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

[Peter Otten]

fl wrote:

> Hi,
> 
> 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.
> It looks very complicated to me. Could you simplified it to a simple
> example?

Just break it into its components.

"(...)" in the context of re.split() keeps the delimiters while just "..." 
does not. Example:

>>> re.split("a+", "abbaaababa")
['', 'bb', 'b', 'b', '']
>>> re.split("(a+)", "abbaaababa")
['', 'a', 'bb', 'aaa', 'b', 'a', 'b', 'a', '']

r"\(" matches the openening parenthesis. The "(" has to be escaped because 
it otherwise has a special meaning (begin group) in a regex.

"[abc]" matches a, b, or c. A leading ^ inverts the set, so "[^abc]" matches 
anything but a, b, or c. Therefore "[^)]" matches anything but the closing 
parenthesis.

The complete regex then is: match two opening parens, then one or more chars 
that are not closing parens, then two closing parens, and make the complete 
group part of the resulting list.

PS: Note that sometimes the re.DEBUG flag may be helpful in understanding 
noisy regexes:

subpattern 1 
  literal 40 
  literal 40 
  max_repeat 1 4294967295 
    not_literal 41 
  literal 41 
  literal 41 
<_sre.SRE_Pattern object at 0x7f5740455c90>

> 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.))" ]