How to Use Regular Expression (Regex) to Filter RSS Feeds

Regex (Regular Expressions) filters are a powerful way to customize your feeds by including or excluding content based on specific patterns. Here’s a step-by-step guide to help you effectively use regex filters in RSS.app based on common use cases:

1. Understanding Regex Filters

Regex filters allow you to:

Include specific posts: Show only posts that match certain patterns.
Exclude specific posts: Hide posts that match certain patterns.
Modify feed content dynamically: Match patterns in titles, descriptions, or other fields.

Regex Operators and Syntax Table

Operator/Syntax	Description	Example	Explanation
`.`	Matches any single character except a newline.	`a.b`	Matches `aab`, `acb`, but not `ab` or `a\nb`.
`*`	Matches 0 or more repetitions of the preceding element.	`ab*`	Matches `a`, `ab`, `abb`, `abbb`.
`+`	Matches 1 or more repetitions of the preceding element.	`ab+`	Matches `ab`, `abb`, `abbb`, but not `a`.
`?`	Matches 0 or 1 repetition of the preceding element.	`colou?r`	Matches `color` or `colour`.
`	`	OR operator (matches either pattern).	`cat
`^`	Matches the start of a string.	`^Hello`	Matches `Hello` only at the beginning of the string.
`$`	Matches the end of a string.	`world$`	Matches `world` only at the end of the string.
`[]`	Matches any one character in the brackets.	`[abc]`	Matches `a`, `b`, or `c`.
`[^]`	Matches any one character not in the brackets.	`[^abc]`	Matches any character except `a`, `b`, or `c`.
`()`	Groups expressions. Used for capturing.	`(ab)+`	Matches `ab`, `abab`, `ababab`. Captures each repetition.
`{n}`	Matches exactly `n` repetitions.	`a{3}`	Matches `aaa`.
`{n,}`	Matches `n` or more repetitions.	`a{2,}`	Matches `aa`, `aaa`, `aaaa`, etc.
`{n,m}`	Matches between `n` and `m` repetitions.	`a{2,4}`	Matches `aa`, `aaa`, or `aaaa`.
`\`	Escapes a special character.	`\.`	Matches the literal `.` character.
`\d`	Matches any digit (0–9).	`\d+`	Matches `123`, `456`, etc.
`\D`	Matches any non-digit.	`\D+`	Matches `abc`, `@#$`, etc.
`\w`	Matches any word character (letters, digits, underscore).	`\w+`	Matches `hello`, `word_1`, etc.
`\W`	Matches any non-word character.	`\W+`	Matches `@#$`, (space), etc.
`\s`	Matches any whitespace character.	`\s+`	Matches spaces, tabs, or newlines.
`\S`	Matches any non-whitespace character.	`\S+`	Matches words or symbols, but not spaces.

2. Common Use Cases

Remove Unwanted Characters or Text

If you want to remove specific parts of a title or description, use a regex pattern.

Example:

Pattern: ^(.*?)\s—.*
Result: Matches the first part of a title before the — symbol and removes everything after it.
Use Case: Clean titles like Breaking News — World Updates to only show Breaking News.

Whitelist Specific Keywords

Include posts that contain specific words or phrases.

Example:

Pattern: .*(AI|Machine Learning|Data Science).*
Result: Matches posts containing AI, Machine Learning, or Data Science.
Use Case: Curate a feed with only tech-related content.

Blacklist Specific Keywords

Exclude posts with unwanted words or phrases.

Example:

Pattern: .*(Sports|Celebrity|Gossip).*
Result: Hides posts containing Sports, Celebrity, or Gossip.
Use Case: Avoid non-business-related content.

Filter by URL or Domain

Use regex to include or exclude posts from specific domains.

Example:

Pattern: .*example\.com.*
Result: Filters posts that mention example.com in the URL.
Use Case: Target posts from a specific website.

Match Specific Date Formats

Include or exclude posts based on date formats.

Example:

Pattern: \d{4}-\d{2}-\d{2}
Result: Matches a date format like 2024-12-31.
Use Case: Focus on posts with structured date formats.

3. How to Apply Regex Filters

Navigate to the feed/bundle filters you want to apply the filter to.
Go to Replace or Remove and Add Replace/Remove rule
Enable Regular expression by clicking on .*
Add Regex Pattern: .*example\.com.*

4. Tips

Use tools like Regex101 to validate your regex patterns before applying them.
Begin with basic patterns and refine as you understand how they affect your feed.
If you're having trouble creating a regex pattern, you can use AI tools like ChatGPT to assist you. Simply describe what you're trying to filter or match, and it can generate the necessary regex filter for you. For example, you can ask:
- "Create a regex pattern to match titles containing the words 'AI' or 'Machine Learning'."
- "Generate a regex to exclude URLs containing 'example.com'."