How to validate this string when we don't have the `|` operator in Lua?

Question

I have strings of the form:

cake!apple!

apple!

cake!juice!apple!cake!

juice!cake!

In other words, these strings are composed of the three sub-strings "cake!", "apple!" and "juice!".

I need to validate these strings. The way to do this with a regular expression is thus:

/^(apple!|juice!|cake!)*$/

But Lua's patterns don't have the | operator, so it seemingly can't be done this way.

How can I validate my strings in Lua?

(I don't care about the contents of the strings: I only care about whether they conform (validate) or not.)

I know to write the code to do this but I can't think of a short way to do this. I'm looking for a short solution. I wonder if there's an elegant solution that I'm not aware of. Any ideas?

Answer 1

if str:gsub("%w+!", {["apple!"]="", ["juice!"]="", ["cake!"]=""}) == "" then
    --do something
end

This solution uses a table as the second parameter to string.gsub. Since the patterns all match %w+, the table will validate for second time, only the real three patterns are replaced with an empty string. If after all the replacement, the string becomes empty, then the match succeeds.

Using a helper table variable can make it more clear:

local t = {["apple!"]="", ["juice!"]="", ["cake!"]=""}
if str:gsub("%w+!", t) == "" then
    --do something
end