I am trying to create an LPeg pattern that would match any Unicode punctuation inside UTF-8 encoded input. I came up with the following marriage of Selene Unicode and LPeg:
local unicode = require("unicode")
local lpeg = require("lpeg")
local punctuation = lpeg.Cmt(lpeg.Cs(any * any^-3), function(s,i,a)
local match = unicode.utf8.match(a, "^%p")
if match == nil
return false
else
return i+#match
end
end)
This appears to work, but it will miss punctuation characters that are a combination of several Unicode codepoints (if such characters exist), as I am reading only 4 bytes ahead, it probably kills the performance of the parser, and it is undefined what the library match
function will do, when I feed it a string that contains a runt UTF-8 character (although it appears to work now).
I would like to know whether this is a correct approach or if there is a better way to achieve what I am trying to achieve.
The correct way to match UTF-8 characters is shown in an example in the LPeg homepage. The first byte of a UTF-8 character determines how many more bytes are a part of it:
Building on this
utf8
pattern we can uselpeg.Cmt
and the Selene Unicodematch
function kind of like you proposed:Note that we return
i
, this is in accordance with whatCmt
expects:This means we should return the same number the function receives, that is the position immediately after the UTF-8 character.