How to validate an IPv6 address using a combination of RegEx and code?

Question

I'd like to validate an IPv6 address using an algorithm that emphasizes readability. The ideal solution combines a dead-simple regular expression with source-code.

Using https://blogs.msdn.microsoft.com/oldnewthing/20060522-08/?p=31113 as an example:

function isDottedIPv4(s)
{
  var match = s.match(/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/);
  return match != null &&
         match[1] <= 255 && match[2] <= 255 &&
         match[3] <= 255 && match[4] <= 255;
}

Notice how Raymond moves the complexity from the regular expression into code. I'd like a solution that does the same for IPv6.

Regular expression that matches valid IPv6 addresses, Javascript regex to validate IPv4 and IPv6 address, no hostnames — Tushar
– Tushar, Commented Jan 3, 2017 at 3:28
@Tushar I solution you linked to is exactly the opposite of what I am looking for. If you read the link I provided you will see that I am asking to move as much of the complexity out of the regex into source-code. — Gili
– Gili, Commented Jan 3, 2017 at 3:30
@Tushar Please... read the link I provided. It's very explicit about what I mean. Essentially I want a regex that only contains capturing groups with minimal validation, and a separate piece of code that validates the capturing groups. — Gili
– Gili, Commented Jan 3, 2017 at 3:34

Community · Accepted Answer · 2017-05-23 12:02:36Z

5

Here is a variant of Brandon's answer:

/**
 * @param {String} a String
 * @return {Boolean} true if the String is a valid IPv6 address; false otherwise
 */
function isIPv6(value)
{
  // See https://blogs.msdn.microsoft.com/oldnewthing/20060522-08/?p=31113 and
  // https://4sysops.com/archives/ipv6-tutorial-part-4-ipv6-address-syntax/
  const components = value.split(":");
  if (components.length < 2 || components.length > 8)
    return false;
  if (components[0] !== "" || components[1] !== "")
  {
    // Address does not begin with a zero compression ("::")
    if (!components[0].match(/^[\da-f]{1,4}/i))
    {
      // Component must contain 1-4 hex characters
      return false;
    }
  }

  let numberOfZeroCompressions = 0;
  for (let i = 1; i < components.length; ++i)
  {
    if (components[i] === "")
    {
      // We're inside a zero compression ("::")
      ++numberOfZeroCompressions;
      if (numberOfZeroCompressions > 1)
      {
        // Zero compression can only occur once in an address
        return false;
      }
      continue;
    }
    if (!components[i].match(/^[\da-f]{1,4}/i))
    {
      // Component must contain 1-4 hex characters
      return false;
    }
  }
  return true;
}


console.log('Expecting true...');
console.log(isIPv6('2001:cdba:0000:0000:0000:0000:3257:9652'));
console.log(isIPv6('2001:cdba:0:0:0:0:3257:9652'));
console.log(isIPv6('2001:cdba::3257:9652'));
console.log(isIPv6('2001:cdba::257:9652'));
console.log(isIPv6('2001:DB8:0:2F3B:2AA:FF:FE28:9C5A'));
console.log(isIPv6('::0:2F3B:2AA:FF:FE28:9C5A'));
console.log('\n');
console.log('Expecting false...');
console.log(isIPv6(':0:2F3B:2AA:FF:FE28:9C5A'));

edited May 23, 2017 at 12:02

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answered Jan 3, 2017 at 5:35

Gili

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2 Comments

wikijames Over a year ago

if (!components[i].match(/^[\da-f]{1,4}/i)) { // Component must contain 1-4 hex characters return false; } when this last condition will pass ?

wikijames Over a year ago

in testcase, i am not able to cover the the condition.

Brandon · Accepted Answer · 2017-01-03 18:56:39Z

1

This still may be too complex, but I think it covers most scenarios with IPv6 addresses. I went through something similar recently, it is really hard to replace a huge RegEx for something as complex as IPv6.

function isIPv6(s)
{
    // Check if there are more then 2 : together (ex. :::)
  if(/:{3,}/.test(s)) return false;
    // Check if there are more then 2 :: (ex. ::2001::)
  if(/::.+::/.test(s)) return false;
    // Check if there is a single : at the end (requires :: if any)
  if(/[^:]:$/.test(s)) return false;
    // Check for leading colon
  if(/^:(?!:)/.test(s)) return false;
    // Split all the part to check each
  var ipv6_parts = s.split(':');
    // Make sure there are at lease 2 parts and no more then 8
  if(ipv6_parts.length < 2 || ipv6_parts.length > 8) return false;
	
  var is_valid = true;
    // Loop through the parts
  ipv6_parts.forEach(function(part) {
      // If the part is not blank (ex. ::) it must have no more than 4 digits
    if(/^[0-9a-fA-F]{0,4}$/.test(part)) return;
      // Fail if none of the above match
    is_valid = false;
  });
  
  return is_valid;
}

console.log(isIPv6('2001:cdba:0000:0000:0000:0000:3257:9652'));
console.log(isIPv6('2001:cdba:0:0:0:0:3257:9652'));
console.log(isIPv6('2001:cdba::3257:9652'));
console.log(isIPv6('::2001:cdba:3257:9652'));

edited Jan 3, 2017 at 18:56

answered Jan 3, 2017 at 5:19

Brandon

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5 Comments

Gili Over a year ago

That's clever, but I think that last check should allow 1-4 digits. See "Leading zero suppression" at 4sysops.com/archives/ipv6-tutorial-part-4-ipv6-address-syntax ... then I believe you can remove if (part === 0) because it's now implied by the 1-4 digits check.

Brandon Over a year ago

Added support for leading 0 check. The (part === 0) allows for :0: which is valid IPv6

Gili Over a year ago

If you changed /[0-9a-fA-F]{4}/.test(part) to /[0-9a-fA-F]{1,4}/.test(part) then you could remove the part === 0 check. Further, currently your code rejects 2001:cdba::257:9652 which I believe is legal. Lastly, the result of your forEach loop is being ignored. If it returns false, your outer function still returns true.

Brandon Over a year ago

I completely missed the return false in the forEach. I have updated to account for all mentioned scenarios, I think :)

Gili Over a year ago

Looks good overall. I'll add any remaining concerns to your testcases.

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