The more sophisticated patterns in Perl and PCRE (regex library used e.g. (Scrape the rendered version, not the markdown, for actual is diagram of finite state machine for above regexp which is more clear than regexp itself The rest of it appears to be consistent with the RFC 5322 grammar and passes several tests using grep -Po, including cases domain names, IP addresses, bad ones, and account names with and without quotes.Ĭorrecting the 00 bug in the IP pattern, we obtain a working and fairly fast regex.

One RFC 5322 compliant regex can be found at the top of the page at but uses the IP address pattern that is floating around the internet with a bug that allows 00 for any of the unsigned byte decimal values in a dot-delimited address, which is illegal. RFC 5322 leads to a regex that can be understood if studied for a few minutes and is efficient enough for actual use. Fortunately, RFC 822 was superseded twice and the current specification for email addresses is RFC 5322. The fully RFC 822 compliant regex is inefficient and obscure because of its length.

Notepad++ regular expression ip address