Regular expression sites I recommend

oniguruma : regular expression library ( Owner: K.Kosako )
https://github.com/kkos/oniguruma

Onigmo : regular expressions library forked from Oniguruma ( Owner: K.Takata )
https://github.com/k-takata/Onigmo

K.Takata's Web Page : bregonig.dll
http://k-takata.o.oo7.jp/

Perl正規表現雑技 ( Owner: OHZAKI Hiroki )
http://www.din.or.jp/~ohzaki/

Perl5 Perl Core Development
https://dev.perl.org/perl5/news/

perlreref - Perl Regular Expressions Reference
https://metacpan.org/dist/perl/view/pod/perlreref.pod