You can extract a URL from a string using the following RegExp:

/(https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*))/

So to get the URL from that string:

const ptr = '<iframe allowFullScreen frameborder="0" height="564" mozallowfullscreen src="https://player.vimeo.com/video/253266360" webkitAllowFullScreen width="640"></iframe>'

const url = /(https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*))/.exec(ptr)[0];

When using RegExp.prototype.exec() you, the first element in the returned array is the full string that was matched, in this case https://player.vimeo.com/video/253266360. You can also remove the [0] from the end of the url expression to get the full returned array in case you need other information about the match.

See MDN Docs for more details on the RegExp.exec function.

Perhaps you can leverage the DOM API and do like;

var div = document.createElement("div"),
    src;

div.innerHTML = "<iframe allowFullScreen frameborder=\"0\" height=\"564\" mozallowfullscreen src=\"https://player.vimeo.com/video/253266360\" webkitAllowFullScreen width=\"640\"></iframe>";
src = div.firstChild.src;

console.log(src);