Question

I need a regex expression to split math expression, like:

-1*x+2*y

to the: -1*x and 2*y

I can’t just split that string basing in the + and - (because there can be negative numbers)

Answer 1

You can split on + or - which is preceded by a letter, digit and potentially other elements that are acceptable in your expressions (parenthesis for example). You can use look-behind to accomplish this.

For example in python you can do it like this:

>>> import re
>>> re.split('(?<=[a-zA-Z0-9)])[+-]', '-a+10*x-2*y+(-2)-b')
['-a', '10*x', '2*y', '(-2)', 'b']

This splits on every + or - not preceded by a small or capital letter or a digit or a closing parenthesis. You may need to tweak this depending on your exact specification of what constitutes a “math expression”.

This or similar regular expression should work in other regexp engines.