Groups in regular expressions don't repeat as expected

John Nagle nagle at animats.com
Wed Apr 20 22:34:16 CEST 2011


On 4/20/2011 12:23 PM, Neil Cerutti wrote:
> On 2011-04-20, John Nagle<nagle at animats.com>  wrote:
>> Here's something that surprised me about Python regular expressions.
>>
>>>>> krex = re.compile(r"^([a-z])+$")
>>>>> s = "abcdef"
>>>>> ms = krex.match(s)
>>>>> ms.groups()
>> ('f',)
>>
>> The parentheses indicate a capturing group within the
>> regular expression, and the "+" indicates that the
>> group can appear one or more times.  The regular
>> expression matches that way.  But instead of returning
>> a captured group for each character, it returns only the
>> last one.
>>
>> The documentation in fact says that, at
>>
>> http://docs.python.org/library/re.html
>>
>> "If a group is contained in a part of the pattern that matched multiple
>> times, the last match is returned."
>>
>> That's kind of lame, though. I'd expect that there would be some way
>> to retrieve all matches.
>
> .findall
>

     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.

				John Nagle



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