Permission denied: The specified file could not be opened

Error message

Permission denied: The specified file could not be opened

What it means

RegExpBuilder::from_file panics with 'Permission denied: The specified file could not be opened' when std::fs::read_to_string returns an io::Error of kind PermissionDenied. The process lacks read access to the file (or a directory on its path), so the builder cannot proceed and aborts.

Solutions

Fix file permissions (chmod a+r file or chown to the running user) and retry
Run the process as a user/group that has read access to the file
Check std::fs::metadata and attempt a probe read (File::open) before calling from_file to fail with a friendlier message

Example fix

// before
let builder = RegExpBuilder::from_file("/var/secure/examples.txt");
// after
let file = std::fs::File::open("/var/secure/examples.txt")
    .map_err(|e| format!("cannot open examples file: {}", e))?;
drop(file);
let builder = RegExpBuilder::from_file("/var/secure/examples.txt");

Defensive patterns

Strategy: validation

Validate before calling

match std::fs::File::open(path) {
    Ok(_) => {},
    Err(e) if e.kind() == std::io::ErrorKind::PermissionDenied =>
        return Err(anyhow!("no read permission for {}", path.display())),
    Err(e) => return Err(e.into()),
}

Try / catch

std::panic::catch_unwind(|| RegExpBuilder::from_file(path))
    .map_err(|_| format!("permission denied reading {} — check file ownership/permissions", path.display()))

Prevention

Run the process as a user with read access to input files
Check file mode/ownership in deployment scripts before launch
Avoid 0600/root-owned files for shared input data

When it happens

Trigger: Calling from_file on a file without read permission bits for the current user; reading inside a container/CI as a non-root user; the parent directory lacking execute (traverse) permission; macOS sandbox or Windows ACLs blocking access.

Common situations: Files created by root in Docker then read by an unprivileged service user; secrets mounted with 0600 owned by another user; CI artifacts with restrictive modes.

Understand the failure class

Background: Permission denied / not authorized / 403 Forbidden: access-control rejections when the caller lacks the required role, grant, or ownership — this error's family across 18 libraries.

Related errors

AI-assisted analysis of pemistahl/grex@99cc347707 (2026-09-13). Data as JSON: /api/errors/5fc7af7270b89c12. Report an issue: GitHub.

Appendix: source

Thrown at src/builder.rs:81

    /// The final line ending is optional.
    ///
    /// ⚠ Panics if:
    /// - the file cannot be found
    /// - the file's encoding is not valid UTF-8 data
    /// - the file cannot be opened because of conflicting permissions
    pub fn from_file<T: Into<PathBuf>>(file_path: T) -> Self {
        match std::fs::read_to_string(file_path.into()) {
            Ok(file_content) => Self {
                test_cases: file_content.lines().map(|it| it.to_string()).collect_vec(),
                config: RegExpConfig::new(),
            },
            Err(error) => match error.kind() {
                ErrorKind::NotFound => panic!("The specified file could not be found"),
                ErrorKind::InvalidData => {
                    panic!("The specified file's encoding is not valid UTF-8")
                }
                ErrorKind::PermissionDenied => {
                    panic!("Permission denied: The specified file could not be opened")
                }
                _ => panic!("{}", error),
            },
        }
    }

    /// Converts any Unicode decimal digit to character class `\d`.
    ///
    /// This method takes precedence over
    /// [`with_conversion_of_words`](Self::with_conversion_of_words) if both are set.
    /// Decimal digits are converted to `\d`, the remaining word characters to `\w`.
    ///
    /// This method takes precedence over
    /// [`with_conversion_of_non_whitespace`](Self::with_conversion_of_non_whitespace) if both are set.
    /// Decimal digits are converted to `\d`, the remaining non-whitespace characters to `\S`.
    pub fn with_conversion_of_digits(&mut self) -> &mut Self {
        self.config.is_digit_converted = true;
        self

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