import re
transcript=r"""12:12:12,234 --> 12:13:46,346
Blah
12:14:12,121 --> 12:15:89,678
Blah2
"""
timestamps = []
linenum = 0
pattern = re.compile(r"\d{2}:\d{2}:\d{2},\d{3}\s-->\s\d{2}:\d{2}:\d{2},\d{3}")
for line in transcript.splitlines():
    linenum += 1
    if pattern.search(line):
        timestamps.append((linenum, line.rstrip('\n')))
print(timestamps)