How to validate that a string is a proper hexadecimal value in Ruby?

Question

I am writing a 6502 assembler in Ruby. I am looking for a way to validate hexadecimal operands in string form. I understand that the String object provides a "hex" method to return a number, but here's a problem I run into:

  "0A".hex #=> 10 - a valid hexadecimal value
  "0Z".hex #=>  0 - invalid, produces a zero
"asfd".hex #=> 10 - Why 10?  I guess it reads 'a' first and stops at 's'?

You will get some odd results by typing in a bunch of gibberish. What I need is a way to first verify that the value is a legit hex string.

I was playing around with regular expressions, and realized I can do this:

true if "0A" =~ /[A-Fa-f0-9]/
#=> true

true if "0Z" =~ /[A-Fa-f0-9]/
#=> true <-- PROBLEM

I'm not sure how to address this issue. I need to be able to verify that letters are only A-F and that if it is just numbers that is ok too.

I'm hoping to avoid spaghetti code, riddled with "if" statements. I am hoping that someone could provide a "one-liner" or some form of elegent code.

Thanks!

Answer 1

!str[/\H/] will look for invalid hex values.

Answer 2

String#hex does not interpret the whole string as hex, it extracts from the beginning of the string up to as far as it can be interpreted as hex. With "0Z", the "0" is valid hex, so it interpreted that part. With "asfd", the "a" is valid hex, so it interpreted that part.

Answer 3

One method:

str.to_i(16).to_s(16) == str.downcase

Another:

str =~ /\A[a-f0-9]+\Z/i   # or simply /\A\h+\Z/ (see hirolau's answer)

About your regex, you have to use anchors (\A for begin of string and \Z for end of string) to say that you want the full string to match. Also, the + repeats the match for one or more characters.

Note that you could use ^ (begin of line) and $ (end of line), but this would allow strings like "something\n0A" to pass.