Created Regular-expression-based filters (markdown)

Regular-expression-based-filters.md

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+[Adblock Plus filter syntax](https://adblockplus.org/en/filters) allows for the use of [regular expression as filters](https://adblockplus.org/en/filters#regexps).
+
+It is not recommended to use regex-based filters, hence why I chose for a long time to not support these in µBlock: to prevent users who write their own filters from acquiring the bad habit of using regular expressions to filter network requests.
+
+However this led to the often-repeated myth that the reason µBlock was efficient memory- and CPU-wise was because it did not support regex-based filters, a completely nonsensical assertion, given that there is a grand total of only 15 such filters in all of _EasyList_ (at time of writing) out of tens of thousands.
+
+It is because of this myth that I finally decided to support regex-based filters with version 0.8.6.0.
+
+Given the way µBlock works internally, the regex-based filters are implemented in a more efficient way than other big-name blockers.
+
+An efficient regex-based filter is one which does not need to be evaluated.
+
+So if you decide to write a regex-based filter, here is the trick to help you make your regex as efficient as possible:
+
+**Use filter options to reduce the likelihood of a regex-based filter of being executed.**
+
+A most-efficient regex-based filter is one which comes with all the following filter options set:
+
+1. type: a filter which apply only to a specific request type will be executed only for request which matches the type.
+    - Example: `/\.filenuke\.com/.*\/[a-zA-Z0-9]{4}/$script` will be executed only for request of type `script` (this filter is found in _EasyList_)
+1. `domain=`: The regular expression won't be executed if a request
+1. `third-party`: the regular expression won't be executed if the request does not fulfill the `third-party` option (or it's complement `~third-party`)