I’m not sure which Boost version is used with SC (I doubt there have many changes in Boost regex recently, but I may be wrong) but note that SC uses the ECMAScript “flavour” of regexes (note that you can also set the regex engine in the regex101.com settings; ECMAScript is not the default there, I don’t think).
Update: looks like SC uses Boost version 1.74, since 2020.