The Miniflux protocol is a fairly well-designed protocol supporting a wide variety of operations on newsfeeds, folders (termed "categories"), and articles; it also allows for user administration, and native OPML importing and exporting. Architecturally it is similar to the Nextcloud News protocol, but has more capabilities.
The Miniflux documentation gives only a brief example of a pattern for its filtering rules; the allowed syntax is described in full [in Google's documentation for RE2](https://github.com/google/re2/wiki/Syntax). Being a PHP application, The Arsse instead accepts [PCRE syntax](http://www.pcre.org/original/doc/html/pcresyntax.html) (or since PHP 7.3 [PCRE2 syntax](https://www.pcre.org/current/doc/html/pcre2syntax.html)), specifically in UTF-8 mode. Delimiters should not be included, and slashes should not be escaped; anchors may be used if desired. For example `^(?i)RE/MAX$` is a valid pattern.
For convenience the patterns are tested after collapsing whitespace. Unlike Miniflux, The Arsse tests the patterns against an article's author-supplied categories if they do not match its title. Also unlike Miniflux, when filter rules are modified they are re-evaluated against all applicable articles immediately.
Nextcloud News' root folder and Tiny Tiny RSS' "Uncategorized" catgory are mapped to Miniflux's initial "All" category. This Miniflux category can be renamed, but it cannot be deleted. Attempting to do so will delete the child feeds it contains, but not the category itself.
Because the root folder does not existing in the database as a separate entity, it will always sort first when ordering by `category_id` or `category_title`.
Tiny Tiny RSS is unique in allowing newsfeeds to be grouped into categories nested to arbitrary depth. When newsfeeds are placed into nested categories, they simply appear in the top-level category when accessed via the Miniflux protocol. This does not affect OPML exports, where full nesting is preserved.
