[Python] How to replace string using regular expression in Python

I need help with replacing characters in a string using regular expressions.

Input :

s3 = ['March/21/2019' , 'Mar/23/2019']


Desired Output :

s3 = ['03/21/2019' , '03/23/2019']


I tried a few things, but none work:

1. 
   s3 = s3.replace(r'Mar[a-z]*', '03')
   
   
   [*]
   s3 = s3.replace(r'(?:Mar[a-z]*)', '03')
   
   

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