fix(grep): match GNU grep 3.11 output, options and POSIX regex dialects

[chaliy]

Requested by Михайло · project thread

What changed

• Regex dialects follow GNU. Basic and extended patterns are translated with POSIX bracket rules, \{m,n\}, \+ \? \|, \< \> \b \B \w \W \s \S and repeated operators.
• Back-references work in plain patterns, e.g. \(abc\)\1. They run on the backtracking engine, which already has a step cap (TM-DOS-025 updated).
• Leftmost-longest -o. Matches are leftmost-longest, reusing awk's POSIX matcher.
• Pattern errors use glibc's exact messages (Unmatched ( or \(, Invalid range end) and exit 2.
• Output order follows GNU:
  • Prefixes print as FILE:LINE:BYTE, with - on context lines.
  • A -- separator goes between groups and between files whenever any context option is given.
  • --group-separator and --no-group-separator are supported.
  • After -m, trailing context is printed in full.
• Options:
  • Added -NUM, -T, --label, -d, -D, -R (follows symlinks; -r does not), -y and --exclude-from.
  • - reads stdin.
  • Long options accept unique prefixes; an ambiguous or unknown one prints GNU's message.
• Inputs:
  • An empty -f file matches nothing.
  • A directory operand without -r reports "Is a directory".
  • A line that isn't valid UTF-8 makes the file binary.
  • \r is part of the line, so x$ no longer matches x\r.
• One line engine now serves both buffered and streaming runs. The old ad-hoc basic-to-extended converter is gone.

Why

grep scored 64% on the Debian-oracle bench from pseudo-linus. Agents write grep -o, back-references and context flags the GNU way, and they hit wrong output or "backreferences are not supported" errors.

Before / After

Each case below was also run through GNU grep 3.11 on the host, and the After output matches it in every case.

$ echo abcabc | grep -o '\(abc\)\1'
before: grep: invalid pattern: ... error: backreferences are not supported
after:  abcabc

$ grep -E '(' f; echo "exit=$?"
before: grep: invalid pattern: regex parse error: ( ... unclosed group / exit=2
after:  grep: Unmatched ( or \( / exit=2

$ printf 'one\ntwo\n' | grep --label=in -H o -
before: grep: unrecognized option '--label=in'
after:  in:one / in:two

$ printf 'ab\n' | grep -o 'a\|ab'
before: a
after:  ab

$ printf 'x\r\n' | grep -c 'x$'
before: 1
after:  0

Debian-oracle bench:

• grep: 60.5% to 96.8% strict (185 cases).
• grep cases in the regex set: 663 to 877 of 879.
• The whole regex set: 64.1% to 81.7% lenient. The sed and awk cases in that set are unchanged.

The 6 grep cases still failing differ only in recursive directory order (L-GREP-003).

Risk

• Medium. grep is heavily used, and this rewrites its option parsing and output path.
• Four old unit tests encoded behavior GNU grep doesn't have and were rewritten. One example: -om1 now prints every match on the first line, because -m counts lines.
• Known gaps are recorded as L-GREP-002: no "stray \" warnings, -o binary detection works per line, and NUL is not a line separator in binary files.

Checklist

• Tests added or updated (new grep_gnu_output_tests, grep_pattern unit tests, limitation evidence tests). The full lib suite (2866) and integration suite (1368) pass. The bash comparison run shows no grep mismatches.
• Backward compatibility considered (behavior now matches GNU grep 3.11)

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