Comment by e3pi

If how to create these giant matchstick regexes interest you, there is a wonderful(famous?) perl script generating a regex 6,598 chars long, more optimized and faster than earlier attempt at 4,724 bytes, in Jeffrey Friedl's book, Mastering Regular Expressions, 1st edition, Oreilly, pp 312-316, Appedix B: Email Regex Program.