local number_cut = 12
local second_one = [[
1283813122344
12
12838138931316
128381383131
]]

local match0 = string.match(second_one, "%f[%w_]" .. number_cut .. "%f[^%w_]")

if (match0) then
    print "this is exactly matched 12"
else
    print "not matching"
end