things that are much harder to describe accurately with a regex than you'd think, an incomplete list:
* floating point numbers
* IPv6 addresses
* IPv*4* addresses (depending on how you define them and how picky you are about the numeric ranges)
* ...
curse you for making me type test cases like this into a REPL
Zack Weinberg
Stylus>>> socket.gethostbyname("10.010.0x10")
'10.8.0.16'
but heck even IBM is guilty of grossly oversimplifying things https://www.ibm.com/docs/en/ts4500-tape-library?topic=functionality-ipv4-ipv6-address-formats
Though it appears that the exact syntax of numeric IPv4 addresses is "whatever inet_aton does" and has never been well specified in a published rfc? https://datatracker.ietf.org/doc/html/draft-main-ipaddr-text-rep-02