String Methods Deep Dive

public class RegexMatch {
    public static void main(String[] args) {
        String email = "user@test.com";
        String phone = "9876543210";
        String zipCode = "560001";

        // matches() checks if ENTIRE string matches the regex
        System.out.println(email.matches(".*@.*\\..*"));      // true
        System.out.println(phone.matches("\\d{10}"));          // true
        System.out.println(zipCode.matches("\\d{6}"));         // true

        // Practical validations
        String password = "Test@123";
        boolean hasUpper = password.matches(".*[A-Z].*");
        boolean hasDigit = password.matches(".*\\d.*");
        boolean hasSpecial = password.matches(".*[@#$%^&+=].*");
        boolean isLongEnough = password.length() >= 8;

        System.out.println("Strong password: " + (hasUpper && hasDigit && hasSpecial && isLongEnough));
    }
}

public class RegexReplace {
    public static void main(String[] args) {
        // Remove all non-numeric characters
        String price = "$1,299.50";
        String cleaned = price.replaceAll("[^0-9.]", "");
        System.out.println(cleaned);  // 1299.50

        // Remove extra whitespace
        String messy = "  Login    Test   Suite  ";
        String clean = messy.trim().replaceAll("\\s+", " ");
        System.out.println(clean);  // "Login Test Suite"

        // Mask sensitive data
        String card = "4111-2222-3333-4444";
        String masked = card.replaceAll("\\d{4}-\\d{4}-\\d{4}", "****-****-****");
        System.out.println(masked);  // ****-****-****-4444
    }
}

public class FormatJoin {
    public static void main(String[] args) {
        // String.format — same as printf but returns a String
        String report = String.format("Test: %s | Status: %s | Time: %.2fs",
                "LoginTest", "PASS", 2.456);
        System.out.println(report);

        // String.join — concatenate with a delimiter
        String csv = String.join(",", "Chrome", "Firefox", "Safari");
        System.out.println(csv);  // Chrome,Firefox,Safari

        // Join an array
        String[] tags = {"smoke", "regression", "critical"};
        String tagString = String.join(" | ", tags);
        System.out.println(tagString);  // smoke | regression | critical
    }
}

public class RealExtractions {
    public static void main(String[] args) {
        // Extract order number from confirmation text
        String confirmation = "Your order #ORD-12345 has been placed successfully";
        int start = confirmation.indexOf("#") + 1;
        int end = confirmation.indexOf(" ", start);
        String orderNumber = confirmation.substring(start, end);
        System.out.println("Order: " + orderNumber);  // ORD-12345

        // Extract all numbers from a mixed string
        String text = "Page 3 of 25 (247 results)";
        String[] numbers = text.replaceAll("[^0-9 ]", "").trim().split("\\s+");
        System.out.println("Current page: " + numbers[0]);   // 3
        System.out.println("Total pages: " + numbers[1]);    // 25
        System.out.println("Total results: " + numbers[2]);  // 247

        // Clean element text for comparison
        String elementText = "  \n  Welcome, John!  \n  ";
        String cleaned = elementText.trim().replaceAll("\\s+", " ");
        System.out.println("\"" + cleaned + "\"");  // "Welcome, John!"

        // URL parameter extraction
        String url = "https://example.com/search?query=laptop&page=2&sort=price";
        String queryString = url.substring(url.indexOf("?") + 1);
        String[] params = queryString.split("&");
        for (String param : params) {
            String[] kv = param.split("=");
            System.out.println(kv[0] + " → " + kv[1]);
        }
    }
}

Order: ORD-12345
Current page: 3
Total pages: 25
Total results: 247
"Welcome, John!"
query → laptop
page → 2
sort → price