Groups in regular expressions don't repeat as expected

John Nagle nagle at animats.com
Sun Apr 24 21:43:41 CEST 2011


On 4/21/2011 6:16 AM, Neil Cerutti wrote:
> On 2011-04-20, John Nagle<nagle at animats.com>  wrote:
>>       Findall does something a bit different. It returns a list of
>> matches of the entire pattern, not repeats of groups within
>> the pattern.
>>
>>       Consider a regular expression for matching domain names:
>>
>>>>> kre = re.compile(r'^([a-zA-Z0-9\-]+)(?:\.([a-zA-Z0-9\-]+))+$')
>>>>> s = 'www.example.com'
>>>>> ms = kre.match(s)
>>>>> ms.groups()
>> ('www', 'com')
>>>>> msall = kre.findall(s)
>>>>> msall
>> [('www', 'com')]
>>
>> This is just a simple example.  But it illustrates an unnecessary
>> limitation.  The matcher can do the repeated matching; you just can't
>> get the results out.
>
> Thanks for the further explantion.
>
> Assuming a fake API that returned multiple group matches as a
> tuple:
>
>>> ? print(re.match(r"^([a-z])+$", "abcdef").groups())
> (('a', 'b', 'c', 'd', 'e', 'f'),)
>
> I was thinking of applying findall something like this, but you
> have to make multiple calls:
>
>>>> m = re.match(r"^[a-z]+$", s)
>>>> if m:
> ...   print(re.findall(r"[a-z]", m.group()))
> ...
> ['a', 'b', 'c', 'd', 'e', 'f']
>
> I can see that getting really annoying. Is there a better way to
> make multiple group matches accessible without adding a third
> element type as a group element?

     The most elegant solution would be to have a regular expression
function that returned a tree of tuples or lists.  Then you could
express an entire language syntax as a regular expression and
get out a parse tree.

     Since the regular expression system is actually doing that work,
then discarding the results, it seems a reasonable extension.
I'm not suggesting extending regular expression matching itself,
just the way the results are stored.

				John Nagle




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