Bash regex to detect IPv6 or none [duplicate] - regex

Question

This question already has answers here:
Regular expression that matches valid IPv6 addresses
(30 answers)
Closed 9 years ago.
How would I modify this IPv6 regex I wrote to either detect the address (ie the way the regex is written right now), but also accept "blank" ie the user did not specify an IPv6 address?
^[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}$
Right now, the regex is looking for a minimum of 0:0:0:0:0:0:0:0 or similar. Infact in addition to a blank address, I probably need to also be able to handle compression such as the following address:
FE80::1
or ::1
etc
Thanks!
* UPDATE *
So let me make sure I have this straight...
(^$|^IPV4)\|(^$|IPV6)\|REST OF STUFF$
That doesn't seem right. I feel like I have misplaced the ^ and $ and the very beginning and end of my entire regex.
Maybe this instead:
^(^$|IPV4)\|(^$|IPV6)\|REST$
* UPDATE *
Still no luck. Here is part of my code with the middles chopped out for sanity:
^(|[0-9]{1,3}.<<<OMIT MIDDLE IPV4>>>.[0-9]{1,3})\|(|(\A([0-9a-f]{1,4}:){1,1}<<<OMIT MIDDLE IPV6>>>[0-1]?\d?\d)){3}\Z))\|[a-zA-Z0-<<<MORE STUFF MIDDLE OMITTED>>>{0,50}$
I hope that isn't confusing. Thats the beginning and end of each regex with the middles omitted so you can see the ( ).
Perhaps I need to enclose the entire gigantic IPV6 regex in parenthesis?
* UPDATE *
Tried last statement above... no luck.

Answer 1

You can specify alternation with the | character, so a|b means "match either a or b". In this case it would look something like this:
^$|^[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}:[0-9a-fA-F]{1,4}$
The regex ^$ will match empty strings, so ^$|<current-regex> means "match either an empty string, or whatever <current-regex> matches (in this case IPv6)". You could use ^\s*$ in place of ^$ if you want strings that only consist of whitespace character to also be considered "empty".
This just handles the first part of the question, handling the compression like FE80::1 is more complex and it looks like there are already some other good answers for that in comments (note that I don't think this question is a dupe, because the "also matching an empty string" part isn't present in those questions).
edit: If it is part of a larger regex, then you should wrap everything in a group and get rid of the ^$, so it would be something like (|<current-regex>). Since there is nothing before the |, it means that the group can match either empty strings or whatever your current regex would match.

Answer 2

According to this post on this site called Stack Overflow this other site has an explanation & example of a huge—but very usable—regex which is this:
(\A([0-9a-f]{1,4}:){1,1}(:[0-9a-f]{1,4}){1,6}\Z)|
(\A([0-9a-f]{1,4}:){1,2}(:[0-9a-f]{1,4}){1,5}\Z)|
(\A([0-9a-f]{1,4}:){1,3}(:[0-9a-f]{1,4}){1,4}\Z)|
(\A([0-9a-f]{1,4}:){1,4}(:[0-9a-f]{1,4}){1,3}\Z)|
(\A([0-9a-f]{1,4}:){1,5}(:[0-9a-f]{1,4}){1,2}\Z)|
(\A([0-9a-f]{1,4}:){1,6}(:[0-9a-f]{1,4}){1,1}\Z)|
(\A(([0-9a-f]{1,4}:){1,7}|:):\Z)|
(\A:(:[0-9a-f]{1,4}){1,7}\Z)|
(\A((([0-9a-f]{1,4}:){6})(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3})\Z)|
(\A(([0-9a-f]{1,4}:){5}[0-9a-f]{1,4}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3})\Z)|
(\A([0-9a-f]{1,4}:){5}:[0-9a-f]{1,4}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A([0-9a-f]{1,4}:){1,1}(:[0-9a-f]{1,4}){1,4}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A([0-9a-f]{1,4}:){1,2}(:[0-9a-f]{1,4}){1,3}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A([0-9a-f]{1,4}:){1,3}(:[0-9a-f]{1,4}){1,2}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A([0-9a-f]{1,4}:){1,4}(:[0-9a-f]{1,4}){1,1}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A(([0-9a-f]{1,4}:){1,5}|:):(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)|
(\A:(:[0-9a-f]{1,4}){1,5}:(25[0-5]|2[0-4]\d|[0-1]?\d?\d)(\.(25[0-5]|2[0-4]\d|[0-1]?\d?\d)){3}\Z)