How it works

Give a feed URL and zero or more filters. Items that match every filter (or don't match, for "exclude") are kept. The result is a new URL you can subscribe to in your RSS reader; it will always reflect the current state of the source feed.

Picking what to filter on

Each filter has three parts: a value to look for, a field to look in, and a mode that decides what to do with matches.

Field	Use it for
`title`	The post title.
`description`	The summary / short text.
`content`	The full post body.
`link`	The post URL.
`author`	The post author.
`category` or `tags`	Tags or categories.
`any`	Look in every text field of the item.
`title,description`	Comma-separated list looks in several fields.
`_reddit.score`	A dotted path into a JSON Feed extension field.

What each mode does

Mode	Behavior	Filter value
Include	Keep items where the value is found.	Regex, e.g. `(?i)golang`
Exclude	Drop items where the value is found.	Regex, e.g. `spam\.example`
Numeric ≥ (gte)	Keep items where the field's number is at least this much.	A number, e.g. `100`
Numeric ≤ (lte)	Keep items where the field's number is at most this much.	A number, e.g. `5`

Regex is the Go flavor. Prefix with (?i) to ignore case.

Recipes

Click any line to copy the URL idea, then adapt it:

Only "golang" posts in a tech blog

/feed?url=FEED&filter=(?i)golang&field=title

Drop everything from a spammy domain

/feed?url=FEED&filter=spam\.example\.com&field=link&mode=exclude

Only posts with at least 100 upvotes

/feed?url=https://old.reddit.com/r/programming/top/.rss?t=week&filter=100&field=_reddit.score&mode=gte

Hide anything flagged NSFW

/feed?url=https://old.reddit.com/r/all/.rss&filter=true&field=_reddit.over_18&mode=exclude

Only items tagged with a specific topic

/feed?url=https://lobste.rs/hottest.json&filter=^rust$&field=_lobsters.tags

Tips

You can paste a reddit.com or lobste.rs link straight into the form. It will be converted into a JSON Feed with score, tags, and other fields you can filter on.
Add &format=rss to the end of the URL if your reader prefers RSS.
The Preview button shows which items would be kept or dropped without saving the URL.
Leave filters empty to just clean up or convert a feed without filtering.

Endpoints

GET /feed?url=FEED returns the (optionally filtered) feed.
GET /preview?url=FEED returns a JSON summary of what would be kept.
GET /fields lists field names the matcher understands.
GET /health returns ok.