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Regex: IPv6 address

An IPv6 address is eight groups of up to four hex digits, and one run of zero groups can be written as ::. Because :: can stand anywhere, the pattern lists every shape the address can take: nothing shortened, :: at the end, at the start, or between groups.

Open in Regex Tester The pattern and every example below are filled in.

Pattern

^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$

How it works

^(
Start of the string, and of the list of shapes.
([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|
Eight groups, nothing shortened.
([0-9a-fA-F]{1,4}:){1,7}:|
One to seven groups, then :: at the end (fe80::).
([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|
One to six groups, ::, one group (2001:db8::1).
([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|
One to five groups, ::, one or two groups.
([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|
One to four groups, ::, one to three groups.
([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|
One to three groups, ::, one to four groups.
([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|
One or two groups, ::, one to five groups.
[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|
One group, ::, one to six groups.
:((:[0-9a-fA-F]{1,4}){1,7}|:)
Starting with ::: up to seven groups after it (::1), or nothing at all (::).
)$
End of the list of shapes, and of the string.

Matches

  • 2001:0db8:85a3:0000:0000:8a2e:0370:7334
  • 2001:db8::1
  • ::1
  • ::
  • fe80::

Doesn't match

  • 2001:db8::1::2
  • 12345::1
  • 192.168.1.1
  • 2001:db8:g::1

In your language

JavaScript
/^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$/

A literal; new RegExp(source, flags) builds the same from a string.

Python
re.compile(r"^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$")

A raw string, so backslashes reach re as written. re.ASCII keeps \d to 0-9, as in JavaScript (Python matches any Unicode digit otherwise). Use re.fullmatch to test a whole string.

Java
Pattern.compile("^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$")

A normal string literal, so every backslash is doubled. matcher(s).matches() tests the whole string.

Go
regexp.MustCompile(`^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$`)

A raw string in backticks. RE2 runs in linear time but has no lookaround and no backreferences.

PHP
preg_match('/^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$/D', $input)

PCRE with / delimiters inside a single-quoted string. Without the D modifier, $ also matches before a final newline, so it is added to patterns that end in $.

C#
new Regex(@"^(([0-9a-fA-F]{1,4}:){7}[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,7}:|([0-9a-fA-F]{1,4}:){1,6}:[0-9a-fA-F]{1,4}|([0-9a-fA-F]{1,4}:){1,5}(:[0-9a-fA-F]{1,4}){1,2}|([0-9a-fA-F]{1,4}:){1,4}(:[0-9a-fA-F]{1,4}){1,3}|([0-9a-fA-F]{1,4}:){1,3}(:[0-9a-fA-F]{1,4}){1,4}|([0-9a-fA-F]{1,4}:){1,2}(:[0-9a-fA-F]{1,4}){1,5}|[0-9a-fA-F]{1,4}:(:[0-9a-fA-F]{1,4}){1,6}|:((:[0-9a-fA-F]{1,4}){1,7}|:))$")

A verbatim string: backslashes stay, a quote is doubled. RegexOptions.ECMAScript keeps \d to 0-9, as in JavaScript. $ also matches before a final newline; to reject one, end the pattern with \z instead.

Common mistakes

  • Zone ids and embedded IPv4

    fe80::1%eth0 (a link-local address with its interface) and ::ffff:192.0.2.1 (an IPv4-mapped address) are valid but fail here. Add those shapes only if your input contains them.

  • Prefer an address parser

    Python ipaddress.ip_address, Go netip.ParseAddr and .NET IPAddress.TryParse handle every form and give you an address you can compare.

  • Compare in canonical form

    2001:DB8:0:0::1 and 2001:db8::1 are the same address. RFC 5952 defines one text form (lower case, the longest run of zeros shortened); normalize before comparing or storing.