Question

For example:

AA33FF = valid hex color

Z34FF9 = invalid hex color (has Z in it)

AA33FF11 = invalid hex color (has extra characters)

Answer 1

Without transparent support:

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

With transparent support :

/^#[0-9A-F]{6}[0-9a-f]{0,2}$/i.test('#AABBCC80')

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
[0-9a-f]{0,2} -> adding support for transparent ( 00..FF)
$ -> match end
i -> ignore case

If you need support for 3-character HEX codes (no transparent supported) , 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