On Microsoft Windows, if when you click or type the name of a .pl file, you get an error "is not recognized as an internal or external command, operable program or batch file", then this means you do not have the .pl file extension associated with the perl interpreter. This is normally done by the installer, but not always.
To associate files:
1) Start Windows explorer
2) Select, from the menu, Tools -> Folder Options
3) Select the File Types tab
4) Look in the list for a PL extension. If it does NOT exist:
4a) Click New
4b) Type pl for extension
5) Select the PL extension in the list
6) In the section "opens with", click Change
7) Select perl.exe
8) Click OK until all Windows are closed.
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern: