Regular Expression Validation in Single line of text field

Question

Hi I need the digit to be displayed as follows

00.11.12aa or

 

00.12.55 or

 

11.48.61d

starts with 2 digit and decimal then 2 digit then decimal then twodigit or twodigit one alpha or twodigit two alpha.

I need to validate in single line of text field. Please tell me how to do it.