Wikipedia:Edit filter/Traps and pitfalls

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This page[note 1] covers some common mistakes made by edit filter managers. For the full documentation, see Wikipedia:Edit filter/Documentation and mw:Extension:AbuseFilter.


When applying a throttle to an edit filter, it is important that you do so using both the ip and user variables wherever possible (as opposed to using either or).

Throttling by user alone throttles by user id, not by username. All logged out editors share one user id, which is 0. This may cause false positives and issues if many anonymous users unrelated to one another match the filter conditions when saving edits.

Throttling by ip alone throttles logged in editors by their underlying IP address. Do not use only the ip variable when applying a throttle, unless the filter specifically targets logged out or anonymous users only.


The user_rights variable only contains the user's current rights. If the user has logged in using a bot password, or is editing with an OAuth application, user_rights may be limited. For example, it looks like we could exclude extended confirmed users, bots, and administrators with[note 2]

!("extendedconfirmed" in user_rights) /* WRONG! */

but this will not work as expected if the user did not grant editprotected when setting up a bot password. Instead, just specify the groups explicitly:

!contains_any(user_groups, "extendedconfirmed", "sysop", "bot")

Test/examine interface and recent changes

Some variables at Special:Abusefilter/test and Special:AbuseFilter/examine[note 3] will have different values from what they would have been had the filter actually tripped at the time of the change.[note 4]

Suppose that Alice, as her first edit, adds the string "Hello, world! ~~~~" to a page that has only ever been edited by Bob. She then makes 20 more edits.

One week later, we look at her edit[note 5] with Special:AbuseFilter/examine. Some results may be surprising:

Variable At save At /examine or /test
added_lines Hello, world! ~~~~ Hello, world! [[User:Alice|Alice]] ([[User talk:Alice|talk]]) 21:07, 14 November 2019 (UTC)[note 6]
user_editcount 0 20
user_groups ["*", "user"] ["*", "user", "autoconfirmed"]
page_recent_contributors Bob Alice

Order of operations

rlike and other keywords have a higher precedence than +. This does not check if added_lines contains "foo" or "bar":

added_lines rlike "foo" + "|bar" /* WRONG! */

Instead use:

added_lines rlike ("foo" + "|bar")

norm() and repeating characters

The norm() function performs the following modifications to the string value to it in the following specific execution order:

  1. It begins by replacing confusing characters, or characters that are often used to spoof or maliciously bypass edit filter conditions.
  2. It then removes any repeating characters that are next to one another, leaving one character remaining. For example: string "ABC12345555556" would become "ABC123456".
  3. All special characters (such as _, +, :, #, $. %, {, etc.) are then stripped and removed.
  4. Lastly, all whitespace characters are stripped and removed from the string.

This can lead to unexpected results if one is unaware of the function's specific execution order:

string_example := "A@ AB,BCC";
norm(string_example) == "ABC"    /*  FALSE  */
norm(string_example) == "AABBC"  /*  TRUE   */

You may be asking yourself, "what happened here?" Take a look below to see how the norm() function's execution order modifies string_example step-by-step:

string_example = "A@ AB,BCC"    //This is the initial string that we originally assigned to string_example. Now we run the norm() function to it...
string_example = "AA AB,BCC"     //The first task (replacement of confusing characters) would result in the '@' being replaced by the letter 'A'.
string_example = "A AB,BC"      //The second task would remove the repeated 'A' and 'C' characters, leaving one of each.
string_example = "A ABBC"       //The third task removes all special characters, meaning that the comma (',') in this string is removed.
string_example = "AABBC"        //The last task would then remove the space.

string_example = "AABBC"        //The resulting string will be "AABBC".

When in doubt, use the debugging tool to assist you.

Creating a tag

Tags are created automatically when a filter is saved. Do not use the interface at the top of Special:Tags, unless you also want to activate the tag for manual use. Mistakenly activated tags may be deactivated from Special:Tags.

Be careful with arrays

The only operation that really works with arrays is length. Other operations will implicitly cast an array to a string first. This could give an unintuitive result. For example, page_namespace in [12, 34] is in fact equivalent to string(page_namespace) in "12\n34\n". Therefore, when page_namespace is 1, 2, 3, or 4, the expression will be evaluated to true as well. In the above case, use equals_to_any(page_namespace, 12, 34) as a workaround instead.

On the other hand, if you want to compute the amount of text added (removed), you might be tempted to use strlen(added_lines), strlen(removed_links) or similar. However, strlen, length and count do not implicitly cast arrays to string and will return the length of the array (i.e., number of lines), not the character count, instead. The cast needs to be explicit, i.e., strlen(string(added_lines)).

Be careful with division

One might expect that page_namespace / 2 === 0 will check if page_namespace is either 0 or 1. However, the division operation in fact doesn't discard the remainder. That means, if the numerator is not divisible by the denominator, the result will be a float. In the above case, use equals_to_any(page_namespace, 0, 1) instead.

Numeric comparisons with null

Like in PHP, null is smaller than any number, i.e. null < -1234567 is true. This is especially problematic when using edit_delta: if the action being filtered is not an edit, edit_delta < -5000 will evaluate to true. Remember to check that action === "edit" when using edit_delta like that.

Disappearing filter logs

Filter logs can disappear under these circumstances: 1) If an edit is saved and then rev-deleted or oversighted, then the filter log disappears from view (including from sysops). 2) Oversighters can remove the logs of either saved or unsaved edits. Edit filter counters will always increment, therefore, a filter may have fewer visible logs than the number of hits.

Inconsistent naming of some variables

For historical reasons, some variable names do not fit the general naming pattern:

Page content variables Pre-save transform variables
Old New Sent variable Transformed variable
old_wikitext new_wikitext added_lines added_lines_pst
old_html (disabled) new_html edit_diff edit_diff_pst
old_links all_links (not new_links) new_wikitext new_pst (not new_wikitext_pst)

See also


  1. ^ The title was shamelessly stolen from C Traps and Pitfalls.
  2. ^ All these groups have extendedconfirmed rights, according to Special:UserGroupRights
  3. ^ When examining recent changes. Examining old filter hits will show the correct values.
  4. ^ See also T102944
  5. ^ Not a filter log entry, if any exists
  6. ^ This is actually the value of added_lines_pst