LUA UTF8 String functions

gistfile1.lua

-- Provides UTF-8 aware string functions implemented in pure lua:
-- * string.utf8len(s)
-- * string.utf8sub(s, i, j)
--
-- All functions behave as their non UTF-8 aware counterparts with the exception
-- that UTF-8 characters are used instead of bytes for all units.
--
-- Note: all validations had been removed due to awesome usage specifics.
--[[
Copyright (c) 2006-2007, Kyle Smith
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-- ABNF from RFC 3629
--
-- UTF8-octets = *( UTF8-char )
-- UTF8-char = UTF8-1 / UTF8-2 / UTF8-3 / UTF8-4
-- UTF8-1 = %x00-7F
-- UTF8-2 = %xC2-DF UTF8-tail
-- UTF8-3 = %xE0 %xA0-BF UTF8-tail / %xE1-EC 2( UTF8-tail ) /
-- %xED %x80-9F UTF8-tail / %xEE-EF 2( UTF8-tail )
-- UTF8-4 = %xF0 %x90-BF 2( UTF8-tail ) / %xF1-F3 3( UTF8-tail ) /
-- %xF4 %x80-8F 2( UTF8-tail )
-- UTF8-tail = %x80-BF
--

-- returns the number of bytes used by the UTF-8 character at byte i in s
-- also doubles as a UTF-8 character validator
function utf8charbytes (s, i)
    -- argument defaults
    i = i or 1
    local c = string.byte(s, i)

    -- determine bytes needed for character, based on RFC 3629
    if c > 0 and c <= 127 then
        -- UTF8-1
        return 1
    elseif c >= 194 and c <= 223 then
        -- UTF8-2
        local c2 = string.byte(s, i + 1)
        return 2
    elseif c >= 224 and c <= 239 then
        -- UTF8-3
        local c2 = s:byte(i + 1)
        local c3 = s:byte(i + 2)
        return 3
    elseif c >= 240 and c <= 244 then
        -- UTF8-4
        local c2 = s:byte(i + 1)
        local c3 = s:byte(i + 2)
        local c4 = s:byte(i + 3)
        return 4
    end
end

-- returns the number of characters in a UTF-8 string
function utf8len (s)
    local pos = 1
    local bytes = string.len(s)
    local len = 0

    while pos <= bytes and len ~= chars do
        local c = string.byte(s,pos)
        len = len + 1

        pos = pos + utf8charbytes(s, pos)
    end

    if chars ~= nil then
        return pos - 1
    end

    return len
end

-- functions identically to string.sub except that i and j are UTF-8 characters
-- instead of bytes
function utf8sub (s, i, j)
    j = j or -1

    if i == nil then
        return ""
    end

    local pos = 1
    local bytes = string.len(s)
    local len = 0

    -- only set l if i or j is negative
    local l = (i >= 0 and j >= 0) or utf8len(s)
    local startChar = (i >= 0) and i or l + i + 1
    local endChar = (j >= 0) and j or l + j + 1

    -- can't have start before end!
    if startChar > endChar then
        return ""
    end

    -- byte offsets to pass to string.sub
    local startByte, endByte = 1, bytes

    while pos <= bytes do
        len = len + 1

        if len == startChar then
            startByte = pos
        end

        pos = pos + utf8charbytes(s, pos)

        if len == endChar then
            endByte = pos - 1
            break
        end
    end

    return string.sub(s, startByte, endByte)
end

-- replace UTF-8 characters based on a mapping table
function utf8replace (s, mapping)
    local pos = 1
    local bytes = string.len(s)
    local charbytes
    local newstr = ""

    while pos <= bytes do
        charbytes = utf8charbytes(s, pos)
        local c = string.sub(s, pos, pos + charbytes - 1)
        newstr = newstr .. (mapping[c] or c)
        pos = pos + charbytes
    end

    return newstr
end