I checked the pcre2 source for changes and saw this:
Quote:
Version 10.43 16-February-2024
------------------------------
There are quite a lot of changes in this release (see ChangeLog and Git log for
a list). Those that are not bugfixes or code tidies
...
* Added support for limited-length variable-length lookbehind assertions, with
a default maximum length of 255 characters (same as Perl) but with a function
to adjust the limit.
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And the version of PCRE2 in Sigil 2.4.2 is version 10.44 so support for limited length lookbehind assertions should be in Sigil 2.4.2.
Are you using an assertion properly? A more specific error message might help if you can get one.