Validate a complete identifier with a regex.
C#: Use absolute anchors and a timeout so the regex checks the complete input with a bounded execution time.
using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.Json;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Globalization;
using System.Security.Cryptography;
using System.Threading;
using System.Threading.Tasks;
var pattern = new Regex(@"\A[A-Za-z_][A-Za-z0-9_]*\z", RegexOptions.CultureInvariant, TimeSpan.FromSeconds(1));
Console.WriteLine(pattern.IsMatch("user_42"));
Console.WriteLine(pattern.IsMatch("42user"));Expected output EXAMPLE
True False
Details
Use Program.cs in a .NET 8 console project and run dotnet run. Uses only the .NET standard libraries. Use absolute anchors and a timeout so the regex checks the complete input with a bounded execution time.
Tested on Ubuntu 24.04.5 LTS · · 8.0.425 · libc6 2.39-0ubuntu8.9
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
dotnet build --no-restore && dotnet out/csharp.dll- Container image ID
sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd- Code SHA-256
bfafb52f8c94df6ef2a8cb7d356912c1179b2eef2dde32bdc3e59407c2b94632- Dependencies
- libc6 2.39-0ubuntu8.9
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.