data = """
[Tue Nov 12 01:30:51 2000]
abcd
[Tue Nov 12 02:30:51 2000]
1234
cmd_prefix a
    cmd output1
cmd_prefix b
[Tue Nov 12 03:30:51 2000]
abcd
[Tue Nov 12 04:30:51 2000]
1234
cmd_prefix a
    cmd output2
cmd_prefix b
"""

timestamp = (
    LineStart()
    + Suppress(Optional(Literal("[")))
    + Regex(r"\w{3} \w{3}\s+\d{1,2} \d{2}:\d{2}:\d{2} \d{4}")
    + Suppress(Optional(Literal("]")))
)
cmd_prefix_cmd = LineStart() + Literal("cmd_prefix")
cmd_prefix_m_cmd = Combine(cmd_prefix_cmd + Literal(" a"))
cmd_prefix_z_cmd = Combine(cmd_prefix_cmd + Literal(" b"))
cmd_prefix_parse = Suppress(SkipTo(cmd_prefix_m_cmd)) + SkipTo(cmd_prefix_z_cmd)

timestamp_with_data = Group(
    OneOrMore(timestamp)("TIMESTAMP")
    + FollowedBy(cmd_prefix_parse)
    + cmd_prefix_parse("DATA")
)

parser = OneOrMore(timestamp_with_data)

results = parser.parseString(data)

for result in results:
    print(result.dump())