c/string/byte/isalpha

Checks if the given character is an alphabetic character, i.e. either an uppercase letter, or a lowercase letter.

In locales other than, an alphabetic character is a character for which isupper or islower returns or any other character considered alphabetic by the locale. In any case, iscntrl, isdigit, ispunct and isspace will return for this character.

The behavior is undefined if the value of is not representable as  and is not equal to.

Return value
Non-zero value if the character is an alphabetic character, zero otherwise.