import re
months=['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec']
s = 'Aug 5 2020: John Doe: Hello Aug 5 2020 Jane Doe: Hi'
rx = re.compile( fr"\s+(?=(?:{'|'.join(months)})\b)", re.I )
print (rx)
print (rx.split(s))
aW1wb3J0IHJlCm1vbnRocz1bJ0phbicsJ0ZlYicsJ01hcicsJ0FwcicsJ01heScsJ0p1bicsJ0p1bCcsJ0F1ZycsJ1NlcCcsJ09jdCcsJ05vdicsJ0RlYyddCnMgPSAnQXVnIDUgMjAyMDogSm9obiBEb2U6IEhlbGxvIEF1ZyA1IDIwMjAgSmFuZSBEb2U6IEhpJwpyeCA9IHJlLmNvbXBpbGUoIGZyIlxzKyg/PSg/OnsnfCcuam9pbihtb250aHMpfSlcYikiLCByZS5JICkKcHJpbnQgKHJ4KQpwcmludCAocnguc3BsaXQocykp