Simple regex-based strip_tags in Javascript

striptags.js

/*
regex example(strips 'i' and 'em' tags): /(<(?!\/?em|\/?i).*?>)/ig
*/

/**
 * @param string html
 * @param string allowed_tags       space-separated allowed tags
 */
function strip_tags(html, allowed_tags){
    allowed_tags = allowed_tags.trim()
    
    if (allowed_tags) {
        allowed_tags = allowed_tags.split(/\s+/).map(function(tag){ return "/?" + tag });
        allowed_tags = "(?!" + allowed_tags.join("|") + ")";
    }

    return html.replace(new RegExp("(<" + allowed_tags + ".*?>)", "gi"), "");
}

var text = "<h1>Title</h1><p>Paragraph with <strong>bold</strong> and <em>italic</em> text. <a href='#'>A link</a></p>";

console.log(strip_tags(text, "p strong em"));
// "Title<p>Paragraph with <strong>bold</strong> and <em>italic</em> text. A link</p>"

console.log(strip_tags(text, "h1 p a"));
// "<h1>Title</h1><p>Paragraph with bold and italic text. <a href='#'>A link</a></p>"

console.log(strip_tags(text, "em"));
// "TitleParagraph with bold and <em>italic</em> text. A link"

console.log(strip_tags(text, ""));
// "TitleParagraph with bold and italic text. A link"