See perlretut and perlre for a start. If you are new to Perl then have a good browse around the Tutorials section here too.

A small nomenclature note: the "regular expression" is the bit that does the matching. In a substitution it is the first expression. In a match it is the only expression. In a substitution the other expression is called the substitution expression.

For complicated regular expressions it is often helpful to use the /x switch which allows you to use white space and comments in your expression so that it is easier to see the different parts and to provide comments for the parts of the expression.