Validate a simple C-style ASCII identifier.

C: Check the first character separately from subsequent characters and reject empty input.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <stdint.h>
#include <stdbool.h>
#include <errno.h>
#include <limits.h>
#include <math.h>
#include <time.h>

int main(void) {
    const char *text="user_42";
    bool valid=(text[0]=='_'||isalpha((unsigned char)text[0]));
    for (size_t i=1;valid && text[i]!='\0';i++) valid=(text[i]=='_'||isalnum((unsigned char)text[i]));
    puts(valid?"valid":"invalid");
    return 0;
}

Expected output EXAMPLE

valid

Details

Save as snippet.c; compile with gcc -std=c17 -Wall -Wextra snippet.c -lm -o snippet, then run ./snippet. Check the first character separately from subsequent characters and reject empty input.

Keep in mind

The default C locale makes this an ASCII-oriented example; it is not a Unicode identifier validator.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:02:40 UTC · gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 · gcc 4:13.2.0-7ubuntu1

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: gcc -std=c17 -Wall -Wextra -Werror snippet.c -lm -o snippet && ./snippet
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 6a321a4a84bf9a0b3c6ccd080f1e6c3ae53e898e6dbd2a5723721ced71ba79fe
Dependencies: gcc 4:13.2.0-7ubuntu1

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

valid

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