example.py

import re

text = "The stable release of python 3.8 was on 05 Aug 2020. The stable release of C++17 was on 12 Jan 2019."

#date_pattern = re.compile(r'(\d+)( )+(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)( )+(\d+)')
date_pattern = re.compile(r'(\d+) +(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) +(\d+)')

def format_date(date_input):
    month_digit = {'Jan':'01','Feb':'02','Mar':'03','Apr':'04','May':'05','Jun':'06','Jul':'07','Aug':'08','Sep':'09','Oct':'10','Nov':'11','Dec':'12'}
    month_d = month_digit[date_input.group(2)]
    return '{}-{}-{}'.format(date_input.group(3), month_d, date_input.group(1))

print(date_pattern.sub(format_date, text))

text = "The stable release of python 3.8 was on 05 Aug 2020. The stable release of C++17 was on 12 Jan 2019." 
 
date_pattern = re.compile(r'(\d+)( )+(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)( )+(\d+)') 
#date_pattern = re.compile(r'(\d+) +(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) +(\d+)') 
 
def format_date(date_input): 
    month_digit = {'Jan':'01','Feb':'02','Mar':'03','Apr':'04','May':'05','Jun':'06','Jul':'07','Aug':'08','Sep':'09','Oct':'10','Nov':'11','Dec':'12'}
    month_d = month_digit[date_input.group(3)] 
    return '{}-{}-{}'.format(date_input.group(5), month_d, date_input.group(1)) 
 
print(date_pattern.sub(format_date, text))