/^(0[1-9]|1[0-2])/?([0-9]{2})$/
/^(0[1-9]|1[0-2]) :
First Digit - 0 second digit will be -0 to 9 ----> for Jan to Sep First Digit - 1 second digit will be -0 to 2 ----> for Oct to Dec
/?([0-9]{2})$/
2 Digit in between 0 to 9.