substring between substrings

[Skaperen]

i am looking for an existing function or i will code my own. given a main string and two substrings A and B ... if A and B are both in the main string and B is found after the end of A, return the substring found between A and B. if B is immediately after A then return an empty string. a way to also do this with byte strings is a big plus, even if it is a different function name. note that A and B can be strings longer than a single character and that A and B may be the same or may be different.

m = 'happy birthday to you'
a = 'happy '
b = ' to'
betweenstr(m,a,b) -> 'birthday'
a = 'b'
b = 'y'
betweenstr(m,a,b) -> 'irthda'
a = 'birth'
b = 'day'
betweenstr(m,a,b) -> ''
a = 'day'
b = 'happy'
betweenstr(m,a,b) -> None or an exception

i am not asking anyone to code this for me; i can do that. i am only interested in something in the Python library or i code on my own, not somehing to be installed.