Verifies whether each input line contains a valid three- or six-digit hexadecimal number, commonly used as color codes in CSS and other technologies. Each number must begin with the "#" sign.

I've seen other regexes for this which use individual expressions for a three-digit number and a six-digit number as alternates for each other. I didn't see any reason to repeat the character class twice, so I used only one pattern of three digits, which may appear one or two times.

Valid Hex Color Codes

#FFF
#025
#F0A1FB

Invalid Hex Color Codes

#fffabg
#abcf
#12365erff