const regex = new RegExp('<img[^>]*alt="([^"]+)"[^>]*>', 'gm')
const str = `<span itemprop="description"> <img src="//res.smzdm.com/images/emotions/30.png" alt="[小怒]" class="wp-smiley"> <img src="//res.smzdm.com/images/emotions/30.png" alt="[小怒]" class="wp-smiley"> <img src="//res.smzdm.com/images/emotions/30.png" alt="[小怒]" class="wp-smiley"> 这文章百度哪里搬来的 </span>
<span itemprop="description"> <img src="//res.smzdm.com/images/emotions/36.png" alt="[喜极而泣]" class="wp-smiley"> 年少无知看了那篇文章好奇买了4瓶 喝了一口差点吐出来 </span>
<span itemprop="description">国内咋就没厂商做点纯气泡水呢，成本应该不高吧，钱都让屈臣氏巴黎水赚去了 <img src="//res.smzdm.com/images/emotions/23.png" alt="[皱眉]" class="wp-smiley"> </span>
<span itemprop="description">小时候就经常煲来喝，利尿排毒 <img src="//res.smzdm.com/images/emotions/45.png" alt="[抽烟]" class="wp-smiley"> </span>
<p class="p_content_166651051"><span itemprop="description">我喜欢香草味的，滑滑的感觉 </span></p>
<p class="p_content_166650868"><span itemprop="description">我喝过好几箱，后来还回购了，再后来就喝不起了，太贵[皱眉] </span></p>`;
const subst = `\g<1>`;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions