Seemingly simple patterns can cause `string.match()` to throw timeout errors near end of time slice

[canny]

Running on server: Luau 2026-01-06.20757451310

In successive calls of string.match, I often get a fatal error if I don't wrap string.match in a pcall:

runtime error

lua_script:3: String pattern too complex, timed out

[C] function match

The problem is that the only thing repeatable is that I may or may not get errors when running the code.

local function getStringMatch(s)

    local status, k, v

    repeat

        status, k, v = pcall(string.match, s, "(%g+)%s*=%s*(%g+)")

        if not status then print(k) end

    until status

end

local keys = {}

for i = 1, 10 do

    table.insert(keys, tostring(ll.GenerateKey()))

end

for k, v in keys do

    print(k)

    getStringMatch(v)

end

This is basically a useless example, except to trigger the issue. gmatch should return the captures, the original string if no captures, or nil.

This example will never find a match, but I don't believe string complexity is the issue, and I don't believe its timing out because it happens very fast.

But, if a failure is triggered, I can recall the very same match again and get success, without providing any delays in the code.

https://secondlife.canny.io/admin/board/slua-alpha/p/seemingly-simple-patterns-can-cause-stringmatch-to-throw-timeout-errors-near-end

[canny]

This issue has been linked to a Canny post: Seemingly simple patterns can cause string.match() to throw timeout errors near end of time slice 🎉

[HaroldCindy]

From my Canny comments:

There doesn't seem to be a lot of prior on optimizing Lua pattern matching behavior, but I think a lot of these are simple enough that it's straightforward to compile them to re2 expressions, and use re2 under the hood. That would allow us to provide APIs for proper regex support everywhere as well.

That being said, I don't like that string.find() and friends can't yield internally if they run too long, but the design of conventional Lua pattern matching implementations doesn't allow for it. It's on my TODO list to rewrite the pattern matching code inherited from Luau to support that, but it's very tricky work.