import re

s = "field1: content asd..\n\nfield2: content\n\nqwe...\n\nfield3: content asfdqegt"
pattern = r"^field\d+:.*(?:\r?\n(?!field\d+:).*)*"
res = [x.replace('\n', '') for x in re.findall(pattern, s, re.MULTILINE)]
print (res)