What RegEx will match all loopback addresses?

Question

I have an application which must warn the user upon use of localhost, 127.0.0.1, ::1, or any loopback address (the target host is used for a database-to-database connection not necessarily inside of the environment of the application). This is made complicated because addresses like the following...

• 127.1
• 127.0.01
• 127.0000.0000.1
• 127.0.0.254
• 127.63.31.15
• 127.255.255.254
• 0::1
• 0000::0001
• 0000:0:0000::01
• 0000:0000:0000:0000:0000:0000:0000:0001

...will parse properly by the consuming code, and will resolve to loopback.

What is a regular expression which will match any permutation of the IPv4 and IPv6 loopback addresses?

Answer 1

After a short time fiddling around in RegexBuddy (which is a truly magnificent tool for test-driven RegEx construction), I have come up with this:

^localhost$|^127(?:\.[0-9]+){0,2}\.[0-9]+$|^(?:0*\:)*?:?0*1$

This RegEx matches

The string "localhost"

• localhost
• LOCALHOST

These permutations of the IPv4 loopback address

• 127.0.0.1
• 127.0.0.001
• 127.0.00.1
• 127.00.0.1
• 127.000.000.001
• 127.0000.0000.1
• 127.0.01
• 127.1
• 127.001
• 127.0.0.254
• 127.63.31.15
• 127.255.255.254

These permutations of the IPv6 loopback address

• 0:0:0:0:0:0:0:1
• 0000:0000:0000:0000:0000:0000:0000:0001
• ::1
• 0::1
• 0:0:0::1
• 0000::0001
• 0000:0:0000::0001
• 0000:0:0000::1
• 0::0:1

This RegEx does not match

A valid server name

• servername
• subdomain.domain.tld

These valid IPv4 addresses

• 192.168.0.1
• 10.1.1.123

These valid IPv6 addresses

• 0001::1
• dead:beef::1
• ::dead:beef:1