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regular expression for nested parentheses

John Machin sjmachin at lexicon.net
Sun Dec 9 17:12:27 EST 2007 
On Dec 10, 8:53 am, Noah Hoffman <noah.hoff... at gmail.com> wrote:
> On Dec 9, 1:41 pm, John Machin <sjmac... at lexicon.net> wrote:
>
> > A pattern that can validly be described as a "regular expression"
> > cannot count and thus can't match balanced parentheses. Some "RE"
> > engines provide a method of tagging a sub-pattern so that a match must
> > include balanced () (or [] or {}); Python's doesn't.
>
> Okay, thanks for the clarification. So recursion is not possible using
> python regular expressions?
>
> > Ummm ... even if Python's re engine did do what you want, wouldn't you
> > need flags=re.VERBOSE in there?
>
> Ah, thanks for letting me know about that flag; but removing
> whitespace as I did with the no_ws lambda expression should also work,
> no?

Under a very limited definition of "work". That technique would not
produce correct answers on patterns that contain any *significant*
whitespace e.g. you want to match "foo" and "bar" separated by one or
more spaces (but not tabs, newlines etc) ....
pattern = r"""
foo
[ ]+
bar
"""
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