1. ([a-z]|[0-9])+ : One or more letters and/or numbers
2. ((\.)+([a-z]|[0-9])+)* : Possibly a dot. Considering that there is a dot one or more letters and/or numbers must follow
3. (\-|\_)* : Possibly a dash or underscore.
4. ([a-z]|[0-9])* : Possibly more letters and/or numbers
5. \@{1} : Must have one (and only one) '@'
6. ([a-z]|[0-9])+ : Must have more letters and /or numbers followed by
7. \.{1} : a dot that must be followed by
8. ([a-z]|[0-9]){2,} : at least two letters and/or numbers
9. (\.([a-z|0-9]){2})* : and possibly followed by a dot and more letters and/or numbers

Please use <code> tags around Perl code so it displays correctly. You may want to use (?: ... ) for grouping so you don't get a bunch of assignments to $1, $2, etc. Also {1} is redundant. In several cases your comments don't match what the regexp is doing.

In 2. the (\.)+ allows two or more dots in a row. I think you probably just want to drop the + (and the redundant parentheses).

In 3. you have almost the same problem. Try changing * to ? unless you mean to allow multiple dashes/underscores in a row.

In 9. I think you want (?:\.(?:[a-z0-9]){2,})*. I'm leaving the * (even though you said "possibly") since I think you can have any number of domain levels in a FQDN.

janitored by ybiC: Balanced <code> tags around regex examples for proper rendering