Regex to find a substring with a specific length that contains minimum occurrences of a specific characters -

is there regex find substring specific length contains minimum number of specific char occurrences?

for example have string such as: aababaaaaa string have substring length 5 contains 2 b => aabab regex should find it.

but aaaabaaaab there not substring length of 5 contains 2 b.

suppose our string contains , b , want find substring length of 5 contains @ least 2 b:

aaaabaaaab -> invalid aaaaaaaabb -> valid aaaaaaaaaabaabaaaaaa -> valid aaaabaaaaaaabaaaaaaa -> invalid 

brute force:

.b..b|b...b|..bb.|.b.b.|..b.b|bb...|b.b..|...bb|b..b.|.bb.. 

well, know such regular expression not parametrizable. on other hand it's possible obtain programmatically (the example in python):

import itertools  def get_regex(char, charnum, strsize):     chars = char * charnum + "." * (strsize - charnum)     return "|".join("".join(x) x in set(itertools.permutations(chars)))  print get_regex("b", 2, 5)