It's not clear what you want to achieve. Maybe the following Perl code will help? Just replace the text in the hash with whatever you want to fill the gaps with.

use strict;
use warnings;

my %fillers = (
    A => "Whatever is required for the A gap here",
    B => "Whatever is required for the B gap here",
    C => "Whatever is required for the C gap here",
    );

while (<DATA>) {
    s/_*(\w)_+/$fillers{$1}/eg;
    print;
}

__DATA__
function start()
    {
      __A________________________________________________;
    }

    function validate( event )
    {
      if ( B_____{
         event.preventDefault();
         C__________________;
         alert( "Wrong format for age!" );
      }
    }
[download]

Prints:

function start()
    {
      Whatever is required for the A gap here;
    }

    function validate( event )
    {
      if ( Whatever is required for the B gap here{
         event.preventDefault();
         Whatever is required for the C gap here;
         alert( "Wrong format for age!" );
      }
    }
[download]