How to decipher :re.split(r"(\(\([^)]+\)\))" in the example

[Cameron Simpson]

On 10Jul2014 08:37, fl <rxjwg98 at gmail.com> wrote:
>This example is from the link:
>
>https://wiki.python.org/moin/RegularExpression
>
>I have thought about it quite a while without a clue yet.
>I notice that it uses
>double quote ", in contrast to ' which I see more often until now.

With raw strings (r', r") this doesn't matter. I tend to use r' myself.

You want raw strings with regular expressions because otherwise their heavy use 
of sloshes "\" overlap with Python's use of sloshes, making everything harder.

>It looks very complicated to me. Could you simplified it to a simple example?
>
>import re
>split_up = re.split(r"(\(\([^)]+\)\))",
>                    "This is a ((test)) of the ((emergency broadcasting station.))")
>
>...which produces:
>
>["This is a ", "((test))", " of the ", "((emergency broadcasting station.))" ]

Rip off the python punctuation and get the regexp itself:

   (\(\([^)]+\)\))

then start from the inside out:

   [^)]  Any character except a closing bracket.
   +     One or more of the preceeding.

Therefore:

   [^)]+ One or more characters which are not closing brackets.
         Also phrased: at least one character which is not a closing bracket.

Outside this are \( and \): these are literal opening and closing bracket 
characters. So:

   \(\([^)]+\)\)
         Two opening brackets, then at least one character which is not a 
         closing bracket, then two closing brackets.

The outermost ( and ) are regexp grouping brackets, not text. On their own you 
don't need them, but they mark out the regexp between them for later reference 
or for use with a repeating modifier like ?, * or +. So in this instance they 
do not add anything special to the regexp.

Given the above inside-to-out explaination, does that explain the re.split 
result for you?

Cheers,
Cameron Simpson <cs at zip.com.au>

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         - David Wood <davewood at teleport.com>