rejects a leading quantifier (`?`/`*`/`+`) that GNU accepts with a warning

[sylvestre]

A repetition operator (?, *, +) at the start of an expression has no preceding atom to repeat. GNU grep treats this leniently: it prints warning: <op> at start of expression to stderr, matches as if the operator were a no-op (so every line is selected), and exits 0. uu_grep rejects the pattern as a hard error and exits 2.

Found by the differential fuzzer (fuzz_grep).

Rust (incorrect)

$ printf 'abc\n' | ./target/release/grep -e '?' -E
# Output (stderr): grep: invalid pattern "?": Oniguruma error: target of repeat operator is not specified
# Exit code: 2

GNU (correct)

$ printf 'abc\n' | LC_ALL=C /usr/bin/grep -e '?' -E
# Output (stderr): grep: warning: ? at start of expression
# Output (stdout): abc
# Exit code: 0

The same divergence occurs for * and + as the leading character (grep -E '*', grep -E '+'): GNU warns and matches every line (exit 0); uu_grep errors (exit 2).

[sylvestre]

The interval form {2} triggers the same thing — GNU warns {...} at start of expression and matches every line, uu_grep errors:

$ printf 'abc\n' | LC_ALL=C /usr/bin/grep -E '{2}'   # warning: {...} at start of expression; matches; exit 0
$ printf 'abc\n' | ./target/release/grep -E '{2}'    # invalid pattern error; exit 2