SPUG: minimal unmatching regex

[Michael R. Wolf]

Question:
What's the smallest (comprehensible) regex that's guaranteed to fail  
against all input?

Motivation:
I'm creating a lab for perl classes.  I want to provide the stub of a  
script that compiles, but always fails to match.  Unfortunately, the  
obvious minimal RE of // matches *everything*.

Here's a stub of the lab...

# TODO... replace the following line with your regular expression to  
match zip codes.
my $zip_re = qr//;

like('98107', $zip_re, 'Simple 5-digit zip'),
like('07748', $zip_re, 'Simple 5-digit zip with leading zero'),

unlike('1234', 		$zip_re, 'Four digits');
unlike('123456', 	$zip_re, 'Six digits');

-- 
Michael R. Wolf
     All mammals learn by playing!
         MichaelRWolf at att.net