Java RegEx

RegEx Components

Pattern pattern = Pattern.compile("\\d+");

Matcher matcher = pattern.matcher(text);

. * + ? ^ $ | [] {} ()

\\d \\w \\s [a-z] [0-9]

🔹 Basic Pattern Matching

Simple examples of pattern matching in Java:

import java.util.regex.*;

public class BasicRegEx {
    public static void main(String[] args) {
        String text = "The price is $25.99";
        
        // Check if text contains digits
        boolean hasDigits = Pattern.matches(".*\\d.*", text);
        System.out.println("Has digits: " + hasDigits);
        
        // Find all numbers
        Pattern numberPattern = Pattern.compile("\\d+\\.?\\d*");
        Matcher matcher = numberPattern.matcher(text);
        
        while (matcher.find()) {
            System.out.println("Found number: " + matcher.group());
        }
        
        // Validate email format
        String email = "[email protected]";
        String emailRegex = "^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$";
        boolean validEmail = Pattern.matches(emailRegex, email);
        System.out.println("Valid email: " + validEmail);
    }
}

🔹 Common RegEx Patterns

Useful regular expression patterns for validation:

import java.util.regex.Pattern;

public class CommonPatterns {
    
    // Phone number validation (US format)
    public static boolean isValidPhone(String phone) {
        String phoneRegex = "^\$$\\d{3}\$$\\s\\d{3}-\\d{4}$";
        return Pattern.matches(phoneRegex, phone);
    }
    
    // Password strength check
    public static boolean isStrongPassword(String password) {
        // At least 8 chars, 1 uppercase, 1 lowercase, 1 digit
        String passwordRegex = "^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d).{8,}$";
        return Pattern.matches(passwordRegex, password);
    }
    
    // Extract words from text
    public static void extractWords(String text) {
        Pattern wordPattern = Pattern.compile("\\b\\w+\\b");
        Matcher matcher = wordPattern.matcher(text);
        
        System.out.println("Words found:");
        while (matcher.find()) {
            System.out.println("- " + matcher.group());
        }
    }
    
    public static void main(String[] args) {
        // Test phone validation
        System.out.println(isValidPhone("(123) 456-7890")); // true
        System.out.println(isValidPhone("123-456-7890"));   // false
        
        // Test password strength
        System.out.println(isStrongPassword("Password123")); // true
        System.out.println(isStrongPassword("password"));    // false
        
        // Extract words
        extractWords("Hello, World! How are you?");
    }
}

🔹 String Methods with RegEx

Using regular expressions with String methods:

public class StringRegEx {
    public static void main(String[] args) {
        String text = "Java is great! Java is powerful. Java is fun.";
        
        // Split string using regex
        String[] sentences = text.split("\\.");
        System.out.println("Sentences:");
        for (String sentence : sentences) {
            System.out.println("- " + sentence.trim());
        }
        
        // Replace using regex
        String replaced = text.replaceAll("Java", "Python");
        System.out.println("Replaced: " + replaced);
        
        // Replace first occurrence only
        String replacedFirst = text.replaceFirst("Java", "C++");
        System.out.println("First replaced: " + replacedFirst);
        
        // Check if string matches pattern
        String number = "12345";
        boolean isNumber = number.matches("\\d+");
        System.out.println("Is number: " + isNumber);
        
        // Remove all non-alphabetic characters
        String messy = "H3ll0 W0rld!@#";
        String clean = messy.replaceAll("[^a-zA-Z\\s]", "");
        System.out.println("Cleaned: " + clean);
    }
}