#!/usr/bin/env python
import re
import sys

# A case of catastrophic backtracking in Python
#
# regex: (a?){n} a{n}
# strings: aa...
#
# call with:
#
#   > ./catastrophic.py 20
#
# or
#
#   > ./catastrophic.py 28
#
#
# call with timing as:
#
#   > time ./catastrophic.py 20



# counter n given on the command line
cn = sys.argv[1]

# constructing the regex
r1 = '((a?){%s})' % cn
r2 = 'a{%s}' % cn

# calling the matching function
m = re.match(r1 + r2 , "a" * int(cn)) 

print m.group(0)
