The BNF rules from RFC 5322 transformed to PCRE by Nikita Popov and described in the post http://nikic.github.io/2012/06/15/The-true-power-of-regular-expressions.html.

style img

Capture src attribute of HTML image tags

capture Nth character from string, 3 times

Credit: https://gist.github.com/FinalAngel/1876898