check textbox text that is a valid mail or no

In PHP there is a function called preg_match[^] which returns 1 if a given string matches the provided pattern:

$email_string; // Contains the user's email address

$simple_email_pattern = "/^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$/";
$match = preg_match($simple_email_pattern, $email_string);

if ($match === 1) {
    // Valid email
}

For a better Regular Expression I would recommend taking a look at this question[^] on SO

In Java the String class offers a method called matches[^]:

String emailString = "test@test.test";
boolean emailIsValid = emailString.matches("^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\.[a-zA-Z0-9-.]+$");