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Hir::is_match_empty returns false for \b, but should return true #859

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BurntSushi opened this issue May 17, 2022 · 0 comments · Fixed by #860
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Hir::is_match_empty returns false for \b, but should return true #859

BurntSushi opened this issue May 17, 2022 · 0 comments · Fixed by #860
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@BurntSushi BurntSushi commented May 17, 2022

The predicate in question: https://docs.rs/regex-syntax/latest/regex_syntax/hir/struct.Hir.html#method.is_match_empty

The issue here is that is_match_empty returns true for \B but not for \b. I had done this because \B matches "" but \b does not. However, as of version 1.5.5, this program runs without panicking:

use regex::Regex;

fn main() {
    let wb = Regex::new(r"\b").unwrap();
    let notwb = Regex::new(r"\B").unwrap();
    
    assert!(!wb.is_match(""));
    assert!(notwb.is_match(""));
    
    let got: Vec<_> = wb.find_iter("a").map(|m| m.range()).collect();
    assert_eq!(vec![0..0, 1..1], got);
    
    let got: Vec<_> = notwb.find_iter("a").map(|m| m.range()).collect();
    assert!(got.is_empty());
}

Playground link.

Thus proving that \b does indeed report matches that correspond to the empty string. Therefore, it is a bug that is_match_empty returns false for \b. The issue here is that neither \B nor \b match every empty string. Instead, they only match a subset of empty strings.

@BurntSushi BurntSushi added the bug label May 17, 2022
BurntSushi added a commit that referenced this issue May 17, 2022
This was incorrectly defined for \b. Previously, I had erroneously made
it return true only for \B since \B matches '' and \b does not match
''. However, \b does match the empty string. Like \B, it only matches a
subset of empty strings, depending on what the surrounding context is.
The important bit is that it can match *an* empty string, not that it
matches *the* empty string.

We were not yet using this predicate anywhere in the regex crate, so we
just fix the implementation and update the tests.

This does present a compatibility hazard for anyone who was using this
function, but as of this time, I'm considering this a bug fix since \b
clearly matches an empty string.

Fixes #859
BurntSushi added a commit that referenced this issue May 18, 2022
This was incorrectly defined for \b. Previously, I had erroneously made
it return true only for \B since \B matches '' and \b does not match
''. However, \b does match the empty string. Like \B, it only matches a
subset of empty strings, depending on what the surrounding context is.
The important bit is that it can match *an* empty string, not that it
matches *the* empty string.

We were not yet using this predicate anywhere in the regex crate, so we
just fix the implementation and update the tests.

This does present a compatibility hazard for anyone who was using this
function, but as of this time, I'm considering this a bug fix since \b
clearly matches an empty string.

Fixes #859
otc-zuul bot pushed a commit to opentelekomcloud-infra/cloudmon-plugin-smtp that referenced this issue Jun 7, 2022
Bump regex from 1.5.4 to 1.5.6

Bumps regex from 1.5.4 to 1.5.6.

Changelog
Sourced from regex's changelog.

1.5.6 (2022-05-20)
This release includes a few bug fixes, including a bug that produced incorrect
matches when a non-greedy ? operator was used.

[BUG #680](rust-lang/regex#680):
Fixes a bug where [[:alnum:][:^ascii:]] dropped [:alnum:] from the class.
[BUG #859](rust-lang/regex#859):
Fixes a bug where Hir::is_match_empty returned false for \b.
[BUG #862](rust-lang/regex#862):
Fixes a bug where 'ab??' matches 'ab' instead of 'a' in 'ab'.

1.5.5 (2022-03-08)
This releases fixes a security bug in the regex compiler. This bug permits a
vector for a denial-of-service attack in cases where the regex being compiled
is untrusted. There are no known problems where the regex is itself trusted,
including in cases of untrusted haystacks.

SECURITY #GHSA-m5pq-gvj9-9vr8:
Fixes a bug in the regex compiler where empty sub-expressions subverted the
existing mitigations in place to enforce a size limit on compiled regexes.
The Rust Security Response WG published an advisory about this:
https://groups.google.com/g/rustlang-security-announcements/c/NcNNL1Jq7Yw




Commits

9aef5b1 1.5.6
2931b07 syntax: bump minimum regex-syntax version to 0.6.26
b41bde0 regex-syntax-0.6.26
d98da65 changelog: 1.5.6
1c19619 syntax: fix literal extraction for 'ab??'
88a2a62 syntax: fix 'is_match_empty' predicate
72f09f1 syntax: fix ascii class union bug
b537286 doc: fix some typos
258bdf7 changelog: 1.5.5
d130381 1.5.5
Additional commits viewable in compare view




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Reviewed-by: Artem Goncharov <Artem.goncharov@gmail.com>
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