fsparse.lua

local strfind, strsub, strgsub = string.find, string.sub, string.gsub
local strlen, ipairs, tinsert = string.len, ipairs, table.insert

local function formspec_unescape(str)
	return (strgsub(str, "\\(.)", "%1"))
end

---
-- Parse a string and return a list of formspec elements.
-- Has limited detection of malformed data and automatically unescapes
-- formspec item parameters.
-- Each returned element is a table with the following format:
--     {
--       type = "label",
--       params = {
--         "1,2",
--         "Hello;] world!",
--       },
--     }
local function parse_formspec(fs)
	local items = { }
	local pos, len = 1, strlen(fs)
	while pos <= len do
		-- Search for `type[`
		local start, startend, type = strfind(fs, "([a-z_]+)%[", pos)
		if not start then break end
		pos = startend + 1
		startend = startend + 1
		start = startend
		local params
		local paramstart, paramend
		while true do
			-- Search for unescaped `]`
			local fin, finend, slashes = strfind(fs, "(\\*)%]", startend)
			if not fin then
				return nil, "malformed data"
			end
			if #slashes%2 == 0 then
				paramstart, paramend = start, finend-1
				params = strsub(fs, paramstart, paramend)
				break
			end
			startend = finend+1
		end
		-- At this point, `type` is the element type (`image`,
		-- `button`, etc.), and `params` is everything between
		-- `[` and `]`.
		local paramlist = { }
		-- Split parameters by `;`.
		paramstart, paramend = 1, strlen(params)
		local paramp = paramstart
		while paramp <= paramend and paramstart <= paramend do
			-- Search for next unescaped `;`
			local sc, scend, slashes = strfind(params, "(\\*);", paramp)
			if not sc then
				-- No more `;`s; add remaining data and bail out.
				tinsert(paramlist, strsub(params, paramstart, paramend))
				break
			end
			if #slashes%2 == 0 then
				-- Unescaped `;`: add data up to that point and continue.
				tinsert(paramlist, strsub(params, paramstart, scend-1))
				paramstart = scend+1
				paramp = paramstart
			else
				-- Escaped `;`: keep searching.
				paramp = scend+1
			end
		end
		-- Unescape parameters.
		for i, v in ipairs(paramlist) do
			paramlist[i] = formspec_unescape(v)
		end
		tinsert(items, {
			type = type,
			params = paramlist,
		})
	end
	return items
end