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Native UTF8 strings with Luart

By Samir Tine, published on December 2021

Lua strings are not strings !

Yes, you read that right. It could have been worthy of a shakespear play but when it comes to strings in Lua, it can turn into a nightmare if you don't pay attention with the true definition of what a string is...

Wait, what do you mean by strings are not strings ?

In Lua, strings could have been called buffers, arrays, or even containers. Indeed strings are only containers. But it does not contain characters. In fact, Lua has no idea what a character is.

So what a string contains ?

You may then wonder what the strings contain: they simply contain bytes. Strings can therefore, in Lua, contain lots of things: an image, a digitized sound, a database ... and characters too.

This is where things get complicated: Lua considers that in strings, a single byte corresponds to a single character. This is fine as long as you are using single-byte encoded characters (as with ASCII encoding), with only 255 character possibilities.

But in the age of the Internet, when the whole world communicates in all languages, that seems rather restrictive ! Fortunately other encodings than ASCII exist, to extend the number of usable characters: UTF8, UCS 2 LE, UCS 2 BE,... They allow to encode a character over several bytes.

Multibytes characters with standard Lua

Multibytes characters can be stored in Lua strings after all, as strings in Lua contains bytes. Yes, that's right. But it does not mean you can use them !

One rule to rule them all

All Lua strings functionnalities (concatenation, length calculation, string.find, string.gmatch, string.sub...) consider that strings contain only single byte characters : the same rule again!

Here is an example that illustrates the problem when using standard Lua (the script must have been saved with UTF8 encoding +/- BOM)

local summer_infrench = "t"

-- outputs 5 !?

-- pos = 3 !?
pos = string.find(summer_infrench, "t"))

What's going on ?

Remember the rule : strings are considered as bytes containers. The UTF8 string "t" (means "summer" in French) is 3 characters long, but occupies 5 bytes in memory :

t = 3 characters
0xC3 0xA9 0x75 0xC3 0xA9 = 5 bytes

That's why the function string.len returns 5 and not 3. The same for string.find : The byte position of the "t" character is 3.

Is there any workaround ?

Hopefully, yes there is one. Since Lua 5.3 a new module "utf8" is available to help developers with UTF8 encoded strings. But this greatly complicates the use of UTF8 strings, as it uses specific functions. A kind of overlay over strings. Not very friendly : in other modern programming languages, strings are containers for characters and support natively multibytes encodings.

Here is the previous example using the "utf8" module :

local utf8 = require "utf8"
local summer_infrench = "t"
-- yes ! outputs 3 !

-- Still pos = 3, no solution for string.find with UTF8 strings
pos = string.find(summer_infrench, "t"))

But as you can see, this module is no help when using most of the strings functions.

If you want to use UTF8 strings with standard Lua, you will have to use a binary module dependency.

It's in Lua philosophy : if Lua lacks something, implement it using binary modules or Lua modules. Search on the net and you will find some of them. But again, for such a simple functionality, this represents a certain degree of complication especially for beginners.

Luart natively support multibytes character strings

For all of these reasons, I decided to natively implement UTF8 strings in Luart.

The string module in Luart provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. That means that all standard Lua strings functions can be used with UTF8 encoded strings.

Here is the previous example with Luart :

 local utf8 = require "utf8"
 local summer_infrench = "t"
 -- yes ! outputs 3 !
 -- yes ! Position of letter "t" is 2 !
 pos = string.find(summer_infrench, "t"))

When you use strings to store binary data, DO NOT use functions from the string module, as they are expecting UTF8 characters, not raw bytes. You should use a Buffer object to store and manipulate binary data with Luart.