import re

string = "It was never going to work, he thought. He did not play so well, so he had to practice some more. Not foobar !"

transformed = re.sub(r'(?:not|never|no)[\w\s]+[^\w\s]', 
  		lambda match: re.sub(r'(\s+)(\w+)', r'\1NEG_\2', match.group(0)), 
			string,
      flags=re.IGNORECASE)
      
print transformed