Oniguruma (used by uu expr) panics on backtracking regexes

[sylvestre]

This is an additional detail relevant to #1145 .

The following regex causes uu expr to panic:

root@03081849be59:/tmp# expr --version
expr (uutils coreutils) 0.6.0
root@03081849be59:/tmp# expr "$(perl -e "print \"a\"x25 . \"c\"")" : "\(a\+a\+\)\+b"

thread 'main' (1403) panicked at /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/onig-6.5.1/src/lib.rs:647:23:
Onig: Regex search error: retry-limit-in-match over
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Aborted

Note that while GNU expr's regex engine supports backreferences, it does not appear to have exponential time complexity for this regex:

root@b082d601e194:/tmp# expr --version
expr (GNU coreutils) 9.7
Packaged by Debian (9.7-3)
Copyright (C) 2025 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Parker, James Youngman, and Paul Eggert.
root@b082d601e194:/tmp# time expr "$(perl -e "print \"a\"x25 . \"c\"")" : "\(a\+a\+\)\+b"


real    0m0.018s
user    0m0.003s
sys     0m0.016s
root@b082d601e194:/tmp# time expr "$(perl -e "print \"a\"x100000 . \"c\"")" : "\(a\+a\+\)\+b"


real    0m0.021s
user    0m0.004s
sys     0m0.017s