1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
//! Regex dialect helpers shared by the pattern-matching builtins.
//!
//! kaish's regex is ERE-always (Rust's `regex` crate, egrep-style). Commercial
//! LLMs, though, reflexively reach for the GNU BRE spellings — `foo\|bar` for
//! alternation, `a\+` for one-or-more, `\(...\)` for a group, `x\{2,5\}` for an
//! interval — because GNU grep/sed accept them as a BRE extension and the habit
//! is baked deep into how models write shell. Under bare ERE those backslash
//! escapes mean a *literal* `|`/`+`/`(`/`{`, so an agent's `grep 'a\|b'`
//! silently matches nothing (issue #60: measured as the single largest source of
//! wasted explorer tool calls).
//!
//! [`bre_metas_to_ere`] makes kaish a *superset*: it rewrites the GNU BRE
//! backslash-metas into their ERE equivalents so both spellings work. The
//! casualty is narrow and documented — a backslash before one of these metas is
//! now always the operator, never a literal; match the character itself with a
//! bracket class (`[+]`, `[|]`, `[{]`).
/// The GNU BRE backslash-metacharacters kaish rewrites to their bare ERE form.
/// `\|`→alternation, `\+`/`\?`→quantifiers, `\(`/`\)`→group, `\{`/`\}`→interval.
const BRE_METAS: &[char] = &['|', '+', '?', '(', ')', '{', '}'];
/// Append a dialect note to a regex compile error, when the rewrite is the
/// likely culprit: `rewrote` is true when [`bre_metas_to_ere`] changed the
/// pattern, so a formerly-literal escape like `:\)` became an operator and the
/// engine's error (`unopened group` on `:)`) describes a pattern the author
/// never wrote. `strict_flag` names the tool's strict-ERE escape hatch (`-E`,
/// `-E/-r`); awk has none and passes `None`.
pub(crate) fn append_dialect_hint(err: String, rewrote: bool, strict_flag: Option<&str>) -> String {
if !rewrote {
return err;
}
let escape_hatch = match strict_flag {
Some(flag) => format!(", or pass {flag} for strict ERE"),
None => String::new(),
};
format!(
"{err} (note: a backslashed |+?(){{}} is a GNU BRE operator in the default \
dialect — match the literal character with a bracket class like [)] or \
[|]{escape_hatch})"
)
}
/// Rewrite GNU BRE backslash-metas (`\| \+ \? \( \) \{ \}`) into the bare ERE
/// operators Rust's `regex` crate understands. Any other escape is passed
/// through verbatim, so `\.`, `\d`, `\b`, `\w`, and an escaped backslash `\\`
/// keep their meaning — `a\\|b` stays "literal backslash, then alternation",
/// never a stray BRE `\|`.
///
/// Bracket-expression interiors are deliberately *not* special-cased: for every
/// meta in [`BRE_METAS`], the escaped and bare forms denote the same literal
/// character inside a class (`[\|]` and `[|]` both match `|`), so rewriting
/// there yields an equivalent pattern and needs no class tracking.
pub(crate) fn bre_metas_to_ere(pattern: &str) -> String {
let mut out = String::with_capacity(pattern.len());
let mut chars = pattern.chars().peekable();
while let Some(c) = chars.next() {
if c != '\\' {
out.push(c);
continue;
}
match chars.peek() {
// `\<meta>` → drop the backslash, keep the operator.
Some(&next) if BRE_METAS.contains(&next) => {
out.push(next);
chars.next();
}
// `\<other>` → preserve both chars (incl. `\\`) untouched.
Some(&next) => {
out.push('\\');
out.push(next);
chars.next();
}
// Trailing lone backslash: leave it for the regex engine to judge.
None => out.push('\\'),
}
}
out
}
#[cfg(test)]
mod tests {
use super::*;
use rstest::rstest;
#[rstest]
// Alternation — the issue's headline case.
#[case(r"foo\|bar", "foo|bar")]
#[case(r"a\|b\|c", "a|b|c")]
// Quantifiers.
#[case(r"a\+", "a+")]
#[case(r"a\?", "a?")]
// Groups and intervals.
#[case(r"\(foo\)\+", "(foo)+")]
#[case(r"x\{2,5\}", "x{2,5}")]
// Non-meta escapes are preserved verbatim.
#[case(r"\d\.\w\b", r"\d\.\w\b")]
// Escaped backslash stays literal; a following bare `|` is already ERE.
#[case(r"a\\|b", r"a\\|b")]
// Escaped backslash then BRE alternation → literal backslash, then `|`.
#[case(r"a\\\|b", r"a\\|b")]
// Bare ERE forms pass straight through.
#[case(r"foo|bar", "foo|bar")]
#[case(r"a+b?", "a+b?")]
// Inside a class the rewrite is equivalent (both match the literal char).
#[case(r"[\|]", "[|]")]
#[case(r"[\{]", "[{]")]
// Trailing lone backslash is left alone.
#[case(r"foo\", r"foo\")]
fn rewrites_gnu_bre_metas(#[case] input: &str, #[case] expected: &str) {
assert_eq!(bre_metas_to_ere(input), expected);
}
}