The Regular Expression check box causes the contents of the Additional Onsite Domains text box to be interpreted as a regular expression pattern. The pattern is passed a parameter to the RegExp constructor as follows:

new RegExp("AdditionalOnsiteDomains","i")

The resulting RegExp object is the compared against the host property of the event object. This is done using the RegExp.test method.

More information on Additional Onsite Domains here. More information on JavaScript regular expressions here.