Really Simple Syndication (RSS) is a family of web feed formats used to publish frequently updated works. You can use torrent clients to read RSS feeds and download certain torrents based on rules you specify.

Flexget - all clients

flexget is a little tool that reads RSS feeds and downloads certain torrent files to your watch directory, read the flexget article to set it up. This tool has a lot of features, but it requires some command-line knowledge to use it.

PluginRSS - a ruTorrent plugin

This plugin is loaded in your ruTorrent by default and is the easiest way to use RSS-download torrents. This article on ruTorrent's Github repository explains the process in detail.

RSS Regex Tutorials

RSS regular expressions (PCRE Regex) is the language used for the creation of RSS rules. Another common use of PCRE Regex is the preg_replace(); function in PHP.

RSS regex can be used to create, filter, organise, and automatically download files. It is recommended to only use RSS feeds served over https:// due to the TLS (Transport Layer Security) encryption. Any feeds or any form of data served over http:// will be sent in clear text meaning they are unencrypted and easily read by anyone monitoring the traffic.

Some quick regex rules are as follows:

/.*/gi       = download/ignore all files case insensitve 
/\.mp4.*/gi  = download/ignore all files containing .mp4 in file name case 
/abcdef.*/g   = download/ignore all files containing abcdef in file name while being case sensitive (i) 

the long version:

/ /     Everything between these slashes will be searched 
\       Escapes a character
.       Wild card will find any single character
\.      Escapes full stop, treats it as a normal . in file name
*       Repeats the preceding character 0 or multiple times
?       Repeats the preceding character 0 or 1 times
+       Repeats the preceding character 1 or more times 
^       Must be at the start of the string.
$       Must be at the end of the string
|       Or for example a|b will find a or b
()      Groups an evaluation together and treat as a single character 
(pie)?  Will find pie instead of just e
[]      Matches one character inside the brackets.
[abc]   Will find any a or b or c character
[^abc]  Will find any character that is not a or b or c
[a-z]   Will find every letter in the alphabet
\w      Will find any "word" character A-Z a-z 0-9 _ 
\W      Will find any non "word" character eg. )(#*^$#+@-
[\w\W]  Will find everything
[\s\S]  Will find everything
\S      Will find any character that is not a space
\s      Will find any space
\d      Will find any digit
\D      Will find any non digit
\b      Will find the start/end of any word
\B      Will find anything but the start/end of a word

at the end you can have modifiers \g Means global match, instead of finding one file/set you find all within feed \i Means case insensitive