Regular expression guaranteed to fail

Jeremy Bowers jerf at
Fri Aug 20 22:10:22 CEST 2004

On Fri, 20 Aug 2004 10:35:18 +0000, Des Small wrote:
> The question is, what regular expression should correspond to the
> empty set?

I would return compiled RE objects instead of strings, and in the empty
case, return a class you write that matches the interface of a compiled RE
but returns what you like. Something like:

def NeverMatch(object):
	def match(*args, **kwargs):
		return None

def set2re(s):
	if s: return re.compile("|".join([e for e in s]))
	else: return NeverMatch()

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