Traceback (most recent call last):
  File "./hamming_sanity", line 9, in <module>
    s = checker.parse_string("s_%d" % i, min_length=N, max_length=N, valid_chars="ACTG")
  File "/usr/local/lib/python2.6/dist-packages/sanity.py", line 252, in parse_string
    if not _re_match("^[%s]{%d,%d}$" % (valid_chars, min_length, max_length), line):
  File "/usr/local/lib/python2.6/dist-packages/sanity.py", line 56, in _re_match
    return re.match(pattern, text)
  File "/usr/lib/python2.6/re.py", line 137, in match
    return _compile(pattern, flags).match(string)
  File "/usr/lib/python2.6/re.py", line 243, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/usr/lib/python2.6/sre_compile.py", line 529, in compile
    groupindex, indexgroup
RuntimeError: invalid SRE code