Standard C Library (libc, -lc)
#include <string.h>
size_t
  
  strspn(const
    char *s, const char
    *charset);
The strspn() function spans the initial part of the
  nul-terminated string s as long as the characters from
  s occur in string charset.
The strspn() function returns the number of characters
  spanned.
The following call to strspn() will return 3, since the
  first three characters of string s are part of string
  charset:
char *s = "foobar";
char *charset = "of";
size_t span;
span = strspn(s, charset);
 
The strspn() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).