How to identify a given string is hex color format

Question

I\'m looking for a regular expression to validate hex colors in ASP.NET C# and
am also looking code for validation on server side.

For instance: #CCCCCC

Answer 1

Based on MSalters' answer, but preventing an incorrect match, the following works

^#(([0-9a-fA-F]{2}){3}|([0-9a-fA-F]){3})$

Or for an optional hash # symbol:

^#?(([0-9a-fA-F]{2}){3}|([0-9a-fA-F]){3})$

And without back references being generated:

^#?(?:(?:[0-9a-fA-F]{2}){3}|(?:[0-9a-fA-F]){3})$