Using Regular Expressions with Pandas Dataframe

I had used regular expressions in many of my previous implementations and had witnesses the efficacy of their usage in overall execution. am quite hands-on with using them with C#. I am new to python and am trying to use the same with the dataframes to perform certain data pre-processing operations.

Currently I am using the code as below and its working fine, just wondering how i can use regex instead of multiple conditions to improvise on the execution time.

df = pd.DataFrame(np.array([[2,4,4],[4,3,3],[5,9,1]]),columns=['a','aa','b'])

   a aa