Cookbooks
Java
Chapter 2. Strings
Using regular expressions

Java:
Using regular expressions

How to:

Java’s built-in support for regex is primarily through the Pattern and Matcher classes in the java.util.regex package. Here’s a simple example to find and print all occurrences of a word in a string, case insensitive:

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

public class RegexExample {
    public static void main(String[] args) {
        String text = "Regex is great for parsing. Parsing with regex is powerful.";
        String wordToFind = "parsing";
        
        Pattern pattern = Pattern.compile(wordToFind, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(text);
        
        while (matcher.find()) {
            System.out.println("Found '" + matcher.group() + "' at position " + matcher.start());
        }
    }
}

Output:

Found 'parsing' at position 16
Found 'Parsing' at position 31

For tasks like splitting strings, you can use the String class’s split() method with a regex:

public class SplitExample {
    public static void main(String[] args) {
        String text = "Java,Python,Ruby,JavaScript";
        String[] languages = text.split(",");
        
        for (String language : languages) {
            System.out.println(language);
        }
    }
}

Output:

Java
Python
Ruby
JavaScript

When working with regex in Java, there might be cases where an external library can simplify complex tasks. One of the popular third-party libraries for working with regex in Java is Apache Commons Lang. It offers utilities like StringUtils that make some regex tasks more straightforward. Here’s how to use it to count matches of a substring:

import org.apache.commons.lang3.StringUtils;

public class CommonsLangExample {
    public static void main(String[] args) {
        String text = "Regex makes text processing easier. Processing text with regex is efficient.";
        String substring = "processing";
        
        int count = StringUtils.countMatches(text, substring);
        System.out.println("'" + substring + "' appears " + count + " times.");
    }
}

To use Apache Commons Lang, you need to include it in your project. If you’re using Maven, add this dependency to your pom.xml:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.12.0</version> <!-- Check for the latest version -->
</dependency>

Output:

'processing' appears 2 times.
Searching and replacing text