★ Web · regex

Regex for IPv6 Address

Covers the eight-group form and every legal placement of the :: shorthand. It does not cover zone IDs (%eth0) or IPv4-embedded forms like ::ffff:192.0.2.1.

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Try the ipv6 address pattern

Read it in plain English (best-effort)

^start of a line
(?:start a group (not captured)
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
{7}exactly 7 of the previous
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
|or
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
{1,7}between 1 and 7 of the previous
:the character ':'
|or
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
{1,6}between 1 and 6 of the previous
:the character ':'
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
|or
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
{1,5}between 1 and 5 of the previous
(?:start a group (not captured)
:the character ':'
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
)end of group
{1,2}between 1 and 2 of the previous
|or
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
{1,4}between 1 and 4 of the previous
(?:start a group (not captured)
:the character ':'
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
)end of group
{1,3}between 1 and 3 of the previous
|or
(?:start a group (not captured)
[0-9A-Fa-f]one character from: 0–9, A–F, a–f
{1,4}between 1 and 4 of the previous
:the character ':'
)end of group
+44 more

Test text

One sample per line works well with the m flag on anchored patterns.

Waiting for a pattern

The pattern, piece by piece

/^(?:(?:[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}|:))$/

Pattern explanation
Part	What it does
`[0-9A-Fa-f]{1,4}`	one group of 1–4 hex digits
`(?:group:){7}group`	the full eight-group form
`…:(?::group){n}`	each alternative places the :: gap in a different position
`:(?:(?::group){1,7}\|:)`	addresses starting with ::, including :: on its own

What it accepts and rejects

Every sample below is checked automatically in JavaScript and Python. The tester above is preloaded with all of them, one per line.

Should match
Should match
`2001:0db8:85a3:0000:0000:8a2e:0370:7334`	✓
`2001:db8::8a2e:370:7334`	✓
`::1`	✓
`::`	✓
`fe80::1`	✓
`2001:db8::`	✓

Should not match
Should not match
`2001:db8::85a3::7334`	✗
`1:2:3:4:5:6:7:8:9`	✗
`12345::1`	✗
`gggg::1`	✗
`192.168.1.1`	✗
`:1:2:3:4:5:6:7`	✗

Copy it into your code

RE2-safeNo lookarounds or backreferences, so the same pattern works in Go's RE2 engine and in Rust's regex crate.

Language

const ipv6Re = /^(?:(?:[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}|:))$/;

export function isValidIpv6(value) {
  return ipv6Re.test(value.trim());
}

When not to use regex for this

Parse IPv6 with your language's IP library (net/netip in Go, ipaddress in Python, a library in JavaScript). It handles zone IDs, embedded IPv4 and canonical formatting, and the code reads better than this pattern.

Questions engineers actually ask

Why is the IPv6 regex so long?

The :: shorthand can replace a run of zero groups anywhere, and each position needs its own alternative so that :: appears at most once.

Does it accept ::ffff:192.168.1.1?

No. IPv4-mapped addresses need an extra branch; use an IP parsing library if you expect them.

What is catastrophic backtracking?

Some patterns, usually nested quantifiers like (a+)+, make a backtracking engine try an exponential number of paths on input that almost matches. The tester stops any run after 300ms, so a bad pattern can't freeze your tab. In production, that same pattern can take down a server (a ReDoS attack).

More web patterns

Or browse the full regex library.

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Fine print

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Engineer: by the hour.

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