Lua - Using string.find to return true if between 2 special characters

Question

I need a pattern that will work with string.find (or string.match if necessary) that will return true if a "table path" string matches. This is my function:

local function FindValueFromPattern(myTable, pattern, previousPath)
    for key, value in pairs(myTable) do
        local path;

        if (not previousPath) then
            path = key;
        else
            path = string.format("%s.%s", previousPath, key);
        end

        if (path:find(pattern)) then
            return value;

        elseif (type(value) == "table") then
            value = FindValueFromPattern(value, pattern, path);

            if (value ~= nil) then
               return value;
            end
        end
    end

    return nil;
end

local tbl = {}
tbl.settings = {};
tbl.settings.module = {};
tbl.settings.module.appearance = {};
tbl.settings.module.appearance.color = "blue";

print(FindValueFromPattern(tbl, "settings.module.appearance.color")); -- prints "blue";

The code above works BUT I want to now change the pattern to:

"module.<ANY>.color" where <ANY> is any child table of "module" and also has a child table called "color", so when traversing down the table, a value will be returned regardless of what table is used (does not have to be the appearance table):

-- should also print "blue" ("setting." should not be required);
print(FindValueFromPattern(tbl, "module.<ANY>.color"));

Rather than returning found values straight away, I may have to change the logic to insert found values in a table and then return the table after the for-loop but I wrote this quickly to illustrate the problem.

So the question is, what would that pattern look like? Thank you.

Are numeric keys allowed? Should "settings.module[3].color" be found? — Egor Skriptunoff
@EgorSkriptunoff Good question. No, because the path is artificially created and I join each key explicitly with "." so if the pairs function returned a number then, for example, it would still look like "module.2.property" and I can use this in the pattern I send to the function. Thanks for asking. — Mayron
To match the "module.<ANY>.color" string you might use a "module%.[^.]+%.color" pattern. If you need to extract the ANY part, wrap the [^.]+ with capturing group: "module%.([^.]+)%.color" — Wiktor Stribiżew
@WiktorStribiżew Thanks, that looks like a great answer. Not necessary for the solution I posted but would using "module%.([^.]+)%.([^.]+)%color" also work for cases where there are more than 1 of these keys? I might need to do something like this in the future. — Mayron
The point is that [^.]+ matches any consecutive chars other than . one or more times. I guess yes. If you need to restrict the chars to letters and digits use %w+ instead. If you need to match any 0+ chars use .- (to get to the first .color) or .+ (to match up to the last .color). — Wiktor Stribiżew

DarkWiiPlayer DarkWiiPlayer · Accepted Answer · 2019-02-08T11:51:10

What you do there is extremely inefficient. A much better approach would be to split the string at each . and just index the table.

A simple version that doesn't accept "any" could look like this

function findintable(tab, path)
    local pos = path:find(".", 1, true)
    if pos then
        local tab = tab[path:sub(1, pos-1)]
        if not type(tab) then error("Expected value to be table, got "..type(tab), 1) end
        return findintable(tab, path:sub(pos+1, -1))
    else
        return tab[path]
    end
end

Adding the possibility for an any key (he... he... he...) would add some complexity and needs a loop, but it's also doable

    function findintable(tab, path)
        local pos = path:find(".", 1, true)
        if not pos then
            return tab[path]
        end
        local key, rest = path:sub(1, pos-1), path:sub(pos+1, -1)
        if key == "*" then
            for k, v in pairs(tab) do
                if type(v)~="table" then return end
                local res = findintable(v, rest)
                if res then return res end
            end
            return
        else
            local tab = tab[path:sub(1, pos-1)]
            if not type(tab) then error("Expected value to be table, got "..type(tab), 1) end
            return findintable(tab, path:sub(pos+1, -1))
        end
    end

This should do what you want. Just change the "*" to whatever you want the placeholder to be.

Lua - Using string.find to return true if between 2 special characters

2 Answers