import re

text = """1
00:00:05.210 --> 00:00:07.710
In this lecture, we're
going to talk about

2
00:00:07.710 --> 00:00:10.815
pattern matching in strings
using regular expressions.

3
00:00:10.815 --> 00:00:13.139
Regular expressions or regexes

4
00:00:13.139 --> 00:00:15.825
are written in a condensed
formatting language."""


regex = r"^\d+\r?\n\d{2}:\d{2}:\d{2}\.\d{3} -->.*\r?\n((?:(?!\d+\r?\n\d\d:).*(?:\r?\n|$))*)"
content = [i for i in re.split(r"(?<=\.)\s+", re.sub(r"[\r\n]+", " ", "".join(re.findall(regex, text, re.M)))) if i]
print(content)