Which standard of regular expressions does Qt use?

Question

I use https://regex101.com and https://www.regextester.com to check my regular expressions. I presume Qt uses PCRE or PCRE2, but sometimes it won't work as expected. This makes me think that Qt uses some other version of regex standard. Which is it?

Answer 1

From docs:

Note: In Qt 5, the new QRegularExpression class provides a Perl compatible implementation of regular expressions and is recommended in place of QRegExp.

This page describes the reasons for the switch, listing the many inadequacies of the old homebrewed regexp engine.

There still seems to be some features of PCRE that are not implemented.