It is our long holiday weekend in Canada, but I finally got some time to test things in Sigil on my only laptop up here at my cottage. It is a pre-release version of the forthcoming Sigil v2.50.
I decided to test the example cited by one of the issues posted at PCRE2 in that link I posted earlier.
In my xhtml file I have:
And as my find I have:
And I get no error and it works to find the y in the text.
Would you please try this test with your Sigil 2.4.2 and let me know if you get the same thing? Perhaps there was a bug in PCRE2 10.44 that got fixed in PCRE2 10.45 which is in the upcoming release of Sigil.
I will try Doitsu's test next.