Regular expressions? Ugh. Encode it properly as XML in the correct namespace, load it so, and take it from that.

Or just serve the SVG file and use <foreignObject> to embed the HTML, and include <link rel="icon" href=""> inside it. In theory you should be able to define a <view id="icon"> and use <link rel="icon" href="#icon">, but in practice neither Firefox nor Chromium seems to be handling that properly in a favicon, which is disappointing.