正则表达式：由 + 分隔，除非在括号内

Question

I'm dealing with equations like 'x_{t+1}+y_{t}=z_{t-1}' . My objective is to obtain all "variables", that is, a list with x_{t+1}, y_{t}, z_{t-1} .

I'd like to split the string by [+-=*/] , but not if + or - are inside {} .

Something like this re.split('(?<!t)[\+\-\=]','x_{t+1}+y_{t}=z_{t-1}') partly does the job by not spliting if it observes t followed by a symbol. But I'd like to be more general. Assume there are no nested brackets.

How can I do this?

Answer 1

Instead of splitting at those characters, you could find sequences of all other characters (like x and _ ) and bracket parts (like {t+1} ). The first such sequence in the example is x , _ , {t+1} , ie, the substring x_{t+1} .

import re

s = 'x_{t+1}+y_{t}=z_{t-1}'

print(re.findall(r'(?:\{.*?}|[^-+=*/])+', s))

Output ( Try it online! ):

['x_{t+1}', 'y_{t}', 'z_{t-1}']

Answer 2

Instead of re.split , consider using re.findall to match only the variables:

>>> re.findall(r"[a-z0-9]+(?:_\{[^\}]+\})?","x_{t+1}+y_{t}=z_{t-1}+pi", re.IGNORECASE)
['x_{t+1}', 'y_{t}', 'z_{t-1}', 'pi']

Try online

Explanation of regex:

[a-z0-9]+(?:_\{[^\}]+\})?
[a-z0-9]+                : One or more alphanumeric characters
         (?:           )?: A non-capturing group, optional
            _\{      \}  : Underscore, and opening/closing brackets
               [^\}]+    : One or more non-close-bracket characters