Here is an example of a regular expression (regex) for email validation using JavaScript:

const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;

This regex checks whether a string is a valid email address or not. Here’s a breakdown of how it works:

• ^ indicates the start of the string.
• [^\s@]+ matches one or more characters that are not whitespace or @.
• @ matches the @ character.
• [^\s@]+ matches one or more characters that are not whitespace or @.
• \. matches a literal dot (.).
• [^\s@]+ matches one or more characters that are not whitespace or @.
• $ indicates the end of the string.

In plain English, this regex checks whether the string starts with one or more characters that are not whitespace or @, followed by an @ character, followed by one or more characters that are not whitespace or @, followed by a dot (.), followed by one or more characters that are not whitespace or @, and ends with one or more characters that are not whitespace or @.

You can use this regex with the test() method to check whether a string is a valid email address or not:

const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
const email = 'john.doe@example.com';
const isValidEmail = emailRegex.test(email);
console.log(isValidEmail); // true

In this example, we create a regex emailRegex and a string email that contains a valid email address. We then use the test() method of the emailRegex object to check whether the email string is a valid email address or not. Finally, we log the result to the console (true, in this case).