http://www.perlmonks.org?node_id=854220

bharatbsharma has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks I am trying to build my know how on perl regular expression from cookbook http://docstore.mik.ua/orelly/perl/cookbook I am getting stuck at some point . How to read?
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
~ s/man(?=\d)/cat/
What does '(?=\d)' signify here . Is there any good link which will explain all nitty gritty of regular expression from basic level? Thanks in advance Bharat