Regex for Hex Color Codes

CSS actually allows four valid hex color lengths, not just the common 6-digit form: #RGB (shorthand), #RGBA (shorthand with alpha), #RRGGBB (standard), and #RRGGBBAA (standard with alpha). This pattern's alternation covers all four — a pattern that only checks for 6 or 3 digits will reject valid 4- and 8-digit alpha colors.