Python regex recursion [duplicate]

Question

I am trying to match brackets using regex in python:

(a (dcsd)) ()

I dont have internet on my desktop at the moment so I cant download new libraries. What is the best way of doing this when regex has no recursive features in python?

More clearly, and for a particular user who cannot read, I do not have internet on my main computer, just some mobile internet that I am using to post this question. I was hoping that someone could suggest a manual algorithm for finding the contents of matched brackets as the only answers I have found using the almighty google use separate libraries.

In that example I would need the result 'a' 'dcsd'

i am sorry,because there is no support of recursion in re library..you need regex library..BTW, how are you asking question without internet? — rock321987
– rock321987, Commented Jun 26, 2016 at 14:21
If you just want to verify they are balanced - without a regular expression: s.count('(') == s.count(')') — wwii
– wwii, Commented Jun 26, 2016 at 14:36
1 - I have no internet on the desktop I need to the run program from. I of course have it on my phone. 2 - Google isnt helping because python does not seem to support recursive regex - hence I am asking for an alternative algorthim that does not involve downloading a new library - something I could not find elsewhere on here, obviously I googled it first — user1212520
– user1212520, Commented Jun 26, 2016 at 14:44

Filip Allberg · Accepted Answer · 2016-06-26 14:31:54Z

1

You cannot match brackets using regex, this sentiment is echoed by the comments you have received.

Regular expressions (regex) is a Chomsky Type 3 grammar, whereas finding matching tokens in a string requires a Chomsky Type 2 grammar (or above). Regular expressions have the same descriptive power as a finite automata.

To find matching tokens such as quote symbols, parentheses, braces etc. requires a computational model capable of describing grammars of Chomsky Type 2, namely a pushdown automaton. You might better recognize this as having a "stack".

answered Jun 26, 2016 at 14:31

Filip Allberg

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