.sh.match[0] coupled with string substitution no longer works

[rgomes1]

Using RedHat 9 RPM: ksh-1.0.6-14.el9.x86_64 (Version AJM 93u+m/1.0.6 2023-06-13), the following .sh.match[0] reference coupled with string substitution no longer works:

digits_only="${.sh.match[0]//[^0-9]/}"

Using ksh93 on RedHat 8/7/6 etc., the above statement works fine.

As a result, presently changing this operation to 2 separate statements as a workaround:

x_temp="${.sh.match[0]}"
digits_only="${x_temp//[^0-9]/}"

[McDutchie]

Thanks for your report.

Your reproducer stopped working in commit b1b8d6f, which backported the new (and much faster) .sh.match code from the abandoned AT&T 93v- beta. It doesn't work on 93v- either, or on ksh2020.

The thing is, you're doing a string substitution operation on .sh.match, but string substitution itself modifies .sh.match. In cases where it appears to work, that is purely an accident of implementation. The effect of any matching operation directly on .sh.match itself should be considered undefined.

The manual page currently says, under Shell Variables > .sh.match: “Note that even matching operations performed on the .sh.match variable itself will overwrite it upon finding a match.” That could probably be put more clearly, so I'll mark this as a documentation issue.

Note that even on 93u+ 2012-08-01, this didn't actually work reliably. In the following, ${.sh.match[0]/a/A} produces A instead of Abc on all ksh versions:

$ /bin/ksh -c 'v=abc; : ${v//*/}; print -r "${.sh.version}: ${.sh.match[0]} ${.sh.match[0]/a/A}"'   
Version AJM 93u+ 2012-08-01: abc A
$ ksh -c 'v=abc; : ${v//*/}; print -r "${.sh.version}: ${.sh.match[0]} ${.sh.match[0]/a/A}"'     
Version AJM 93u+m/1.0.11-beta 2026-03-07: abc A

So, the workaround that you've implemented in your script is actually the correct fix. There is no bug in ksh here.