C# Code Sample:

var subject = "RE: RE:Fw:             RE:FW: FWD: re: This is a message from a long thread re: asdf ";
var pattern = @"((?:[\t ]*(?:R|RE|F|FW|FWD):[\t ]*)*)(.*)";
var regex = new Regex(pattern, RegexOptions.ECMAScript | RegexOptions.IgnoreCase);
var match = regex.Match(subject);
var extracted = match.Groups[2].Value;

extracted value: This is a message from a long thread re: asdf