import re
def transform(text):
    dct = {'January':'01','February':'02','March':'03','April':'04','May':'05','June':'06','July':'07','August':'08','September':'09','October':'10','November':'11','December':'12'}
    output, n = re.subn(rf'\b(?:{"|".join(dct.keys())})\b', lambda x: dct[x.group()], text)
    if not n:
        return('This is a string without a month in it')
    else:
        return output
 
 
print( transform('I was born on June 24 and my sister was born on May 17') )
# expected output: 'I was born on 06 24 and my sister was born on 05 17'
 
print( transform('This is a string without a month in it') )
# expected output: 'This is a string without a month in it'

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