javaintermediate

Java Regex — Pattern Matching Examples

Common regex patterns with Pattern and Matcher: email, URL, phone, extraction, and replacement.

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import java.util.regex.*;
import java.util.List;
import java.util.stream.Collectors;

public class RegexDemo {
    // Compiled patterns (reuse for performance)
    static final Pattern EMAIL = Pattern.compile(
        "^[\\w.+-]+@[\\w-]+\\.[\\w.]+$");
    static final Pattern URL = Pattern.compile(
        "https?://[\\w.-]+(?:/[\\w./-]*)?");
    static final Pattern PHONE = Pattern.compile(
        "\\+?\\d{1,4}[-.\\s]?\\(?\\d{1,3}\\)?[-.\\s]?\\d{3,4}[-.\\s]?\\d{4}");

    // Validate
    static boolean isValidEmail(String email) {
        return EMAIL.matcher(email).matches();
    }

    // Extract all matches
    static List<String> extractUrls(String text) {
        return URL.matcher(text).results()
            .map(MatchResult::group)
            .toList();
    }

    // Named groups
    static void parseLog() {
        Pattern logPattern = Pattern.compile(
            "\\[(?<timestamp>[^]]+)] (?<level>\\w+) (?<message>.+)");
        String log = "[2024-01-15 10:30:00] ERROR Database connection failed";

        Matcher m = logPattern.matcher(log);
        if (m.matches()) {
            System.out.println("Time:  " + m.group("timestamp"));
            System.out.println("Level: " + m.group("level"));
            System.out.println("Msg:   " + m.group("message"));
        }
    }

    // Replace with transform
    static String maskEmails(String text) {
        return EMAIL.matcher(text).replaceAll(mr -> {
            String email = mr.group();
            int atIdx = email.indexOf('@');
            return email.charAt(0) + "***" + email.substring(atIdx);
        });
    }

    // Split with regex
    static String[] tokenize(String csv) {
        return csv.split(",\\s*");
    }

    public static void main(String[] args) {
        System.out.println(isValidEmail("alice@test.com")); // true
        System.out.println(isValidEmail("invalid@")); // false

        String text = "Visit https://example.com and http://test.org/page";
        System.out.println(extractUrls(text));

        parseLog();
    }
}