feedcel

feedcel 🙄

A proxy for filtering RSS, Atom, and JSON feeds using CEL expressions and a package for including that filtering in other applications (typically reader clients).

Usage

CLI

Inspect feeds directly:

go run ./cmd/cli -feed <url_or_path> -expr "item.Title.contains('Go')"

Options:

• -feed: URL or local path to the feed (required)
• -expr: CEL expression to filter items (optional)
• -version: Print version information as JSON

Run without -expr for interactive input and live validation.

Proxy

Start the server:

go run ./cmd/proxy -port 8080

Send a request with a url and a CEL expression.

GET:

curl "http://localhost:8080/filter?url=https://news.ycombinator.com/rss&expression=item.Title.contains('Go')"

POST:

curl -X POST http://localhost:8080/filter \
  -H "Content-Type: application/json" \
  -d '{"url": "https://news.ycombinator.com/rss", "expression": "item.Title.contains('\''Go'\'')"}'

Options

• format: Output format. Supports json (default), rss, or atom.
• expression: CEL expression. The item variable exposes fields like Title, URL, Author, Tags, and Content.

For more details on available fields and expression syntax, see pkg/cel/env.go and the test suite in cmd/proxy/main_test.go.

CEL environment

Fields

Available fields on the item variable. See pkg/cel/env.go for the canonical definition.

• URL (string)
• Title (string)
• Author (string)
• Tags (string)
• Content (string)
• Published (timestamp)
• Updated (timestamp)

Global variables:

• now (timestamp)

String Functions

Built-in functions from CEL standard:

• contains(string) — substring check
• startsWith(string), endsWith(string) — prefix/suffix check
• matches(regex) — RE2 regular expression match
• size() — string length

Additional functions from cel-go/ext#Strings:

• charAt, indexOf, lastIndexOf
• lowerAscii, upperAscii
• replace, split, substring, trim

Examples

See pkg/cel/env_test.go and cmd/proxy/main_test.go for the test suite.

// Substring match
item.Title.contains("Go")

// Case-insensitive match
item.Title.lowerAscii().contains("rust")

// Regex match (RE2 syntax)
item.Title.matches("(?i)release|announcement")

// Filter by specific tag
item.Tags.split(",").exists(t, t.trim() == "golang")

// Recent items only
now - item.Published < duration("24h")

// Composite logic
(item.Title.contains("release") || item.Tags.contains("v1")) &&
now - item.Published < duration("24h")