parseFonts.js

const fontFaceRegex = /\@font\-face\s?\{([\s\S]+?)\}/m
const fontFamilyRegex = /font\-family\:\s?['"](.+?)['"]\;?/
const urlRegex = /url\(['"]?(.+?)['"]?\)/;

const currentHost = window.location.host;

function parseFontsFromCSSString(css) {
  const matches = css.matchAll(fontFaceRegex);

  if(!matches) {
    return
  }

  const results = {}

  for(const match of matches) {
    const rules = match[1];

    // Split our declaration by line
    const lines = rules
      .split('\n')
      .filter(v => v)
      .map(line => line.trim());

    // Find the font-family rule:
    const familyLine = lines.find(line => line.test(fontFamilyRegex);
    const family = familyLine.match(familyLine)[1];
    
    const familyUrls = [];

    // Extract the URLs from the raw lines
    for(const line of lines) {
      const urlMatches = line.matchAll(urlRegex);

      if(!urlMatches) { continue; }

      for(const urlMatch of urlMatches) {
        let url = urlMatch[1];

        // Add a host if one isn't specified
        if(url.startsWith('/')) {
          url = `${currentHost}${url}`  
        }

        familyUrls.push(url);
      }
    }

    results[family] = familyUrls;
  }

  return results;
}