Regular Expressions

Java Regular Expressions

In Java, regular expressions (regex) are sequences of characters that define a search pattern. They are widely used for string manipulation and text processing tasks such as searching, splitting, replacing, and validating strings. Java provides the java.util.regex package for working with regular expressions. Here's an overview of how to use regular expressions in Java:

1. Pattern and Matcher: The Pattern class represents a compiled regular expression, and the Matcher class is used to perform matching operations on a character sequence.

import java.util.regex.*;

public class RegexExample {
    public static void main(String[] args) {
        String text = "The quick brown fox jumps over the lazy dog";
        String patternString = "fox";

        // Compile the regular expression pattern
        Pattern pattern = Pattern.compile(patternString);

        // Create a matcher object
        Matcher matcher = pattern.matcher(text);

        // Find matches
        while (matcher.find()) {
            System.out.println("Found match '" + matcher.group() + "' at index " + matcher.start());
        }
    }
}
     

2. Matches: The matches() method of the String class checks whether the entire string matches the given regular expression.

        public class MatchesExample {
            public static void main(String[] args) {
                String text = "Hello123";
                String patternString = "[a-zA-Z]+[0-9]+";
        
                boolean isMatch = text.matches(patternString);
                System.out.println("Is there a match? " + isMatch);
            }
        }
     

3. Splitting: The split() method of the String class uses regular expressions to split a string into an array of substrings.

        public class SplitExample {
            public static void main(String[] args) {
                String text = "apple,banana,orange";
                String[] fruits = text.split(",");
        
                for (String fruit : fruits) {
                    System.out.println(fruit);
                }
            }
        }
     

4. Replacement: The replaceAll() and replaceFirst() methods of the String class use regular expressions to replace substrings in a string.

        public class ReplacementExample {
            public static void main(String[] args) {
                String text = "The cat in the hat";
                String newText = text.replaceAll("cat", "dog");
        
                System.out.println(newText);
            }
        }