strcspn - get length of complementary substring

Syntax	#include <string.h> size_t strcspn(const char *s1, const char *s2);
Description	Starting at the beginning of string s1, strcspn() calculates the length of the segment that does not contain a single character from string s2. The terminating null byte (\0) is not treated as part of string s2. The function is terminated and the segment length is returned on encountering a character in s1 that matches a character in s2. If the first character in s1 already matches a character in s2, the segment length is equal to 0.
Return val.	Integer value that indicates the segment length (number of non-matching characters) as of the beginning of string s1.
Notes See also	Strings terminated with the null byte (\0) are expected as arguments. strspn(), string.h.