Hacker News Reader

Comment by burntsushi | Hacker News Reader

Author of Rust's regex crate here.

> Also regex / variable interpolation should be added to regexes in languages probably

I've often wondered and desired this myself. It feels like the single most useful change one might make, because it enables (albeit very basic) logical decomposition. It comes in handy when a regex works for your use case, but the regex is somewhat long but with many repeated parts. In my experience, that isn't too rare, and the regex readability would likely be helped quite a bit by some kind of decomposition.

View on HN

Perl's been doing this for ages with the qx/.../ operator. It indeed works great for logical composition although Raku's first-class grammars are a much better evolutionary step IMO.

You mean the qr// operator. qx// is the generic operator for backticks.

You are right! Guess who hasn't used Perl for a few years!

For a Raku Grammar tutorial, see https://docs.raku.org/language/grammar_tutorial

You can do that by building up a string and converting that to a regex. Pseudo-code:

  let day = "[0-9]{2}"
  let month = "jan|feb|…|dec"
  let year = "[0-9]{4}"
  let regex = Regex(month + " " + day + ", " + year)

Disadvantage is that, if your programming language has automatic syntax check of regular expressions, you probably lose that.

The string concatenations also make things a bit slower, but that almost always needs to be done only once.

Advantage is that this makes testing easier. For example, you can do

  let dayRegex = Regex(day)

and then test that regex in isolation. That probably is overkill for this example, but if you change the regex to only accept numbers in [1,31], that can be useful.

If you decide to have your regex to treat 30-day and 31 day months separately, things get more complicated. I wouldn’t go that far, though, and certainly not as far as handling leap years correctly. When regexes get complex, it’s time to go for a proper lexer/parser.

Of course I'm aware of that technique and I've used it many times. :-) Doesn't change what I said.

Surely, you can use Rust's enums for this.

As I said above, I'm the author of Rust's regex crate. I know what Rust enums are.

Still doesn't change what I said.

I mean that it's typesafe.

I still don't know what you're talking about. I didn't say anything about type safety? Nobody else did? I guess you're probably talking about a combinator API... Like this one: https://docs.rs/regex-syntax/latest/regex_syntax/hir/struct....

My original comment was talking about adding variables to the regex language itself. String interpolation and combinator APIs and enums and type safety are all beside the point. They surely solve a similar problem as regex variables in certain contexts, but they go about it in a very different way and are specific to the context in which the regex is created.

It's easy to see the usages on github:

https://github.com/search?l=Rust&p=5&q=%22regex%3A%3Anew%22&...

For the importance of numeric range it was quite easy to find actually a good example:

x0rz3q/advent-of-code-2020

Regex::new(r"^19[2-8][0-9]|199[0-9]|200[0-2]$").unwrap(), ); comparitor.insert("iyr", Regex::new(r"^(201[0-9]|2020)$").unwrap());

There are lots of number parsing.

I would enable both [[:1-12:]] and [[:01-12:]] as options without / with leading zeros.

About the variables:

This file would look much more readable with variables that are reusing other regexes:

https://github.com/spcan/common-regex/blob/3238bc8ee85e0e000...

Reply on news.ycombinator.com