Java Regular Expressions Examples

Introduction to Java Regular Expressions

Regular expressions (regex) are powerful tools in programming that allow developers to match strings of text, such as specific characters, words, or patterns. In Java, regular expressions are implemented through the java.util.regex package, which provides classes for pattern matching with regular expressions. This article presents diverse examples of Java regular expressions to help you understand their applications in real-world scenarios.

Example 1: Email Validation

Use Case

Validating email addresses is a common requirement in many applications. A regular expression can help determine if an email address has the correct format.

import java.util.regex.*;

public class EmailValidator {
    public static void main(String[] args) {
        String email = "example@test.com";
        String regex = "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$";

        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(email);

        if (matcher.matches()) {
            System.out.println("Valid email address.");
        } else {
            System.out.println("Invalid email address.");
        }
    }
}

Notes

• The regex pattern checks for a valid format, including characters before and after the @ symbol and a domain.
• Variations can include stricter rules for domain names or specific TLDs.

Example 2: Phone Number Formatting

Use Case

When dealing with user inputs, ensuring consistent phone number formats is essential. This regex example formats a phone number to a standard format.

import java.util.regex.*;

public class PhoneNumberFormatter {
    public static void main(String[] args) {
        String phone = "1234567890";
        String regex = "^(\d{3})(\d{3})(\d{4})$";
        String formatted;

        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(phone);

        if (matcher.matches()) {
            formatted = String.format("(%s) %s-%s", matcher.group(1), matcher.group(2), matcher.group(3));
            System.out.println("Formatted phone number: " + formatted);
        } else {
            System.out.println("Invalid phone number.");
        }
    }
}

Notes

• This regex captures three groups: area code, central office code, and line number.
• Users can modify the regex and output format based on regional standards.

Example 3: Password Strength Checker

Use Case

Ensuring strong passwords is crucial for security. This regex checks if a password meets specific criteria, such as length and character diversity.

import java.util.regex.*;

public class PasswordStrengthChecker {
    public static void main(String[] args) {
        String password = "P@ssw0rd123";
        String regex = "^(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z])(?=.*[!@#$%^&*])(?=.{8,})";

        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(password);

        if (matcher.matches()) {
            System.out.println("Strong password.");
        } else {
            System.out.println("Weak password.");
        }
    }
}

Notes

• The regex checks for at least one digit, one lowercase letter, one uppercase letter, one special character, and a minimum length of 8 characters.
• Developers may adjust the criteria to fit their security policies.