import re
p = re.compile(r"([\d\W]+)\n.*\[(.*)\].*\n.*", re.MULTILINE | re.UNICODE)
test_str = u"""00:00:05,233 --> 00:00:08,133
GAT[xd]fES: Amelia Earhart
is on one of the final legs
00:00:05,333 --> 00:00:08,433
[xd]fES: Amelia Earhart
is on one of the final legs
"""
subst = r"\1 \2"
result = re.sub(p, subst, test_str)
print result
aW1wb3J0IHJlCnAgPSByZS5jb21waWxlKHIiKFtcZFxXXSspXG4uKlxbKC4qKVxdLipcbi4qIiwgcmUuTVVMVElMSU5FIHwgcmUuVU5JQ09ERSkKdGVzdF9zdHIgPSB1IiIiMDA6MDA6MDUsMjMzIC0tPiAwMDowMDowOCwxMzMKR0FUW3hkXWZFUzogQW1lbGlhIEVhcmhhcnQKaXMgb24gb25lIG9mIHRoZSBmaW5hbCBsZWdzCgowMDowMDowNSwzMzMgLS0+IDAwOjAwOjA4LDQzMwpbeGRdZkVTOiBBbWVsaWEgRWFyaGFydAppcyBvbiBvbmUgb2YgdGhlIGZpbmFsIGxlZ3MKIiIiCgpzdWJzdCA9IHIiXDEgXDIiCnJlc3VsdCA9IHJlLnN1YihwLCBzdWJzdCwgdGVzdF9zdHIpCnByaW50IHJlc3VsdA==