Regex (Regular Expressions) in Java

Regex (Regular Expressions) in Java

Java provides java.util.regex package with two main classes:

  • Pattern → Compiles the regex.
  • Matcher → Used to match patterns against text.

Basic Example

import java.util.regex.*;

public class RegexExample {
    public static void main(String[] args) {
        String text = "My email is test@example.com";
        String regex = "\\w+@\\w+\\.\\w+";  // Simple email regex

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

        if (matcher.find()) {
            System.out.println("Found: " + matcher.group());
        } else {
            System.out.println("No match found.");
        }
    }
}

✅ Output:

Found: test@example.com

Common Regex Patterns in Java

TaskRegexExample Match
Digits only\\d+1234
Letters only[a-zA-Z]+Hello
Alphanumeric[a-zA-Z0-9]+User123
Whitespace\\s+space, tab
Word\\w+hello_123
Email (basic)\\w+@\\w+\\.\\w+abc@xyz.com
Phone (10 digits)\\d{10}9876543210
URL (basic)https?://\\S+http://site.com

Checking Full Match

import java.util.regex.*;

public class FullMatchExample {
    public static void main(String[] args) {
        String regex = "\\d{4}";  // exactly 4 digits
        String input = "2025";

        boolean isMatch = Pattern.matches(regex, input);
        System.out.println("Match? " + isMatch);
    }
}

✅ Output:

Match? true

Splitting Strings with Regex

public class SplitExample {
    public static void main(String[] args) {
        String text = "apple,banana;orange grape";
        String[] parts = text.split("[,;\\s]+"); // split by comma, semicolon, or space

        for (String word : parts) {
            System.out.println(word);
        }
    }
}

✅ Output:

apple
banana
orange
grape

Replacing Text with Regex

public class ReplaceExample {
    public static void main(String[] args) {
        String text = "The price is $100.";
        String result = text.replaceAll("\\$\\d+", "$XXX"); // hide price
        System.out.println(result);
    }
}

✅ Output:

The price is $XXX.

⚡ Notes:

  • In Java strings, backslashes must be escaped.
    • Regex \d → Java string "\\d".
  • Use Pattern.CASE_INSENSITIVE flag for case-insensitive matches: Pattern pattern = Pattern.compile("hello", Pattern.CASE_INSENSITIVE);

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