Regex expression to recognize XdY+Z OR XdY - regex

Question

I've been trying to develop a program that will be used for DMing in an MMORPG but I'm having trouble parsing for the actual regex expression I need.
To quote myself from another thread on a less active forum:
I've officially taken over the DiceRoller addon from years and years ago and I've reworked it a lot since I've taken it over and done a lot of testing in game. While I haven't uploaded anything yet, I've been struggling on a piece of regex expression that is currently crucial to the design of the addon.
Some background: the newest iteration of the DiceRoller addon makes it so you can type "!XdY" (where X is the number of dice, Y is the dice value) into raid chat and the DM who has the addon will go through some logic in the addon (random number lua protocol) and then spit out an input after adding up the dice.
It is as follows:
local count, size = string.match(message, "^!(%d+)[dD](%d+)$")
Now the functionality I need it to do is parse for both "!XdY" OR "XdY+Z", but it seems as if I can't get close to "XdY+Z" no matter which regex expression I use since I need it to do both expressions. I can provide more source code context if necessary.
This is the closest I've ever gotten:
http://i.imgur.com/eMhPHQB.png
and this is with the regex expression:
local count, size, modifier = string.match(message, "^!(%d+)[dD](%d+)+?(%d+)$")
As you can see, with the modifier it will work just fine. However, remove the modifier the regex expression still thinks that it is "XdY+Z" and so with "1d20" it think it is "1d2+0". It will think 1d200 is "1d20+0", etc. I've tried moving around the optional character "?" but it just causes the expression to not work at all. If I do !1d2 it doesn't work. It's almost as if the optional character NEEDS to be there?
Thanks for the help ahead of time, I've always struggled with regex.

Answer 1

local function dice(input)
local count, size, modifier = input:match"^!(%d+)[dD](%d+)%+?(%d*)$"
if count then
return tonumber(count), tonumber(size), tonumber("0"..modifier)
end
end
for _, input in ipairs{"!1d6", "!1d24", "!1d200", "!1d2+4", "!1d20+24"} do
print(input, dice(input))
end
Output:
!1d6 1 6 0
!1d24 1 24 0
!1d200 1 200 0
!1d2+4 1 2 4
!1d20+24 1 20 24

Answer 2

Lua regular expressions are very limited. You would need to use ^!(%d+)[dD](%d+)(?:+(%d+))?$ but this wouldn't be supported because of (?:+(%d+))? that uses a non-capturing group and a modifier on a group, both are not supported by Lua Patterns.
Consider using a regex library like this one that allows you to use PCRE, PHP regex engine, one of the most complete engine. But that would be overkill if you only want to use it for this regex. You can do it by code then, wouldn't be so hard for a simple task like this.

Answer 3

While Lua patterns are not powerful enough to parse this with one expression (as they don't support optional groups), there is an easy option to handle it with two expressions:
-- check the longer expression first
local count, size, modifier = string.match(message, "^!(%d+)[dD](%d+)+(%d+)$")
if not count then
count, size = string.match(message, "^!(%d+)[dD](%d+)$")
end