Regular expression based filters

Adblock Plus filter syntax allows for the use of regular expression as filters.

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)
2. domain=: The regular expression won't be executed if the hostname of a request does not match the hostnames declared in the domain filter option
3. 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)