Regular Expressions

1import java.util.regex.*;
2
3// Simple pattern matching with String.matches()
4String email = "user@example.com";
5boolean valid = email.matches("[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}");
6System.out.println("Valid email: " + valid);  // true
7
8// Phone number validation
9String phone = "123-456-7890";
10boolean validPhone = phone.matches("\\d{3}-\\d{3}-\\d{4}");
11System.out.println("Valid phone: " + validPhone);  // true
12
13// Check if string contains only letters
14String name = "John";
15boolean onlyLetters = name.matches("[a-zA-Z]+");
16System.out.println("Only letters: " + onlyLetters);  // true
17
18// Check if string contains digits
19String text = "Hello123";
20boolean hasDigits = text.matches(".*\\d.*");
21System.out.println("Has digits: " + hasDigits);  // true

1// Pattern and Matcher for complex operations
2String text = "Email me at john@example.com or jane@test.org";
3String regex = "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}";
4
5Pattern pattern = Pattern.compile(regex);
6Matcher matcher = pattern.matcher(text);
7
8// Find all matches
9while (matcher.find()) {
10    System.out.println("Found: " + matcher.group());
11    System.out.println("At position: " + matcher.start() + "-" + matcher.end());
12}
13// Found: john@example.com
14// Found: jane@test.org
15
16// Check if entire string matches
17String email = "test@example.com";
18boolean matches = Pattern.matches(regex, email);
19System.out.println("Full match: " + matches);  // true

1// Groups and capturing
2String date = "2024-12-25";
3Pattern pattern = Pattern.compile("(\\d{4})-(\\d{2})-(\\d{2})");
4Matcher matcher = pattern.matcher(date);
5
6if (matcher.matches()) {
7    System.out.println("Year: " + matcher.group(1));   // 2024
8    System.out.println("Month: " + matcher.group(2));  // 12
9    System.out.println("Day: " + matcher.group(3));    // 25
10    System.out.println("Full: " + matcher.group(0));   // 2024-12-25
11}
12
13// Named groups (Java 7+)
14String regex = "(?<year>\\d{4})-(?<month>\\d{2})-(?<day>\\d{2})";
15Pattern p = Pattern.compile(regex);
16Matcher m = p.matcher("2024-12-25");
17
18if (m.matches()) {
19    System.out.println("Year: " + m.group("year"));
20    System.out.println("Month: " + m.group("month"));
21    System.out.println("Day: " + m.group("day"));
22}

1// Replacement operations
2String text = "Hello World! Hello Java!";
3
4// Replace all occurrences
5String result1 = text.replaceAll("Hello", "Hi");
6System.out.println(result1);  // "Hi World! Hi Java!"
7
8// Replace first occurrence
9String result2 = text.replaceFirst("Hello", "Hi");
10System.out.println(result2);  // "Hi World! Hello Java!"
11
12// Replace with groups (back-reference)
13String numbers = "123-456-7890";
14String formatted = numbers.replaceAll("(\\d{3})-(\\d{3})-(\\d{4})", "($1) $2-$3");
15System.out.println(formatted);  // "(123) 456-7890"
16
17// Split with regex
18String csv = "apple, banana,  cherry,   date";
19String[] fruits = csv.split(",\\s*");  // Split on comma + optional spaces
20// ["apple", "banana", "cherry", "date"]
21
22// Common patterns
23String ALPHANUMERIC = "^[a-zA-Z0-9]+$";
24String IP_ADDRESS = "^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$";
25String URL = "^(https?://)?[\\w.-]+\\.[a-zA-Z]{2,}(/\\S*)?$";