Regex : /^(\d{2}:\d{2}:\d{2}[.,]\d{3})\s-->\s(\d{2}:\d{2}:\d{2}[.,]\d{3})\n(.*(?:\r?\n(?!\r?\n).*)*)/gm
Demo : https://regex101.com/r/TH1JhU/1
Code example :
const regex = /^(\d{2}:\d{2}:\d{2}[.,]\d{3})\s-->\s(\d{2}:\d{2}:\d{2}[.,]\d{3})\n(.*(?:\r?\n(?!\r?\n).*)*)/gm;
const str = `WEBVTT
00:00:00.000 --> 00:00:04.469
Blah Bla bla 1
Multine