islower : char functions « Data Types « C++ Tutorial

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#include <iostream>
using std::cout;
using std::endl;

#include <cctype>
using std::islower;
using std::isupper;
using std::tolower;
using std::toupper;

int main()
{
   cout << "According to islower:\n"
      << ( islower( 'p' ) ? "p is a" : "p is not a" )    
      << " lowercase letter\n"
      << ( islower( 'P' ) ? "P is a" : "P is not a" ) 
      << " lowercase letter\n"
      << ( islower( '5' ) ? "5 is a" : "5 is not a" ) 
      << " lowercase letter\n"
      << ( islower( '!' ) ? "! is a" : "! is not a" ) 
      << " lowercase letter\n";

   return 0;
}

According to islower:
p is a lowercase letter
P is not a lowercase letter
5 is not a lowercase letter
! is not a lowercase letter