Validate String by digits and colon

Question

I need to Validate the string . The string has format like "V02:01:02:03:04".
What I am trying to do :
1) I want to check correct length is provided
2) First character is 'V'
3) All other characters are 2 letter digits and separated by colon and at correct position.
4) There is no blank space tabs et.c
4) Cant think of any more validation .

What I have done so far
1) Function has easily check len and fist char not verify that subsequent chars after first does not contain alpha or space. But I am bit stuck about how to check colon positions and 2 letter char after every colon.

Below is my attempt.

 int firmwareVerLen = 15;
const std::string input = "V02:01:02:03:04"; // this is the format string user will input  with changes only in digits.
bool validateInput( const std::string& input )
{
    typedef std::string::size_type stringSize;
    bool result = true ;
    if( firmwareVerLen != (int)input.size() )
    {       
      std::cout <<" len failed" << std::endl;
      result = false ;
    }

    if( true == result )
   {

        if( input.find_first_of('V', 0 ) )
        {
             std::cout <<" The First Character is not 'V' " << std::endl;
             result = false ;
         }
   }

   if( true == result )
   {
        for( stringSize i = 1 ; i!= input.size(); ++ i )
       {
            if( isspace( input[i] ) )
           {
               result = false;
               break;
           }
           if( isalpha(input[i] ) )
            {
               cout<<" alpha found ";
               result = false;
               break;
            }

             //how to check further that characters are digits and are correctly  separated by colon
        }
    }

   return result;
 }

Answer 1

If it's such a strict validation, why not check character by character? for example (given you've done size already)

  if (input[0] != 'V')
    return false;
  if (!std::isdigit(input[1]) || !std::isdigit(input[2]))
    return false;
  if (input[3] != ':')
    return false;
  // etc...