Java Regular Expressions
What is a Regular Expression in Java?
A Regular Expression (Regex) is a sequence of characters that defines a search pattern. It is widely used for text processing, including searching, validating, extracting, and replacing data within strings.
In Java, regular expressions are handled using the java.util.regex package, which provides powerful tools for pattern matching.
Java Regex Classes
The java.util.regex package includes three key classes:
- Pattern → Compiles a regular expression into a reusable pattern
- Matcher → Performs match operations on input text
- PatternSyntaxException → Handles invalid regex patterns
Basic Example: Pattern Matching in Java
Example: Check if an Email Pattern Exists
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegexExample {
public static void main(String[] args) {
String text = "Contact us at support@example.com";
// Regex pattern for a simple email
String regex = "\\w+@\\w+\\.\\w+";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(text);
if (matcher.find()) {
System.out.println("Email found: " + matcher.group());
} else {
System.out.println("No email found");
}
}
}
Explanation
- Pattern.compile() → Compiles the regex pattern
- matcher() → Applies the pattern to the input string
- find() → Checks if a match exists
- group() → Retrieves the matched substring
Using Flags in Regex
Flags modify how pattern matching behaves.
Commonly Used Flags
Example: Case-Insensitive Search
import java.util.regex.*;
public class CaseInsensitiveDemo {
public static void main(String[] args) {
String input = "Java Programming";
Pattern pattern = Pattern.compile("java", Pattern.CASE_INSENSITIVE);
Matcher matcher = pattern.matcher(input);
System.out.println("Match found: " + matcher.find());
}
}
Common Regex Patterns
Character Classes (Brackets)
Example: Extract Digits
import java.util.regex.*;
public class DigitExtractor {
public static void main(String[] args) {
String data = "Order ID: 4589";
Pattern pattern = Pattern.compile("[0-9]+");
Matcher matcher = pattern.matcher(data);
while (matcher.find()) {
System.out.println("Number found: " + matcher.group());
}
}
}
Metacharacters in Regex
Metacharacters have special meanings in regex patterns:
Example: Validate a Phone Number
import java.util.regex.*;
public class PhoneValidator {
public static void main(String[] args) {
String phone = "9876543210";
String regex = "^[0-9]{10}$";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(phone);
System.out.println("Valid phone number: " + matcher.matches());
}
}
Quantifiers in Regex
Quantifiers define how many times a pattern should occur.
Example: Matching Repeated Characters
import java.util.regex.*;
public class QuantifierExample {
public static void main(String[] args) {
String input = "aaab";
Pattern pattern = Pattern.compile("a+");
Matcher matcher = pattern.matcher(input);
while (matcher.find()) {
System.out.println("Match: " + matcher.group());
}
}
}
