To create a character class in Java regular expressions, you enclose the characters you want to match within square brackets []. Here are a few examples:

1. Matching a single character in a range:

   String regex = "[a-z]";
   

   This regex will match any lowercase letter from ‘a’ to ‘z’.

2. Matching multiple characters in a range:

   String regex = "[A-Za-z0-9]";
   

   This regex will match any uppercase letter, lowercase letter, or digit.

3. Matching a specific set of characters:

   String regex = "[aeiou]";
   

   This regex will match any vowel (a, e, i, o, or u).

4. Matching characters based on Unicode properties:

   String regex = "\\p{Lu}";
   

   This regex will match any uppercase letter according to Unicode properties.

5. Excluding specific characters from a set:

   String regex = "[^aeiou]";
   

   This regex will match any character that is not a vowel.

Character classes can also be combined with other regex constructs, such as quantifiers and anchors, to create more complex patterns. For example, you can use the following regex to match a string that starts with an uppercase letter followed by one or more lowercase letters:

String regex = "^[A-Z][a-z]+$";

By utilizing character classes in Java regular expressions, you can create more flexible and concise patterns to match specific characters or groups of characters. These classes are incredibly useful when dealing with string validation, pattern extraction, or data cleaning tasks in your Java applications. Mastering character classes will enhance your ability to work with regular expressions effectively.