Orphan trailing backslash in pattern

[magicant]

(Issue migrated from https://osdn.net/projects/yash/ticket/40752)

XCU 2.13.1 Patterns Matching a Single Character:

If a pattern ends with an unescaped backslash, it is unspecified whether the pattern does not match anything or the pattern is treated as invalid.

This means the pattern should not match with any word in bs='\\\'; case word in $bs) echo match; esac. In existing shells, however, one of \, \\ and \\\ matches.

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Seems the future version of POSIX is going to say that the entire behavior is unspecified in such a case.

https://www.austingroupbugs.net/view.php?id=1234#c4564

If a pattern ends with an unescaped backslash, the behavior is unspecified.

[magicant]

The current yash is unique in that it matches in bs='\\\'; case '\' in $bs) echo match; esac while bash, dash, ksh, mksh, busybox, and zsh do not.