1. var a = `<p class="active aa"><img /></p>`;
    3. a = addClass(a, "bbb");
    5. function addClass(el, clas) {
    6.   return el.replace(/<p\s*(?:(?:class=["]([^]*?)["])|())\s*>/g, (match, $1) => {
    7.     if ($1) {
    8.       return `<p class="${$1} ${clas}">`;
    9.     } else {
    10.       return `<p class="bbb">`;
    11.     }
    12.   });
    13. }
    15. 转换结果 <p class="active aa bbb"><img /></p>