const regex = /<\/body>(?![\s\S]*<\/body>[\s\S]*$)/i;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('<\\\/body>(?![\\s\\S]*<\\\/body>[\\s\\S]*$)', 'i')
const str = `<html>
<body>
</body>
</html>
<html>
<body>
</body>
</html>`;
const subst = `<tag/>`;
// 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