islower —
lower-case character test
Standard C Library (libc, -lc)
#include <ctype.h>
int
  
  islower(int
    c);
The islower() function tests for any lower-case letter
  for which none of iscntrl(3),
  isdigit(3),
  ispunct(3), or
  isspace(3) is true. In the
  ``C'' locale, islower() returns true
  only for the characters defined as lower-case letters.
The islower() function returns zero if the character
  tests false and returns non-zero if the character tests true.
ctype(3),
  isalnum(3),
  isalpha(3),
  isascii(3),
  isblank(3),
  iscntrl(3),
  isdigit(3),
  isgraph(3),
  isprint(3),
  ispunct(3),
  isspace(3),
  isupper(3),
  isxdigit(3),
  stdio(3),
  toascii(3),
  tolower(3),
  toupper(3),
  ascii(7)
The islower() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).
The argument to islower() must be
  EOF or representable as an unsigned
  char; otherwise, the behavior is undefined. See the
  CAVEATS section of
  ctype(3) for more details.