String Indexing

wiki

From Lua 5.1 on, yes. Thus:

getmetatable('').__index = function(str,i) return string.sub(str,i,i) end
-- demo
a='abcdef'
return a[4]      --> d

| I experience problems after this: code s:sub(2) stops to work with error "bad argument #2 to 'sub' (number expected, got string)" Tested with lua 5.2.4 /d9k

local strdefi=getmetatable('').__index
getmetatable('').__index=function(str,i) if type(i) == "number" then
    return string.sub(str,i,i)
    else return strdefi[i] end end

But what about substrings, say a[3,5]? No, that's illegal. We have to use the __call metamethod instead.

getmetatable('').__call = string.sub
-- demo
a='abcdef'
return a(3,5)    --> cde
return a(4)      --> def -- equivalent to a(4,-1)

Let's get really fancy and implement a suggestion of Luiz himself. [1]

getmetatable('').__index = function(str,i) return string.sub(str,i,i) end
getmetatable('').__call = function(str,i,j)  
  if type(i)~='table' then return string.sub(str,i,j) 
    else local t={} 
    for k,v in ipairs(i) do t[k]=string.sub(str,v,v) end
    return table.concat(t)
    end
  end
-- demo
a='abcdef'
return a[4]       --> d
return a(3,5)     --> cde 
return a{1,-4,5}  --> ace

Note: using this simple __index method you will lose the ability to call methods on strings, such as a:match('abc'). You need to modify __index as follows:

getmetatable('').__index = function(str,i)
  if type(i) == 'number' then
    return string.sub(str,i,i)
  else
    return string[i]
  end
end

If you don't like that, you can omit the redefinition of __index and use a{4} instead of a[4].

Characters versus bytes

Always remember that these indexing functions select bytes, not characters. For example, UTF-8 characters occupy a variable number of bytes: see the discussion ValidateUnicodeString.

Lua = 'Ｌｕａ'
print (Lua(1,3))    -->   Ｌ