When not to use an RE -- an example

[John Machin]

I needed a check for strings consisting of repeated characters -- like
when users type "ZZZZZZZ" instead of "UNKNOWN" into a database field.
After implementing the obvious overlapping-substring comparison, I got
to thinking how this could be done with REs. The following resulted:

import re
repeats1 = re.compile(r"^(?:(.)(?=\1))+\1\Z", re.DOTALL).match
def repeats2(s):
   return len(s) > 1 and s[1:] == s[:-1]
for testvalue, expected in zip(
   ['','x','xx','xxx','xxxxxx','xy','xxy','xyy','\n\n\n','aaa\n'],
   [0,  0,  1,   1,    1,       0,   0,    0,    1,       0     ]):
   print repr(testvalue), not not repeats1(testvalue),
repeats2(testvalue), expected

Pergly/phugly, eh? Note the effort required to ensure the newline
cases worked.