strcspn —
span the complement of a string
Standard C Library (libc, -lc)
#include <string.h>
size_t
  
  strcspn(const
    char *s, const char
    *charset);
The strcspn() function spans the initial part of the
  nul-terminated string s as long as the characters from
  s do not occur in string charset
  (it spans the complement of charset).
The strcspn() function returns the number of characters
  spanned.
The following call to strcspn() will return 3, since the
  first three characters of string s do not occur in
  string charset:
char *s = "foobar";
char *charset = "bar";
size_t span;
span = strcspn(s, charset);
 
The strcspn() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).