不要使用正则表达式来分析/解析HTML字符串。使用DOM解析器。您可以使用DOMParser并执行以下操作：

function replaceWithDivs(html, selector) {
    const doc = new DOMParser().parseFromString(html, "text/html");
    for (const elem of doc.querySelectorAll(selector)) {
        const div = doc.createElement("div"); // Create the new element
        div.innerHTML = elem.innerHTML;       // Copy the contents
        elem.replaceWith(div);                // And replace old with new
    }
    return doc.body.innerHTML;
}

// demo
let html = `<span class="line"><span>// Before</span></span> 
<span class="line">
  <span>import</span>
<span> </span><span>db</span>
<span> </span><span>from</span><span>    </span>
  <span>'../../lib/db-admin'</span><span>;</span>
</span> 
<span class="line"><span>// After</span></span> 
<span class="line"><span>import</span>
<span> </span><span>db</span><span> </span>
<span>from</span><span> </span>
<span>'lib/db-admin'</span><span>;</span></span>`;

html = replaceWithDivs(html, "span.line");
console.log(html);