Counting how many chars equal to a given char are in the beginning of a string

[Stormbringer]

Hi,

Given a string s and a char c, is there a short & elegant way to
determine how many consecutive occurences of c are in the beginning of
s ?

For example if s = ">>>>message1" and c = ">" then the number I am
looking for is 4 (the string begins with 4 '>').

Thanks,
Andrei