Validate a complete identifier.

Java: String.matches requires the regular expression to match the entire input.

import java.util.*;
import java.util.stream.*;
import java.util.regex.*;
import java.nio.file.*;
import java.nio.charset.StandardCharsets;
import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.*;
import java.math.*;
import java.io.*;
import java.net.*;
import java.security.MessageDigest;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        String pattern = "[A-Za-z_][A-Za-z0-9_]*";
        System.out.println("user_42".matches(pattern));
        System.out.println("42user".matches(pattern));
    }
}

Expected output EXAMPLE

true
false

Details

Save as Main.java, compile with javac Main.java, and run java Main. Uses the Java 21 standard library. String.matches requires the regular expression to match the entire input.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:01:41 UTC · openjdk 21.0.12.1 2026-08-18 · openjdk-21-jdk-headless 21.0.12.1+1-1~24.04.4

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: javac Main.java && java Main
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: c5e099d783a60577f1f17c8fc0712a14e2c6709971edf7c74b38505eda24f912
Dependencies: openjdk-21-jdk-headless 21.0.12.1+1-1~24.04.4

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

true
false

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.