Regex Character Classes in Java

Regular Expressions in Java provide a way to match, search, and manipulate text. One of the most important building blocks of regex is the Character Class, which allows you to match one character from a defined set of characters.

• Match multiple characters using a single pattern
• Reduce complex conditions
• Write shorter and more readable regex patterns
• Validate inputs like usernames, phone numbers, and passwords

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

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.

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.

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.

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 always apply to the preceding character class
• [0-9]{3} -> digit class repeated 3 times
• -? -> optional hyphen using quantifier on a character