

verb
  • To exclude; to specify as being an exception. 

  • To take exception, to object (to or against). 

if not

adv
  • Used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the former. 

How often have the words except and if not occurred in a corpus of books? (source: Google Ngram Viewer )