CODEWITHSUNDEEP
ruby
Peter Black
Peter Black

Reputation: 1172

In ruby, how do I turn a text representation of a byte in to a byte?

What is the best way to turn the string "FA" into /xFA/ ? To be clear, I don't want to turn "FA" into 7065 or "FA".to_i(16).

In Java the equivalent would be this:

byte b = (byte) Integer.decode("0xFA");

Upvotes: 4

Views: 325

Answers (1)

jrochkind
jrochkind

Reputation: 23347

So you're using / markers, but you aren't actually asking about regexps, right?

I think this does what you want:

['FA'].pack('H*')
# => "\xFA"

There is no actual byte type in ruby stdlib (I don't think? unless there's one I don't know about?), just Strings, that can be any number of bytes long (in this case, one). A single "byte" is typically represented as a 1-byte long String in ruby. #bytesize on a String will always return the length in bytes. 

"\xFA".bytesize
# => 1

Your example happens not to be a valid UTF-8 character, by itself. Depending on exactly what you're doing and how you're environment is set up, your string might end up being tagged with a UTF-8 encoding by default. If you are dealing with binary data, and want to make sure the string is tagged as such, you might want to #force_encoding on it to be sure. It should NOT be neccesary when using #pack, the results should be tagged as ASCII-8BIT already (which has a synonym of BINARY, it's basically the "null encoding" used in ruby for binary data). 

['FA'].pack('H*').encoding
=> #<Encoding:ASCII-8BIT

But if you're dealing with string objects holding what's meant to be binary data, not neccesarily valid character data in any encoding, it is useful to know you may sometimes need to do str.force_encoding("ASCII-8BIT") (or force_encoding("BINARY"), same thing), to make sure your string isn't tagged as a particular text encoding, which would make ruby complain when you try to do certain operations on it if it includes invalid bytes for that encoding -- or in other cases, possibly do the wrong thing

Actually for a regexp

Okay, you actually do want a regexp. So we have to take our string we created, and embed it in a regexp. Here's one way:

representation = "FA"
str = [representation].pack("H*")
# => "\xFA"
data = "\x01\xFA\xC2".force_encoding("BINARY")
regexp = Regexp.new(str)
data =~ regexp
# => 1 (matched on byte 1; the first byte of data is byte 0)

You see how I needed the force_encoding there on the data string, otherwise ruby would default to it being a UTF-8 string (depending on ruby version and environment setup), and complain that those bytes aren't valid UTF-8.

In some cases you might need to explicitly set the regexp to handle binary data too, the docs say you can pass a second argument 'n' to Regexp.new to do that, but I've never done it.

Upvotes: 4

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