Convert WebVTT (*.vtt) to SubRip (*.srt) with Python 2/3

webvtt_to_srt.py

import re
import os

def webvtt_to_srt(webvtt):
    srt = ""
    counter = 1

    for line in webvtt.splitlines():
        if line.startswith("WEBVTT"):
            continue
        if counter == 1 and line == "":
            continue

        matches = re.match(r"^(?P<fh>\d{2}:)?(?P<fm>\d{2}):(?P<fs>\d{2}).(?P<fms>\d{3})\s-->\s(?P<th>\d{2}:)?(?P<tm>\d{2}):(?P<ts>\d{2}).(?P<tms>\d{3})", line)
        if matches:
            srt += str(counter) + os.linesep
            srt += "{}{}:{},{} --> {}{}:{},{}{}".format(
                matches.group('fh') or "00:",
                matches.group('fm'),
                matches.group('fs'),
                matches.group('fms'),
                matches.group('th') or "00:",
                matches.group('tm'),
                matches.group('ts'),
                matches.group('tms'),
                os.linesep
            )
            counter += 1
        else:
            srt += line + os.linesep

    return srt