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Java Regular Expressions (RegEx)

Regular Expressions (RegEx) in Java provide a powerful way to perform pattern matching and manipulate strings. Java's java.util.regex package contains classes for working with regular expressions, primarily Pattern and Matcher.

1. Key Classes

  • Pattern: Represents a compiled regular expression.
  • Matcher: An engine that performs match operations on a character sequence.

2. Basic Usage

To use RegEx in Java, you compile a pattern and then create a matcher to perform operations.

Example: Checking if a String Matches a Pattern

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

public class RegExExample {
    public static void main(String[] args) {
        String text = "hello123";
        String patternString = "\\w+\\d+";

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

        if (matcher.matches()) {
            System.out.println("Pattern matches!");
        } else {
            System.out.println("Pattern does not match.");
        }
    }
}
// Output: Pattern matches!

3. Common Methods

  • matches(): Attempts to match the entire input sequence against the pattern.
  • find(): Attempts to find the next subsequence that matches the pattern.
  • replaceAll(): Replaces every subsequence that matches the pattern with a replacement string.

Example: Finding All Occurrences

String text = "Email me at example@test.com or admin@test.org";
String patternString = "\\b\\w+@\\w+\\.\\w+\\b";

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

while (matcher.find()) {
    System.out.println("Found email: " + matcher.group());
}
// Output:
// Found email: example@test.com
// Found email: admin@test.org

4. Predefined Character Classes

  • \d: A digit (0-9).
  • \w: A word character (letters, digits, underscore).
  • \s: A whitespace character.
  • .: Any character.

5. Quantifiers

  • *: Matches zero or more occurrences.
  • +: Matches one or more occurrences.
  • ?: Matches zero or one occurrence.
  • {n}: Matches exactly n occurrences.

6. Key Takeaways

  • Regular expressions provide powerful pattern matching capabilities.
  • Use Pattern and Matcher classes for regex operations.
  • Understand regex syntax and constructs for effective use.
  • Be cautious with special characters; use double backslashes \\ in Java strings.
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