local MacFormat ={[1] = "0A:BC:DE:FA:BC:DE",[2] = "00:BC:DE:FA:BC:DE",[3] = "00:0C:DE:FA:BC:DE",[4] = "00:00:DE:FA:BC:DE",[5] = "00:00:0E:FA:BC:DE"}
local idNumbers = {[1] = "1",[2]="12",[3]="123",[4]="1234",[5]="12345"}
 
local p = "^" .. ("0"):rep(string.len(idNumbers[3])):gsub("(..)", "%1:")
local repl = idNumbers[3]:gsub("(..)", "%1:")
local str = MacFormat[3]:gsub(p, repl)
 
print(str)
---output 33:3C:DE:FA:BC:DE
--- Desired Output 12:3C:DE:FA:BC:DE

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