Regex to match JWT

I created a regex which might not be the most elegant but it appears to works.

(^[A-Za-z0-9-_]*\.[A-Za-z0-9-_]*\.[A-Za-z0-9-_]*$)

A more concise version could be also:

(^[\w-]*\.[\w-]*\.[\w-]*$)

However, I believe that also non-latin characters would be allowed which would be disallowed as JWT.

If you are working with an HTML document as a string and you are using Javascript to run your regular expression, you could do something like the following:

const html = '<div>stuff</div>something "contextJwt": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJIZWxsbyB5b3UiLCJuYW1lIjoiV2h5IGFyZSB5b3UgY2hlY2tpbmcgbXkgdG9rZW4_ICggzaHCsCDNnMqWIM2hwrApIiwiaWF0IjoxNTE2MjM5MDIyfQ.yAP0xiTwp6vqIYbLKLVBRv-gTyMvU17rT3H8uErLjHA" <div> other stuff</div>';
var regex = /"contextJwt":\s*"(.*)"/;
console.log(html.match(regex)[1]);

/* yields the encoded JWT string:
 eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJIZWxsbyB5b3UiLCJuYW1lIjoiV2h5IGFyZSB5b3UgY2hlY2tpbmcgbXkgdG9rZW4_ICggzaHCsCDNnMqWIM2hwrApIiwiaWF0IjoxNTE2MjM5MDIyfQ.yAP0xiTwp6vqIYbLKLVBRv-gTyMvU17rT3H8uErLjHA

*/

You can tighten up your match from the simple (.*) to the specific characters that are allowed in a valid encoded JWT (per Helio Santo's answer), but since regexes are finicky, I usually start with the simplest solution and only tighten it down when necessary.

What you do with the string that represents an encoded JWT is perhaps another question entirely.

For the records (error checking omitted for brevity) here is an alternative that doesn't use Regex

const html = `<section>
  <p>Your token is <code>"contextJwt": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJIZWxsbyB5b3UiLCJuYW1lIjoiV2h5IGFyZSB5b3UgY2hlY2tpbmcgbXkgdG9rZW4_ICggzaHCsCDNnMqWIM2hwrApIiwiaWF0IjoxNTE2MjM5MDIyfQ.yAP0xiTwp6vqIYbLKLVBRv-gTyMvU17rT3H8uErLjHA"</code>. Have a nice day.</p>
</section>`;
const fragment = document.createElement("div");
fragment.innerHTML = html;
const input = fragment.querySelector("section p code").innerHTML;
const output = JSON.parse("{" + input + "}");
console.log(output.contextJwt);