In Squid, neighbor usually means the same thing as peer. A neighbor cache is one that you have defined with the cache_peer configuration option. Neighbor refers to either a parent or a sibling.
In Harvest 1.4, neighbor referred to what Squid calls a sibling. That is, Harvest had parents and neighbors. For backward compatability, the term neighbor is still accepted in some Squid configuration options.
Regular expressions are patterns that used for matching sequences of characters in text. For more information, see A Tao of Regular Expressions and Newbie's page.