string.gsub returning the text over and over, help?

So, I'm creating a script, and I've found a bug that I don't understand.

This is where it's bugging:

function Filter(txt)
    txt=txt:gsub("[PLRNAME]",plr.Name:upper())
    print(txt)
    for i = 1, #txt do
        ChatLabel.Text=txt:sub(1,i)
        wait(.05)
    end
end

Filter("Testing. [PLRNAME]")

ChatLabel is a TextLabel, and plr is the LocalPlayer the text is turned into "SUPERSONICFAN111" about 7 times, with a [ and ] surrounding the 7 ones I need help with this, so if anyone can help me, that'd be great