What does \d match in a regex?

Earlier tonight, I was playing with Hypothesis’s from_regex strategy. This strategy generates strings that match a given regex, and I thought it might be a good way to debug especially gnarly regexes – by seeing examples of matching strings, maybe I could understand a complex regex better.

I was trying to find strings that matched the regex \d+\(\d+\) (a random example from Google), and it exposed a bug in my understanding.

I know that \d matches any digits, and I’d always treated it as synonymous with [0-9] – but it turns out that’s not always the case. There are all sorts of Unicode characters which are digits and which match \d, but which aren’t Arabic numerals. Here are just a couple:

Different languages handle this differently – for example, using \d in Python 3 will match all these numeric characters that aren’t Arabic numerals, but in Python 2 it won’t. No doubt it varies among other programming languages as well.

How much this distinction matters depends on your use case, and in most cases I imagine the answer is “very little”. I present it more as an intellectual curiosity than something which needs serious consideration when writing regexes.

This isn’t the first time Hypothesis has exposed one of my faulty assumptions – language is incredibly complicated, and I still have a lot to learn.