lpeg grammar to parse comma separated groups that may have internal groups

Question

I need to parse comma separated groups(enclosed in brackets) that may have internal groups inside the groups. It should only separate the outside groups.

I have a function that does this:

function lpeg.commaSplit(arg)
    local P,C,V,sep = lpeg.P, lpeg.C, lpeg.V, lpeg.P(",")
    local p = P{
        "S";
        S = lpeg.T_WSpace * C(V"Element") * (lpeg.T_WSpace * sep * lpeg.T_WSpace * C(V"Element"))^0 * lpeg.T_WSpace,
        Element = (V"Group")^0 * (1 - lpeg.T_Group - sep)^0 * (V"Group" * (1 - lpeg.T_Group - sep)^0)^0 * (1 - sep)^0,
        Group = lpeg.T_LGroup * ((1 - lpeg.T_Group) + V"Group")^0 * lpeg.T_RGroup
    }^-1
    return lpeg.match(lpeg.Ct(p), arg)

end

But the problem is to remove the extra brackets that may enclose the group.

Here is a test string:

[[a,b,[c,d]],[e,[f,g]]]

should parse to

[a,b,[c,d] & [e,[f,g]]

Notice the internal groups are left alone. A simple removal of the extra brackets on the end does not work since you'll end up with a string like a,b,[c,d]],[e,[f,g].

Any ideas how to modify the lpeg grammar to allow for the outside groups?

Perhaps, but it is better to give their definition to increase your chances to have an answer... I can guess but easing work of those willing to answer is better and increases your chance of having an accurate answer, if you still want one. — PhiLho, May 28 '12 at 08:15

PhiLho · Answer 1 · 2012-05-29T11:20:32.037

As I am not expert in making grammars in LPeg, I found this exercise interesting to do...

I couldn't manage to use your grammar, so I went ahead and made my own, with smaller chunks easier to understand and where I could put the captures I needed.

I think I got a decent empirical result. It works on your test case, I don't know if groups can be more deeply nested, etc. The post-processing of the capture is a bit ad hoc...

require"lpeg"

-- Guesswork...
lpeg.T_WSpace = lpeg.P" "^0
lpeg.T_LGroup = lpeg.P"["
lpeg.T_RGroup = lpeg.P"]"
lpeg.T_Group = lpeg.S"[]"

function lpeg.commaSplit(arg)
  local P, C, Ct, V, sep = lpeg.P, lpeg.C, lpeg.Ct, lpeg.V, lpeg.P","
  local grammar =
  {
    "S";
    S = lpeg.T_WSpace * V"Group" * lpeg.T_WSpace,
    Group = Ct(lpeg.T_LGroup * C(V"Units") * lpeg.T_RGroup),
    Units = V"Unit" *
        (lpeg.T_WSpace * sep * lpeg.T_WSpace * V"Unit")^0,
    Unit = V"Element" + V"Group",
    Element = (1 - sep - lpeg.T_Group)^1,
  }
  return lpeg.match(Ct(P(grammar)^-1), arg)
end

local test = "[[a,b,[c,d]],[e,[f,g]]]"
local res = lpeg.commaSplit(test)
print(dumpObject(res))
print(res[1], res[1][1], res[1][2])
local groups = res[1]
local finalResult = {}
for n, v in ipairs(groups) do
  if type(v) == 'table' then
    finalResult[#finalResult+1] = "[" .. v[1] .. "]"
  end
end
print(dumpObject(finalResult))

dumpObject is just a table dump of my own. The output of this code is as follows:

local T =
{
  {
    "[a,b,[c,d]],[e,[f,g]]",
    {
      "a,b,[c,d]",
      {
        "c,d"
      }
    },
    {
      "e,[f,g]",
      {
        "f,g"
      }
    }
  }
}

table: 0037ED48 [a,b,[c,d]],[e,[f,g]]   table: 0037ED70

local T =
{
  "[a,b,[c,d]]",
  "[e,[f,g]]"
}

Personally, I wouldn't pollute the lpeg table with my stuff, but I kept your style here.

I hope this will be useful (or will be a starting point to make you to advance).

lpeg grammar to parse comma separated groups that may have internal groups

1 Answers1