How to embed hex values in a lua string literal (i.e. \x equivalent)

Question

In various languages, you can embed hex values in a string literal by using the \x escape sequence:

"hello \x77\x6f\x72\x6c\x64"

How can I do the same thing in Lua 5.1?

Solution 1

Since Lua 3.1, you can use decimal escapes in strings liberals.

Starting with Lua 5.2, you can use hex escapes in string literals.

In Lua 5.1, you can convert hex escapes a posteriori:

s=[[hello \x77\x6f\x72\x6c\x64]]
s=s:gsub("\\x(%x%x)",function (x) return string.char(tonumber(x,16)) end)
print(s)

Note the use of long strings, which do not interpret escape sequences. If you use short strings (in quotes) as in your original code, then \x will be silently converted to x, because Lua 5.1 does not understand \x. Lua 5.2 and later complains about escape sequences that it does not understand.

Solution 2

(From the Lua 5.1 reference)

You can embed decimal values in a string literal in Lua by using the \ddd escape sequence, where ddd is a sequence of up to three decimal digits. For example:

"\72ell\111" is the same as "hello"

Comments

[Paul Kulchenko]

If you are using decimal value on strings that mix control and regular characters, I suggest you always use 3 positions (and not up-to-3) or convert all characters, otherwise you are risking turning "\n3" into "\103", which is not the same as "\0103"

[Amr Bekhit]

Agreed, I was surprised when I saw the "up to three" statement in the documentation and felt it was nothing but trouble!

[Amr Bekhit]

I like this answer because you explain how to go about doing exactly what I asked, even though the language doesn't support it be default. I think if you can reference the existing ability to embed decimal values in string literals (i.e. combine my answer and yours), then I'll accept this answer.

[Nas Banov]

any idea why this seems to work for me in 5.1?! print(_VERSION, '\xff') >>> Lua 5.1 \255

[lhf]

@NasBanov, perhaps you're running LuaJIT instead of Lua from lua.org.

[Nas Banov]

@lhf you got it! I was trying in ZeroBrane Studio console and when not debugging it is indeed LuaJIT (has jit table). When i debug at a breakpoint though, no more JIT and the output is "Lua 5.1" "xff" - perfect match!