size_t strspn_P(const char *__s, const char *__accept) __ATTR_PURE__
The strspn_P() function calculates the length of the initial segment of s which consists entirely of characters in accept. This function is similar to strspn() except that accept is a pointer to a string in program space.
The strspn_P() function returns the number of characters in the initial segment of s which consist only of characters from accept. The terminating zero is not considered as a part of string.