I’ve noticed a common problem with regular expressions and Hex Characters, so I thought I’d blog about it. The most common way to regex a UUID, or SHA1 or some other hex encoded binary value is this (and I’ve seen this in Perl libraries and StackOverflow answers). [a-f0-9] or [A-F0-9] Neither of these are correct as Hex is case insensitive and both of these regex’s are. Hex is most commonly lowercase (unless you’re Data::UUID), but that’s an aesthetic, not a requirement.