bug#57507: Regular expression matching depends on locale encoding

Regular expressions do funky things with Unicode if a non-Unicode-aware
locale is set. Yet, they're purely string operations, so I don't think
it's expected that they depend on the locale encoding.



$ LC_ALL=C guile3.0
GNU Guile 3.0.7
Copyright (C) 1995-2021 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (use-modules (ice-9 regex))
scheme@(guile-user)> (match:substring (string-match "\u203f" "\u3091"))
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure make-regexp: Invalid preceding regular expression

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> ,q
scheme@(guile-user)> (match:substring (string-match "[\u203f]" "\u3091"))
$1 = "\u3091"
scheme@(guile-user)>