Dart - Split String by RegEx

Question

I am trying to split a string like this:

x^-5 - 3

into a list like this:

[x^-5, -, 3]

The first minus after the ^ must be at the same list index as the x, because it's just the negative exponent. However, I want other minuses, which are not an exponent of anything, to be on their own index.

When splitting by -, obviously my x^-5 gets split into two as well.

So is there any way I can achieve this using RegEx or something like that?

Thanks in advance

Did you try splitting with whitespace? `.split(new RegExp(r"\s+"))` — Wiktor Stribiżew, Apr 26 '17 at 11:43
Sorry, I forgot to mention that it should work without whitespace... But If I don't find another solution, I'll stick to whitespace, thanks. — OhMad, Apr 26 '17 at 11:50
It turns out, in Dart, you cannot use a JS approach to capture subpatterns to return with the split chunks. You'd better write a parser or use any ready-made libraries. — Wiktor Stribiżew, Apr 26 '17 at 11:56

score 6 · Accepted Answer · edited May 23 '17 at 12:26

6

If you use allMatches instead of split, you can use a pattern like this:

(?:\^\s*-|[^\-])+|-

Working example: DartPad

We match tokens that consist of anything except -, or ^-.
If we reach a - that is not an exponent, we match it alone, similar to a split.

Some notes:

There are similar patterns, this may not be the most efficient, but it is short.
If you are matching mathematical expressions, there are many things that can go wrong (for example parentheses), regular expressions are not a good way to achieve that.
This is basically the match to skip trick.

edited May 23 '17 at 12:26

Community

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answered Apr 26 '17 at 12:25

Kobi

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In other languages you could've used a lookbehind, [`(?<!\^)-`](https://regex101.com/r/2vlDCy/1), but that isn't supported on JavaScript (and Dart). – Kobi Apr 26 '17 at 12:30
Wow, awesome. Exactly what I was looking for, thank you :) – OhMad Apr 26 '17 at 12:31
Btw what would you use instead of regex? – OhMad Apr 26 '17 at 12:34
@Rechunk - For example, in JavaScript, I found a library called Math.js, that can parse expressions: http://mathjs.org/docs/expressions/parsing.html#parse . For Dart, there's this: [Is there a math parser for petitparser?](http://stackoverflow.com/q/18924498/7586), and [math_expressions (beta)](https://pub.dartlang.org/packages/math_expressions) – Kobi Apr 26 '17 at 12:39
how would I extend this regex to be able to split something like this: x^-5 + 3 (with a plus) into this: [x^-5, +, 3]? – OhMad Apr 26 '17 at 18:36
@Rechunk - That's pretty simple. You need to change all places you have `-` to also accept `+`. For example (also with `*` and `/`): `(?:[\^*/]\s*[\-+]|[^\-+])+|[\-+]` - [example](https://regex101.com/r/O2QaDh/1) – Kobi Apr 26 '17 at 18:49

Dart - Split String by RegEx

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