Feeds Tamper

I am importing CSV records using Feeds module and Feeds Tamper. I need help figuring out a proper REGEX search and replace expression.

The first character is a numerical digit to represent the year. Example: the "3" in "3M" is a code for the year 2013 (or 2023, 2033, etc.).

The second character is a letter representing the month as follows:

A JAN
B FEB
C MAR
D APR
E MAY
F JUN
G JUL
H AUG
I (NOT USED)
J SEP
K OCT
L NOV
M DEC

So the code "3M" is actually 01-DEC-2013 (The day is always the first of the month)