Maximum match length of a regular expression

Question

What is the easiest way to determine the maximum match length of a regular expression?

Specifically, I am using Python's re module.

E.g. for foo((bar){2,3}|potato) it would be 12.

Obviously, regexes using operators like * and + have theoretically unbounded match lengths; in those cases returning an error or something is fine. Giving an error for regexes using the (?...) extensions is also fine.

I would also be ok with getting an approximate upper bound, as long as it is always greater than the actual maximum length, but not too much greater.

Answer 1

Using pyparsing's invRegex module:

import invRegex
data='foo(bar{2,3}|potato)'    
print(list(invRegex.invert(data)))
# ['foobarr', 'foobarrr', 'foopotato']    
print(max(map(len,invRegex.invert(data))))
# 9

Another alternative is to use ipermute from this module.

import inverse_regex
data='foo(bar{2,3}|potato)'
print(list(inverse_regex.ipermute(data)))
# ['foobarr', 'foobarrr', 'foopotato']
print(max(map(len,inverse_regex.ipermute(data))))
# 9

Answer 2

Solved, I think. Thanks to unutbu for pointing me to sre_parse!

import sre_parse

def get_regex_max_match_len(regex):
    minlen, maxlen = sre_parse.parse(regex).getwidth()
    if maxlen >= sre_parse.MAXREPEAT: raise ValueError('unbounded regex')
    return maxlen

Results in:

>>> get_regex_max_match_len('foo((bar){2,3}|potato)')
12
>>> get_regex_max_match_len('.*')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in get_regex_max_match_len
ValueError: unbounded regex