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Updated Regular expression based filters (markdown)

Raymond Hill 2015-06-16 10:11:35 -04:00
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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.
It is not recommended to use regex-based filters, hence why I chose for a long time to not support these in uBlock: 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.
However this led to the often-repeated myth that the reason uBlock 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:
![regex-based filter at work](https://cloud.githubusercontent.com/assets/585534/5883114/606b7838-a31d-11e4-98e5-1e9ea47e3308.png)
Given the way µBlock works internally, the regex-based filters are implemented in a more efficient way than other big-name blockers.
Given the way uBlock 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.
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1. `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
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`)
An example of a regex-based filter found in _EasyList_ which is handled very efficiently by µBlock:
An example of a regex-based filter found in _EasyList_ which is handled very efficiently by uBlock:
/http:.*(?:\+|\@|\=|\;|\_|\-|\!|\?|\&|\%|\#|\^|\:).*\/\//$script,third-party,domain=allenbwest.com
This filter contains all the filter options which makes it very unlikely that the regular expression will have to be executed. The regular expression will execute **only** if the request is of type `script`, originates from `allenbwest.com`, and is 3rd-party to `allenbwest.com`.
If this sounds like basic common sense, it's because it is. However I've seen other big-name blockers out there execute all regex-based filters unconditionally for every request.
If this sounds like basic common sense, it's because it is. However I've seen other big-name blockers out there execute all regex-based filters unconditionally for every request. (**Edit:** [fixed in ABP](https://issues.adblockplus.org/ticket/2177))