create a regular expression to validate the format of a comma-separated list of email addresses.

gistfile1.txt

\w+@\w+\.\w+(,\s*\w+@\w+\.\w+)*

from: https://dzone.com/articles/regular-expression-to-validate-a-comma-separated-l?edition=206671&utm_source=Daily%20Digest&utm_medium=email&utm_campaign=dd%202016-08-27

// Compile pattern
Pattern emailAddressPattern = Pattern.compile(String.format("%1$s(,\s_%1$s)_", "\w+@\w+.\w+"));
// Validate addresses
System.out.println(emailAddressPattern.matcher("xyz").matches()); // false
System.out.println(emailAddressPattern.matcher("foo@bar.com").matches()); // true
System.out.println(emailAddressPattern.matcher("foo@bar.com, xyz").matches()); // false
System.out.println(emailAddressPattern.matcher("foo@bar.com, foo@bar.com").matches()); // true