can1357/oh-my-pi · error · TacError

invalid regular expression: {0}

Error message

invalid regular expression: {0}

What it means

This error from the tac builtin (crates/pi-builtins/src/tac.rs, InvalidRegex) means the separator expression supplied via tac's -r/--regex option failed to compile with the regex crate. tac reverses files using a user-provided separator pattern; an invalid pattern cannot be used. The regex::Error is embedded and describes the exact syntax problem.

Source

Thrown at crates/pi-builtins/src/tac.rs:30

use clap::{Arg, ArgAction, ArgMatches, Command};
use memchr::memmem;
use memmap2::Mmap;
use thiserror::Error;
use uucore::display::Quotable;

use crate::host::{Host, Utility, format_usage, matches_parser, util};

mod options {
	pub static BEFORE: &str = "before";
	pub static REGEX: &str = "regex";
	pub static SEPARATOR: &str = "separator";
	pub static FILE: &str = "file";
}

#[derive(Debug, Error)]
enum TacError {
	/// A regular expression given by the user is invalid.
	#[error("invalid regular expression: {0}")]
	InvalidRegex(regex::Error),
	/// An error opening a file for reading.
	#[error("failed to open {} for reading: {}", .0.quote(), strip_errno(.1))]
	Open(OsString, std::io::Error),
	/// An error reading the contents of a file or stdin.
	#[error("{}: read error: {}", .0.maybe_quote(), strip_errno(.1))]
	Read(OsString, std::io::Error),
	/// An error writing the reversed contents of a file or stdin.
	#[error("failed to write to stdout: {}", strip_errno(.0))]
	Write(std::io::Error),
}

fn strip_errno(error: &std::io::Error) -> String {
	let mut message = error.to_string();
	if let Some(position) = message.find(" (os error ") {
		message.truncate(position);
	}
	message

View on GitHub (pinned to 9690622007)

Solutions

1. Fix the regex syntax reported by the wrapped error (e.g. close the bracket, escape specials).
2. Test the pattern in a regex tool (e.g. `regex` crate syntax) before passing it to tac.
3. If you meant a literal separator string, drop -r/--regex so the argument is treated literally.
4. Quote the pattern properly in the shell: use single quotes to avoid backslash mangling.

Example fix

// before
tac -r '[sep]'  // invalid regular expression: unclosed character class

// after
tac -r '\[sep\]'

const { Regex } = require('regex');
function isValidRegex(pattern) {
  try { new Regex(pattern); return true; } catch { return false; }
}

function isInvalidRegexError(err) {
  return err instanceof Error && err.message.startsWith('invalid regular expression: ');
}

try {
  await tac.run(['-r', pattern, file]);
} catch (err) {
  if (String(err).startsWith('invalid regular expression')) {
    console.error(`bad separator pattern '${pattern}', falling back to literal newline`);
  } else throw err;
}

When it happens

Trigger: Running tac with `-r/--regex` and a pattern that fails regex compilation, e.g. `tac -r '[unclosed'` or `tac -r 'a{2,1}'` (invalid repetition bounds).

Common situations: Hand-written patterns with unescaped special characters (`(`, `[`, `*` in the wrong place); shell quoting stripping backslashes (`tac -r '\n'` semantics); patterns ported from other regex dialects with unsupported syntax.

Related errors

• Invalid log regex: ${error instanceof Error ? error.message
• err.to_string() (invalid regex pattern)
• err.to_string() (timestamp parse error)
• duration is too large: {value}
• invalid date: {value}

AI-assisted analysis of can1357/oh-my-pi@9690622007 (2026-08-31). Data as JSON: /api/errors/1b512f4cba7d38e0. Report an issue: GitHub.