Validate a whole ASCII identifier.
JavaScript: Use an anchored expression without the multiline flag and reject a trailing line break explicitly.
const valid = /^[A-Za-z_][A-Za-z0-9_]*$/;
console.log(valid.test("user_42") && !/[\r\n]/.test("user_42"));
console.log(valid.test("42user"));Expected output EXAMPLE
true false
Details
Save as snippet.js and run node snippet.js. Uses built-in Node.js and JavaScript APIs. Use an anchored expression without the multiline flag and reject a trailing line break explicitly.
JavaScript $ can also match before a final line terminator; the extra line-break check prevents that acceptance.
Tested on Ubuntu 24.04.5 LTS · · v18.19.1 · nodejs 18.19.1+dfsg-6ubuntu5
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
node snippet.js- Container image ID
sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd- Code SHA-256
0d76b130009f3d3c35452a28a7377d41d62e5ed2ff6f5e450f1571f057192728- Dependencies
- nodejs 18.19.1+dfsg-6ubuntu5
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.