Go forward to isprint
Go backward to isdigit
Go up to Ctype
Go to the top op libc

`islower'--lower-case character predicate

     #include <ctype.h>
     int islower(int C);
`islower' is a macro which classifies ASCII integer values by table
lookup.  It is a predicate returning non-zero for minuscules
(lower-case alphabetic characters), and 0 for other characters.  It is
defined only when `isascii'(C) is true or C is EOF.
   You can use a compiled subroutine instead of the macro definition by
undefining the macro using ``#undef islower''.
`islower' returns non-zero if C is a lower case letter (`a'-`z').
`islower' is ANSI C.
   No supporting OS subroutines are required.