open (MYFILE, 'RegExTest.txt');

#Init variables
$searchTerm = "searchForThis";
$pre 		= "NotBefore";
$post       = "NotAfter";

#Loop through the file
while (<MYFILE>)
{
  $line = $_;              
  
  if($line =~ m/(?<!$pre)$searchTerm?!$post/)
	   {print "Match found: " . $line};        
}                      

close(MYFILE);