Regex Character Classes in Java

Regular Expressions (Regex) in Java are used to search, match, and manipulate text based on defined patterns. Character classes are an important part of regex that allow a pattern to match one character from a specified set or range of characters.

• Match characters from a defined set, such as [abc] or [0-9].
• Simplify patterns by representing multiple possible characters concisely.
• Help validate structured input such as usernames, phone numbers, and passwords.

public class GFG{
    
    public static void main(String[] args){
        
        String input = "abc123";
        System.out.println(input.matches(".*[a-z]+.*")); // true

    }
}

true

Explanation:

• .* allows any characters before and after the pattern.
• [a-z]+ checks that the string contains at least one lowercase letter, so "abc123" matches and returns true.

Syntax

[characters]

Types of Character Classes in Java Regex

1. Simple Character Class

A simple character class matches exactly one character from the given set inside square brackets. It is used when you want to allow multiple specific characters at a single position

public class GFG{
    
    public static void main(String[] args){
        
        String regex = "[abc]";

        System.out.println("a".matches(regex));
        System.out.println("b".matches(regex)); 
        System.out.println("d".matches(regex)); 
    }
}

true
true
false

Explanation:

• The regex "[abc]" allows only one character, and it must be either a, b, or c, which is why "a" and "b" return true.
• The string "d" does not belong to the defined character set in the regex, so "d".matches(regex) returns false.

2. Range Character Class

Range Character Class matches one character within a specified range using a hyphen (-). It is commonly used to validate digits, lowercase letters, or uppercase letters efficiently.

public class Main {
    public static void main(String[] args) {
        String regex = "[0-9]";

        System.out.println("5".matches(regex)); 
        System.out.println("a".matches(regex)); 
    }
}

true
false

Explanation:

• The regex "[0-9]" matches any single digit from 0 to 9, so "5" returns true.
• The character "a" is outside the numeric range, so the match returns false.

3. Multiple Ranges in a Character Class

Multiple Range Character Class allows combining several character ranges in one set. This is useful when validating inputs that can contain letters and digits together.

public class Main {
    public static void main(String[] args) {
        String regex = "[a-zA-Z0-9]";

        System.out.println("A".matches(regex)); 
        System.out.println("7".matches(regex)); 
        System.out.println("@".matches(regex)); 
    }
}

true
true
false

Explanation:

• The regex "[a-zA-Z0-9]" accepts uppercase letters, lowercase letters, or digits, so "A" and "7" match successfully.
• The character "@" is not included in any specified range, so the match returns false.

4. Negated Character Class

A Negated Character Class is used to match any single character except the ones specified inside the brackets. It is identified by the caret (^) placed at the beginning of the character set.

public class Main {
    public static void main(String[] args) {
        String regex = "[^0-9]";

        System.out.println("a".matches(regex)); 
        System.out.println("5".matches(regex)); 
    }
}

true
false

Explanation:

• The regex [^0-9] matches any non-digit character, so "a" returns true.
• The character "5" is a digit and is explicitly excluded, so the match returns false.

5. Predefined Character Classes

Java provides predefined character classes for commonly used patterns.

Quantifiers with Character Classes

Quantifiers are used to specify how many times a character from a defined set or range should appear. They make regex patterns more powerful for validating repeated characters like digits, letters, or symbols.

Note:

• Quantifiers apply to the preceding regex element or atom, such as a character, character class, or group.
• [0-9]{3} → Matches exactly three digits.
• -? → Matches an optional hyphen (-), meaning zero or one occurrence.