To validate credit card numbers using regular expressions in Java, you can use the following regular expression pattern:

String creditCardPattern = "^(?:4[0-9]{12}(?:[0-9]{3})?|5[1-5][0-9]{14}|6(?:011|5[0-9][0-9])[0-9]{12}|3[47][0-9]{13})$";

This regular expression pattern matches most of the major credit card companies, including Visa, Mastercard, American Express, and Discover. It also accounts for different lengths of credit card numbers for each company, as well as any additional digits (such as a CVV code) that may be included.

To use this regular expression to validate a credit card number, you can use the following code:

String creditCardNumber = "4111 1111 1111 1111";
creditCardNumber = creditCardNumber.replaceAll("[^0-9]+", "");
if (creditCardNumber.matches(creditCardPattern)) {
    System.out.println("Valid credit card number: " + creditCardNumber);
} else {
    System.out.println("Invalid credit card number: " + creditCardNumber);
}

In the example above, the "creditCardNumber" string is first cleaned by removing any non-numeric characters using the "replaceAll()" method. This is necessary because the regular expression pattern only matches digits. Then the "creditCardNumber" is checked if it matches the "creditCardPattern" regular expression. If it does, it is considered a valid credit card number.

Note that while this regular expression pattern is a good starting point for validating credit card numbers, it may not catch all invalid credit card numbers. It's always a good idea to double-check any credit card numbers entered by a user, and to use additional validation methods (such as a checksum algorithm) to ensure the number is valid.