A Regular Expression for Validating An Internet Domain

Despite the power and truth of Jamie Zawinski's law:

Regular Expressions: Now You Have Two Problems Jeff Atwood's Perspective

Like Jeff, I too really, really love regular expressions or regexes. I use this one a lot and I finally learned to use \S (Any non-whitespace character) so here's a regex

^\S+\.\S+$

that I wrote yesterday to "validate" the permitted characters in an Internet domain. I was all proud of this and wrote this blog post only to realize that pride really does goeth before a fall – this will NOT correctly validate an Internet domain. As I write this post, I realize that the number of allowed characters in an Internet domain are actually NOT any non-whitespace characters and here's the proof that I actually got that wrong yesterday when I put something online using it:

Note: The fact that Rubular allows through an _ which is NOT a valid character in domains is problematic.

So the right way to do this, DAMN IT, is something like this:

^[A-Za-z0-9\-]+\.[A-Za-z0-9\-]+$

And this actually works:

The [A-Za-z0-9-]+ is a "character class" which says "Any uppercase or lowercase letter plus 0-9 plus a -" are allowed (any order, any quantity)".

Regular Expressions – Now you have two problems.