strcspn: returns the index of the first character in *str1 that matches any of the characters in *str2
//Declaration: size_t strcspn(const char *str1, const char *str2);
//Return: returns the index of the first character in *str1 that matches any of the characters in *str2.
#include <string.h>
#include <stdio.h>
int main(void)
{ int len;
len = strcspn("this is a test", "ab");
printf("%d", len);