Validate FQDN in C#

Question

Does anyone have a Regular Expression to validate legal FQDN?

Now, I use on this regex:

(?=^.{1,254}$)(^(?:(?!\d+\.|-)[a-zA-Z0-9_\-]{1,63}(?!-)\.?)+(?:[a-zA-Z]{2,})$)

However this regex results in "aa.a" not being valid while "aa.aa" is valid.

Does anyone know why?

Answer 1

Here's a shorter pattern:

(?=^.{1,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{2,})$)

As for why the pattern determines "aa.a" as invalid and "aa.aa" as valid, it's because of the {2,} - if you change the 2 to a 1 so that it's

(?=^.{1,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{1,})$)

it should deem both "aa.a" and "aa.aa" as valid.

string pattern = @"(?=^.{1,254}$)(^(?:(?!\d+\.)[a-zA-Z0-9_\-]{1,63}\.?)+(?:[a-zA-Z]{1,})$)";
bool isMatch = Regex.IsMatch("aa.a", pattern);

isMatch is TRUE for me.

Answer 2

I think this could also be an option especially if the FQDN will later be used along with System.Uri:

var isWellFormed = Uri.CheckHostName(stringToCheck).Equals(UriHostNameType.Dns);

Note that this code considers partially qualified domain names to be well formed.