Regular Expression Flavors
Perl
https://perldoc.perl.org/perlre.html
PCRE
http://www.pcre.org/current/doc/html/pcre2syntax.html
POSIX BRE/ERE
http://pubs.opengroup.org/onlinepubs/9699919799/
GNU BRE/ERE
https://www.gnu.org/software/sed/manual/sed.html#Regular-Expressions-Overview
References:
http://www.regular-expressions.info/refrepeat.html