To validate International Standard Book Number (ISBN) codes using regular expressions in Java, you can use the following regular expression pattern:

String isbnPattern = "^(?:(?:\\d{10})|(?:(?:\\d{9}[Xx])|(?:\\d{13})))$";

This regular expression pattern matches ISBN codes in three formats:

• 10-digit ISBN codes (with or without dashes)
• 9-digit ISBN codes with an "X" or "x" check digit (with or without dashes)
• 13-digit ISBN codes (with or without dashes)

To use this regular expression to validate an ISBN code, you can use the following code:

String isbnCode = "978-0-306-40615-7";
if (isbnCode.replaceAll("-", "").matches(isbnPattern)) {
    System.out.println("Valid ISBN code: " + isbnCode);
} else {
    System.out.println("Invalid ISBN code: " + isbnCode);
}

In the example above, the "isbnCode" string is checked if it matches the "isbnPattern" regular expression after removing any dashes. If it does, it is considered a valid ISBN code.

Note that while this regular expression pattern is a good starting point for validating ISBN codes, it may not catch all invalid codes. It's always a good idea to double-check any ISBN codes entered by a user, and to use additional validation methods (such as checking the code against a database of valid ISBN codes) to ensure the code is valid.