Regexp Matching Hex Color Syntax (and Shorten form)

/^#[0-9a-f]{3,6}$/i

would match #abc, #abcd, #abcde, #abcdef

/^#([0-9a-f]{3}|[0-9a-f]{6})$/i

would match #abc and #abcdef but not #abcd

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

would match #abc and #abcdef but not #abcd

/^#(?:[0-9a-f]{3}){1,2}$/i

would match #abc and #abcdef but not #abcd

Have a look at RegExp – MDN to learn more about regular expressions in javascript.