Extract the first letter of a UTF-8 string with Lua

Question

Is there any way to extract the first letter of a UTF-8 encoded string with Lua?

Lua does not properly support Unicode, so string.sub("ÆØÅ", 2, 2) will return "?" rather than "Ø".

Is there a relatively simple UTF-8 parsing algorithm I could use on the string byte per byte, for the sole purpose of getting the first letter of the string, be it a Chinese character or an A?

Or is this way too complex, requiring a huge library, etc.?

"*simple Unicode parsing algorithm*" What *kind* of "Unicode" is this? Is it UTF-8, UTF-16, something else? What's the encoding? — Nicol Bolas, Nov 05 '12 at 16:07
http://www.joelonsoftware.com/articles/Unicode.html Read this. Please. — Cubic, Nov 05 '12 at 16:18
Here's also a [page for Lua users](http://lua-users.org/wiki/LuaUnicode) — Dmitry Ledentsov, Nov 05 '12 at 20:43

score 18 · Accepted Answer · answered Nov 05 '12 at 18:39

18

You can easily extract the first letter from a UTF-8 encoded string with the following code:

function firstLetter(str)
  return str:match("[%z\1-\127\194-\244][\128-\191]*")
end

Because a UTF-8 code point either begins with a byte from 0 to 127, or with a byte from 194 to 244 followed by one or several bytes from 128 to 191.

You can even iterate over UTF-8 code points in a similar manner:

for code in str:gmatch("[%z\1-\127\194-\244][\128-\191]*") do
  print(code)
end

Note that both examples return a string value for each letter, and not the Unicode code point numerical value.

answered Nov 05 '12 at 18:39

prapin

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Brilliant! This was exactly the answer I was looking for. Short and precise. – forthrin Nov 05 '12 at 19:17
This is reasonable for data that's already been validated but you might want to be careful with data which hasn't been. – bames53 Nov 05 '12 at 19:20

score 4 · Answer 2 · answered Jan 14 '15 at 11:12

Lua 5.3 provide a UTF-8 library.

You can use utf8.codes to get each code point, and then use utf8.char to get the character:

local str = "ÆØÅ"
for _, c in utf8.codes(str) do
  print(utf8.char(c))
end

This also works:

local str = "ÆØÅ"
for w in str:gmatch(utf8.charpattern ) do
  print(w)
end

where utf8.charpattern is just the string "[\0-\x7F\xC2-\xF4][\x80-\xBF]*" for the pattern to match one UTF-8 byte sequence.

Extract the first letter of a UTF-8 string with Lua

2 Answers2

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