Get length of substring composed of given characters

#include <string.h>
#include <stdio.h>

int main(void)
{
  int len;

  len = strspn("this is a test", "siht ");
  printf("%d", len);

  return 0;
}

1.	String length
2.	Computes the length of a line
3.	Lengths of strings
4.	Get the string length
5.	Add null terminator to string end
6.	print the string forward and backwards
7.	String operator: get length, compare and find a char
8.	Return string length: how to use strlen
9.	Get length of substring composed of given characters: how to use strspn
10.	Check string length and set string end

Get length of substring composed of given characters : String Length « String « C / ANSI-C