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