/sys$common/syshlp/helplib.hlb CC, Run-time functions, strcspn  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns the length of the prefix of a string that consists entirely
  of characters that are not in a specified set of characters.

  Syntax:

       #include <string.h>

       size_t strcspn(const char *str, const char *charset);