Only allow numbers in text box?

SummerEquinox's answer probably works, but here's a much simpler way

TextBox:GetPropertyChangedSignal("Text"):Connect(function()
    if tonumber(TextBox.Text) then
        -- send the donation
    else
        TextBox.Text = "Enter a number to donate" -- do not send the donation
    end
end)

tonumber is a Lua function that checks if the string can be turned into a number. It is related to tostring if you've heard of that

Note: tonumber returns nil if it can't convert the string to a number, but if it can then it returns the string converted to a number

Say you have a TextBox - you can allow only numbers (where num >0) to be typed in it with the following code:

function Format(t)
    return t:gsub("%D", "")
end

script.Parent:GetPropertyChangedSignal("Text"):Connect(function()
    script.Parent.Text = Format(script.Parent.Text)
end)

If you wanted to check the text afterwards - you could do:

script.Parent.FocusLost:Connect(function()
    if string.match(script.Parent.Text, "%D+") then
        print('Text is not a positive number')
        script.Parent.Text = "TextBox"
    end
end)