String.gsub doesn't work correctly ?

Hi. I recently tried out gsub and figuring how it works, and I ran into a problem. I have a script like this

local charmap = {
    ["k"] = 'x',
    ["kh"] = 'x',
    ["c"] = 'k',
    ["c?"] = 'k',
    ["c!"] = 'k',
    ["ch"] = 'k',
    ["cq"] = 'k',
    ["t"] = 'c',
    ["tr"] = 'c',
    ["c"] = 'c',
    ["ch"] = 'c',
    ["d"] = 'z',
    ["g"] = 'z',
    ["gi"] = 'z',
    ["r"] = 'z',
    ["g"] = 'z',
    ["gi"] = 'z',
    ["gì"] = 'z',
    ["gí"] = 'z',
    ["g?"] = 'z',
    ["g?"] = 'z',
    ["g?"] = 'z',
    ["?"] = 'd',
    ["p"]= 'f',
    ["ph"]= 'f',
    ["n"] = 'q',
    ["ng"] = 'q',
    ["nh"] = 'q',
    ["n?"] = 'q',
    ["t"] = 'w',
    ["th"] = 'w'
}

print(string.gsub("tieng", ".", charmap))

but instead of printing "tieq", it prints "tieqw" (look at the character map for reference). Please fix it for me. Thanks in advance.