STRCSPN(3) Library Functions Manual STRCSPN(3)

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);

index(3), memchr(3), rindex(3), strchr(3), strpbrk(3), strrchr(3), strsep(3), strspn(3), strstr(3), strtok(3)

The strcspn() function conforms to ANSI X3.159-1989 (“ANSI C89”).
August 11, 2002 NetBSD 9.2