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The other day, I tweeted a link to a regex-based mute filter for Tweetbot. The full URL looks like:

tweetbot:///mute/keyword?regex=1&text=(%3Fi)%23%3Fbreaking(%3F%3D%20%3Fbad)

It uses Tweetbot’s URL scheme¹, to provide a predefined mute filter. This way, a filter can be shared without copy and pasting it, which is quite nice. In trying to work out how to do this, I ended up browsing around Twitter trying to find an example of how to pass along a regex in the URL, as I had seen it done before. As I imagine I’ll do this again, I thought I’d write it up.

Regex

Before it is encoded, the regex looks like this:

(?i)#?breaking(?= ?bad)

It’s designed to match;

• “breaking bad”
• “breakingbad”
• “#breakingbad”
• “Breaking Bad”

and similar variations, but not;

• “breaking”
• “bad”

when used alone.

The first bit, (?i) sets the regex to be case insensitive. #? means it may or may not start with a hash (matching hashtags or not). Then, the rest comprises of a lookahead assertion.

A lookahead assertion tests to see if a given set of characters are followed by another set. It’s considered an assertion because it doesn’t consume these characters, it will only match them. For testing regular expressions, I use Patterns, so you get something that looks like this:

You will see that whilst this matches the appropriate lines, it doesn’t match the whole term. In this situation, this is fine (Tweetbot filters any tweet that would match). So, breaking(?= ?bad) looks for the word “breaking” followed by “bad”, with or without a space between them. The lookahead assertion is the bit in brackets, (?=).

Assembling the URL

The next bit is to make it valid inside a URL. I cheated and used Eric Meyer’s URL Decoder/Encoder, but the invalid characters are below:

There are obviously many tools to help do that bit.

So now, the next time I want to avoid spoilers to the finale of a pretty good TV show, I’ll be save in the knowledge that once, I wrote down how it did it.