Case-Insensitive String comparison

It is often a problem that we ask someone to input some string for comparison but they often put different case and the comparison fails. For example if we are expecting a string, say, 'true'. The user may input 'True' or 'TRUE'. A simple comparison will fail in this case.

The following is a simple program to convert the input string to lower case. What this would allow is to do a case-insensitive search.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>
#include<string>

using namespace std;

//First approach for converting into lower case
string toLowerString(string& input)
{
  int length = input.length();
  string output;
  for(int i = 0; i < length; i++)
  {
    output += tolower(input[i]);
  }
  return output;
}

//Second approach for converting into lower case
string toLowerString(string& input, bool approach2) //Overloaded method
{
  int length = input.length();
  string output;
  for(int i = 0; i < length; i++)
  {
    if(input[i] >= 0x41 && input[i] <= 0x5A) //Ascii for A-Z
      output += (input[i] + 0x20); //ascii for a-z
    else 
      output += input[i];
  }
  return output;
}

int main()
{
  string s1="true";
  string s2("TRUE");
  string s3;
  s3 = "True";

  //Check if s1 = s2
  if(s1 == s2)
    cout<<"s1 == s2"<<endl;
  else
    cout<<"s1 != s2"<<endl;

  //Check if s1 = s2 with first approach
  if(s1 == toLowerString(s2))
    cout<<"s1 == Approach1::toLowerString(s2)"<<endl;
  else
    cout<<"s1 != Approach1::toLowerString(s2)"<<endl;

  //Check if s1 = s2 with second approach
  if(s1 == toLowerString(s2,true))
    cout<<"s1 == Approach2::toLowerString(s2)"<<endl;
  else
    cout<<"s1 != Approach2::toLowerString(s2)"<<endl;


  //Check if s1 = s3 with second approach
  if(s1 == s3)
    cout<<"s1 == s3"<<endl;
  else
    cout<<"s1 != s3"<<endl;

  if(s1 == toLowerString(s3,true))
    cout<<"s1 == Approach2::toLowerString(s3)"<<endl;
  else
    cout<<"s1 != Approach2::toLowerString(s3)"<<endl;

  //Check if s1 = s3 with second approach
  if(s1 == toLowerString(s3)) 
    cout<<"s1 == Approach1::toLowerString(s3)"<<endl;
  else
    cout<<"s1 != Approach1::toLowerString(s3)"<<endl;

  return 0;
}

The output is as follows:


I am aure much better approaches are possible. If you have a better example please share.