Re: Lua idiom for RE-style alternation?

[David Manura <dm.lua@...>]

On Wed, Jan 6, 2010 at 3:33 PM, duck wrote:
> What is "Lua idiom" -- using the regular string library, not lpeg or any
> of the RE add-ons -- when doing string matching with alternative
> strings in it. For example, something like this:
>  "eat (green|yellow)? (bananas|apples) daily"

There's xpattern [1].  Example:

  local XP = require "xpattern"
  XP.debug = true  -- prints Lua function code
  local P = XP.P

  local pat = P'eat ' * (P'green' + P'yellow')^-1 * P' '
              * (P'bananas' + P'apples') * ' daily'

  local m = pat:compile()
  print('1:', m('eat blue apples daily'))
  print('2:', m('eat green bananas daily'))

Output:

  DEBUG:
  local match = ...
  return function(s,pos)
    for pos1=(pos or 1),#s do
      local pos2
      pos2 = match(s, "^eat ()", pos1)
      if pos2 then
        do
         local pos3=pos2
         local pos4 = pos3
        pos4 = match(s, "^green()", pos3)
        if not pos4 then
          pos4 = match(s, "^yellow()", pos3)
        end
        pos3 = pos4
          if pos3 then
            pos2=pos3
          else
            pos2=pos2
          end
        end
      end
      if pos2 then
        pos2 = match(s, "^ ()", pos2)
      end
      if pos2 then
        local pos3 = pos2
        pos3 = match(s, "^bananas()", pos2)
        if not pos3 then
          pos3 = match(s, "^apples()", pos2)
        end
        pos2 = pos3
      end
      if pos2 then
        pos2 = match(s, "^ daily()", pos2)
      end
      if pos2 then return s:sub(pos1,pos2-1) end
    end
  end

  1:
  2:      eat green bananas daily


[1] http://lua-users.org/wiki/ExPattern