regex to find email address from a String

The correct code is

Pattern p = Pattern.compile("\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\\.[A-Z]{2,4}\\b",
    Pattern.CASE_INSENSITIVE);
Matcher matcher = p.matcher(input);
Set<String> emails = new HashSet<String>();
while(matcher.find()) {
  emails.add(matcher.group());
}

This will give the list of mail address in your long text / html input.

You can validate e-mail address formats as according to RFC 2822, with this:

(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])

and here's an explanation from regular-expressions.info:

This regex has two parts: the part before the @, and the part after the @. There are two alternatives for the part before the @: it can either consist of a series of letters, digits and certain symbols, including one or more dots. However, dots may not appear consecutively or at the start or end of the email address. The other alternative requires the part before the @ to be enclosed in double quotes, allowing any string of ASCII characters between the quotes. Whitespace characters, double quotes and backslashes must be escaped with backslashes.

And you can check this out here: Rubular example.