using System;

using System.Text.RegularExpressions;

​

public class Example

{

    public static void Main()

    {

        string pattern = @"^#([A-F0-9]{3}){1,2}$";

        string input = @"Only matches when there are 3 & 6 digits

#

#1

#12

#123

#1234

#12345

#123456

#1234567

Only matches if the letters used are: A, B, C, D, E & F

#ABCDEF

#BCDEFG

#0A1B2C

#0E1F2Z

Must use the 'i' flag to match lower & upper case.

#AaBbCc

​

Arbitrary examples

#15F

#C88

#9264FA

#fF0eE1

#FFFXFF

#FFFYFF

#ABCDEF

#ABC*EF

#778657

#E9B3C4";

        RegexOptions options = RegexOptions.IgnoreCase | RegexOptions.Multiline;

        foreach (Match m in Regex.Matches(input, pattern, options))

        {

            Console.WriteLine("'{0}' found at index {1}.", m.Value, m.Index);

        }

    }

}

​