All the systems that I know about manage the list, somewhat manually.
Using web filtering proxies (e.g. WebSense) for inspiration, you can scan for keywords contained in a domain name, or in web content / meta tags at a specified location. However, there are always elements that seem to correspond to more than one category or category, and they need deeper analysis.
In the end, you end up creating your rather complex logic, maintaining the list manually, or buying the list from someone else.
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