next.js/packages/next/src/server/lib/is-ipv6.ts

// Regex from `node/lib/internal/net.js`: https://github.com/nodejs/node/blob/9fc57006c27564ed7f75eee090eca86786508f51/lib/internal/net.js#L19-L29
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const v4Seg = '(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])'
const v4Str = `(${v4Seg}[.]){3}${v4Seg}`
const v6Seg = '(?:[0-9a-fA-F]{1,4})'
const IPv6Reg = new RegExp(
  '^(' +
    `(?:${v6Seg}:){7}(?:${v6Seg}|:)|` +
    `(?:${v6Seg}:){6}(?:${v4Str}|:${v6Seg}|:)|` +
    `(?:${v6Seg}:){5}(?::${v4Str}|(:${v6Seg}){1,2}|:)|` +
    `(?:${v6Seg}:){4}(?:(:${v6Seg}){0,1}:${v4Str}|(:${v6Seg}){1,3}|:)|` +
    `(?:${v6Seg}:){3}(?:(:${v6Seg}){0,2}:${v4Str}|(:${v6Seg}){1,4}|:)|` +
    `(?:${v6Seg}:){2}(?:(:${v6Seg}){0,3}:${v4Str}|(:${v6Seg}){1,5}|:)|` +
    `(?:${v6Seg}:){1}(?:(:${v6Seg}){0,4}:${v4Str}|(:${v6Seg}){1,6}|:)|` +
    `(?::((?::${v6Seg}){0,5}:${v4Str}|(?::${v6Seg}){1,7}|:))` +
    ')(%[0-9a-zA-Z-.:]{1,})?$'
)

export function isIPv6(s: string) {
  return IPv6Reg.test(s)
}