^ means the regular expression starts with the characters following it.
\. is escaping the period.
Putting this regular expression back together and interpreting it in English, it means:
An expression that begins with the characters "ad." (without the quotes), and is followed by one of the following words:
banner
banners
doubleclick
links
forum
affiliates
Expressed another way, it's looking for one of the following strings of characters:
ad.banner
ad.banners
ad.doubleclick
ad.links
ad.forum
ad.affiliates
Hope this helps.
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