kovalenko3331,
var value ="<iframe src='http://sait.ru/embed/1234567891123/video' frameborder='0' width='607' height='360' scrolling='no' allowfullscreen></iframe>" ;
value = value.replace(/<iframe.*?src=.*?(\d+).*?<\/iframe>/gim, 'http://sait.com/video/$1/index.m3u8');
alert(value)