import re
message = """[key    X] value
[key    X]  value value
[key    X]  value
value
value
value
[key     ] value
[key     ] ?
[key     ] ?"""

messageRegex = re.compile(r"^\[([^][]*)]\s+(.*(?:\n(?!\[[^][]*]).*)*)", re.M)

for value in messageRegex.findall(message):
    print(value)