Parsing Lua 5.2 strings with patterns

Question

I wrote some code to parse Lua 5.2 strings in Lua 5.1, using patterns. It works perfectly as far as I tested.

local squote = "'" local dquote = '"' -- escape "sequences" local escapeSequences = { a = '\a', b = '\b', f = '\f', r = '\r', n = '\n', t = '\t', v = '\v', ['"'] = '"', ["'"] = "'", ['\\'] = '\\' } local pads = { z = "\\z", x = "\\x", ['0'] = '\\0', ['1'] = '\\1', ['2'] = '\\2', ['3'] = '\\3', ['4'] = '\\4', ['5'] = '\\5', ['6'] = '\\6', ['7'] = '\\7', ['8'] = '\\8', ['9'] = '\\9' } setmetatable(pads, { __index = function(t,k) return "\\v" .. k .. "/" end }) -- Parse a string like it's a Lua 5.2 string. local function parseString52(s) -- "validate" string local startChar = string.sub(s,1,1) assert(startChar==squote or startChar==dquote) assert(string.sub(s, -1, -1) == startChar) -- remove quotes local str = string.sub(s, 2, -2) -- TODO check for unescaped quotes -- replace "normal" escapes with a padded escape str = string.gsub(str, "\\(.)", function(c) -- swap startChar with some invalid escape if c == startChar then c = "m" -- swap the invalid escape with startChar elseif c == "m" then c = startChar end return pads[c] end) -- check for a padded escape for startChar - remember this is actually our invalid escape assert(not string.find(str, "\\v" .. startChar .. "/"), "invalid escape sequence near '\\m'") -- then check for non-escaped startChar assert(not string.find(str, startChar), "unfinished string") -- pad 1-digit numerical escapes str = string.gsub(str, "\\([0-9])[^0-9]", "\\00%1") -- pad 2-digit numerical escapes str = string.gsub(str, "\\([0-9][0-9])[^0-9]", "\\0%1") local t = {} local i = 1 local last = 1 -- split on \z for from,to in function(x,y) return string.find(x, "\\z", y+1) end, str, 0 do t[i] = string.sub(str, last, from - 1) last = to+1 i = i + 1 end t[i] = string.sub(str, last, #str) -- parse results local nt = {} for x,y in ipairs(t) do nt[x] = string.gsub(y, "\\(([vx0-9])((.).))", function(a,b,c,d) if b == "v" then return escapeSequences[d] or (d == "m" and startChar or assert(false, "invalid escape sequence near '\\" .. d .. "'")) elseif b == "x" then local n = tonumber(c, 16) assert(n, "hexadecimal digit expected near '\\x" .. c .. "'") return string.char(n) else local n = tonumber(a) assert(n < 256, "decimal escape too large near '\\" .. a .. "'") return string.char(n) end end) if x > 1 then -- handle \z nt[x] = string.gsub(nt[x], "^[%s]*", "") end end -- merge return table.concat(nt, "") end return { parse52 = parseString52, } 

(For test cases see here, I decided to keep them separate so to not bloat up the post)

Things I'm not too sure about:

• Should I use if not (condition) then error(msg) end instead of assert(condition, msg)? (Usually assertions are reserved for tests, but I'm not sure if this holds in Lua? I'm also not sure if assert() calls are JITted by LuaJIT.)
  • Should I return nil, msg instead of erroring?
• Should I handle long strings?
• Should I error if the unparsed string literal is smaller than the passed buffer? (That is, if you pass in something like [["<unparsed string literal>"<rest of buffer>]]) Or should I find the end of the string myself?
  • If so, should I return the end position?
• Should I take an optional start position?
• Can this be made faster?