Re: string.gsub - possible enhancement

[Mark Edgar <medgar@...>]

Shmuel Zeigerman wrote:
> I'd like to propose the following extension for future versions of 
> string.gsub (it is described below in the form of an indented insertion 
> into the Manual text):
>
> ------------------------------------------------------------
> If the value returned by the table query or by  the function
> call  is  a  string  or  a number,  then it  is used  as the
> replacement string
>     (if the function returns a second  value and  that value
>     is  true,  then  the  first  return  value  is  not used
>     directly  but rather as if it was the `repl' parameter);
> otherwise,  if  it  is  false  or  nil,  then  there  is  no
> replacement  (that  is, the  original match  is kept  in the
> string).
> ------------------------------------------------------------
>
> Such a change may be helpful for interactive use, say, in text editor 
> Replace operation, when the user specifies the `repl' string (e.g. 
> "%2%1") and wants to make (or not to make) replacements individually for 
> every match.

IMHO, the interface is generic enough as it is.  For your specific 
example of replacing the match with "%2%1", you can simply use this 
function as the repl parameter:

function repl(capture1, capture2)
  return capture2 .. capture1
end


For an arbitrarily complicated pattern, this function will do what you want:

local trynum = {}
function trynum:__index(k)
  return self[tonumber(k)]
end
function generate_replace(repl_func)
  return function(...)
    local repl = repl_func(...)
    if type(repl) == "string" then
      repl = string.gsub(repl, "%%(.)",
        setmetatable({["%"] = "%", ...}, trynum))
    end
    return repl
  end
end


The generate_replace function takes a function which returns replacement 
strings like "%2%1" and returns a transformation of that function which 
is compatible with string.gsub().

An example:

-- replace letter-digit pairs
function sample(let,dig)
    if let == "a" then
        return false
    elseif let == "b" then
        return "%2"
    elseif let == "c" then
        return "%%%2%1%%"
    else
        return "%2%1"
    end
end

for line in io.lines() do
    -- match all letter-digit pairs
    line = string.gsub(line, "(%l)(%d)", generate_replace(sample))
    io.write(line, '\n')
end


And running it:

$ echo :a1:b2:c3:d4: |lua51 repl.lua
:a1:2:%3c%:4d:


Note that this method can't handle %0 replacements, as string.gsub does 
not pass the whole match to the function, only the captures.

					-Mark