/^#[0-9A-F]{6}$/i.test('#AABBCC')

To elaborate:

^ -> match beginning

# -> a hash

[0-9A-F] -> any integer from 0 to 9 and any letter from A to F

{6} -> the previous group appears exactly 6 times

$ -> match end

i -> ignore case

If you need support for 3-character HEX codes, use the following:

/^#([0-9A-F]{3}){1,2}$/i.test('#ABC')

The only difference here is that

 [0-9A-F]{6}

is replaced with

([0-9A-F]{3}){1,2}

This means that instead of matching exactly 6 characters, it will match exactly 3 characters, but only 1 or 2 times. Allowing ABC and AABBCC, but not ABCD

Combined solution :

var reg=/^#([0-9a-f]{3}){1,2}$/i;

console.log(reg.test('#ABC')); //true

console.log(reg.test('#AABBCC')); //true