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The change filter is a tool used to allow administrators and other trusted users to set controls on certain types of bad changes. This page calls it the "change filter" instead of the "abuse filter" because not everything that it stops is abuse.
This extension allows rules to be written to apply to all changes that are made. These rules can include things like "users with less than 500 changes are stopped from moving pages to titles which contain this regular expression: /poop/". Of course, the rules can get quite a bit more complicated.
Log entries (a record of what changes have been caught by filters) can be seen by all users, but some filters cannot. Every user will be able to see a short description of what each filter is designed to stop.
Change filters can cause a lot of damage if they stop changes that are helpful. Because of this, new filters can be tested for a few days (in "log only" mode, which only records the changes that the filter detects) before being used (in "warn", "disallow" or "throttle" modes). If the change filter makes a mistake, it can be reported here.