Sample WebVTT file (valid)

Handle it in code

Each snippet below was run against this exact file with Python 3.14.4 and Node 24.15.0 when this page was built, and the output shown is what it printed.

Python

SRT puts a comma before the milliseconds and WebVTT a full stop, and WebVTT files must start with WEBVTT. Players differ on the rest: overlapping cues are legal in WebVTT, but many SRT players show only one of them.

import re

TIME = r"(?:(\d+):)?(\d{2}):(\d{2})([.,])(\d{3})"
CUE = re.compile(TIME + r"\s+-->\s+" + TIME)

def ms(h, m, s, sep, frac):
    return ((int(h or 0) * 60 + int(m)) * 60 + int(s)) * 1000 + int(frac)

name = "valid.vtt"
vtt = name.lower().endswith(".vtt")
with open(name, encoding="utf-8-sig") as f:
    text = f.read()
problems, cues = [], []
if vtt and not text.startswith("WEBVTT"):
    problems.append("line 1: no WEBVTT signature")
for n, line in enumerate(text.splitlines(), 1):
    m = CUE.search(line)
    if not m:
        continue
    g = m.groups()
    right = "." if vtt else ","
    if g[3] != right or g[8] != right:
        problems.append(f"line {n}: {'WebVTT' if vtt else 'SRT'} needs {right!r} before the milliseconds")
    start, end = ms(*g[:5]), ms(*g[5:])
    if end < start:
        problems.append(f"line {n}: the cue ends before it starts")
    elif end == start:
        problems.append(f"line {n}: zero length, so it never shows")
    if cues:
        prev_start, prev_end = cues[-1]
        if start < prev_start:
            problems.append(f"line {n}: starts before the previous cue (out of order)")
        overlap = min(end, prev_end) - max(start, prev_start)
        if overlap > 0:
            problems.append(f"line {n}: overlaps the previous cue by {overlap} ms")
    cues.append((start, end))
print(f"{len(cues)} cues:", "valid" if not problems else f"{len(problems)} problem{'s' * (len(problems) != 1)}")
for p in problems:
    print("  " + p)

Run against this file, it prints: 4 cues: valid