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Editorial Team
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Asked: 2026-05-27T13:10:05+00:00

I have an application which must warn the user upon use of localhost ,

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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?

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1 Answer

  1. Editorial Team

    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
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