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

Minor disagreement with the other solution. I'd say

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

The reason is that this (correctly) captures the individual RGB components. The other expression broke #112233 in three parts, '#' 112 233. The syntax is actually '#' (RR GG BB) | (R G B)

The slight disadvantage is more backtracking is required. When parsing #CCC you don't know that the second C is the green component until you hit the end of the string; when parsing #CCCCCC you don't know that the second C is still part of the red component until you see the 4th C.