How To Check If Color Hex Value Valid On Keyup

I try to make a script which check's input's value every time when value changes. There must be ready example somewhere, but I just could not found it. The value shoud be hex-color-value (like #FFFFFF), so script should check if the first character is # and after that only characters a-f, A-F and 0-9 are ok. I guess it goes something like this:

function checkHex(value) {
// if value string includes only characters # a-f A-F 0-9 value is ok
// else value is not ok
}
<input type="text" id="hex_input" onkeyup="checkHex(this)" />