Auto Increment Row number in repeating table?

Hi, Everyone

I have a dropdown list to choose project No, and a repeating table for user to insert all sub projects under the existing projects. All sub projects Under each unique projects should get an Identification No auto incresed from "01" to 02, 03, 04...

e.g:

In Project NO dropdown, I have 3 project NO to choose: 

2010Jan1
2010Jan2
2010Jan3

If I chose "2010Jan1" from project No drop down list, Records in Repeating Table to list sub projects should be based on every insersion to be:

(ID NO in repeating table)

2010Jan1_01
2010Jan1_02
2010Jan1_03

If I chose "2010Jan2" from project No drop down list, Records in Repeating Table to list sub projects should be based on every insersion to be:

(ID NO in repeating table)

2010Jan2_01
2010Jan2_02
2010Jan2_03

How can I do this?

Thank you so much.