How to validate an email address

Short answer

A simple regular expression checks the general shape: something, @, a domain with a dot:

void main() {
    String pattern = "^[\\w.+-]+@[\\w-]+(\\.[\\w-]+)+$";
 
    IO.println("anna@example.com".matches(pattern));
    IO.println("anna@example".matches(pattern));
    IO.println("anna example.com".matches(pattern));
}

Output:

true
false
false

A precompiled pattern

String.matches() compiles the expression again every time. If you validate many values, create the Pattern once:

static final Pattern EMAIL = Pattern.compile("^[\\w.+-]+@[\\w-]+(\\.[\\w-]+)+$");
 
void main() {
    var emails = List.of("oleg@mail.de", "bad@", "a.b+tag@site.co.uk");
    for (String email : emails) {
        IO.println(email + " -> " + EMAIL.matcher(email).matches());
    }
}

Output:

oleg@mail.de -> true
bad@ -> false
a.b+tag@site.co.uk -> true

Before validation

Users often type spaces and capital letters. Normalize the address before checking it:

void main() {
    String input = "  Anna@Example.COM ";
    String email = input.strip().toLowerCase(Locale.ROOT);
 
    IO.println(email);
}

Output:

anna@example.com

Common mistake

Trying to write a "perfect" expression. The email standard is very complex, and strict checks reject real addresses. Check the shape with a simple pattern and the existence of the address with a confirmation email. On the other hand, a too simple .+@.+ lets obvious mistakes through:

void main() {
    IO.println("a@b@c".matches(".+@.+"));
}

Output:

true

Thanks for reading!

Short answer

A precompiled pattern

Before validation

Common mistake

Functions used in this recipe

Thanks for reading!