? Regular Expressions

? Quick Overview

Regular Expressions (Regex) in Java provide a powerful way to search, match, validate, and manipulate text using patterns. In Advanced Java, regex is heavily used for validation, parsing logs, processing user input, and backend text handling.

? Key Concepts

• Pattern Compiled representation of a regex
• Matcher Engine that matches patterns against text
• Metacharacters Special symbols like ., *, +
• Quantifiers Control repetition
• Character Classes Sets like [a-z]

? Syntax & Theory

Java regex is provided by the java.util.regex package. The two core classes are:

• Pattern — defines the regex
• Matcher — applies the pattern to text

// Import regex classes
import java.util.regex.Pattern;
import java.util.regex.Matcher;

? Code Example — Basic Matching

// Check if a string contains only digits
String input = "12345";
String regex = "\\d+";

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

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

? Code Example — Email Validation

// Validate an email address using regex
String email = "user@example.com";
String regex = "^[\\w.-]+@[\\w.-]+\\.[a-zA-Z]{2,6}$";

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

System.out.println(matcher.matches());