Java Regex Concepts in Java

Introduction

Regex (Regular Expression) is a sequence of characters used to search, match, validate, or manipulate text based on a specific pattern.

Java provides the Pattern and Matcher classes in the java.util.regex package.

1. Regex Syntax

Basic syntax:

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

Example:

Pattern pattern = Pattern.compile("Java");
Matcher matcher = pattern.matcher("I am learning Java");

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

Output:

true

2. Common Regex Patterns

3. Check Digits

String input = "12345";

System.out.println(input.matches("\\d+"));

Output:

true

4. Check Alphabetic Characters

String input = "Java";

System.out.println(input.matches("[a-zA-Z]+"));

Output:

true

5. Validate Mobile Number

Example for a 10-digit number:

String mobile = "9876543210";

System.out.println(mobile.matches("\\d{10}"));

Output:

true

6. Validate Email

String email = "user@gmail.com";

String regex = "^[A-Za-z0-9+_.-]+@[A-Za-z0-9.-]+$";

System.out.println(email.matches(regex));

Output:

true

7. Using Pattern and Matcher

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegexExample {
    public static void main(String[] args) {

        String text = "Java is powerful";

        Pattern pattern = Pattern.compile("Java");
        Matcher matcher = pattern.matcher(text);

        if (matcher.find()) {
            System.out.println("Pattern found");
        }
    }
}

Output:

Pattern found

8. Useful Regex Methods

matches()

Checks whether the complete string matches the pattern.

System.out.println("12345".matches("\\d+"));

find()

Searches for the pattern inside a string.

Matcher matcher = Pattern.compile("Java")
        .matcher("I am learning Java");

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

replaceAll()

Replaces matching characters.

String result = "Java123".replaceAll("\\d", "");

System.out.println(result);

Output:

Java

9. Common Regex Examples

\\d+              → One or more digits
[a-zA-Z]+         → Only alphabets
\\d{10}            → 10 digits
^[0-9]+$           → Only numbers
^[A-Z].*           → Starts with uppercase letter
\\s+               → One or more spaces

Conclusion

Java Regex is useful for pattern matching, text searching, input validation, and text replacement.

The main classes are:

• Pattern → Defines the regex pattern

• Matcher → Finds and matches the pattern

• matches() → Checks the complete string

• find() → Searches for a pattern

• replaceAll() → Replaces matching text