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From: Peter Otten <__peter__@web.de>
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Subject: Re: Help for a complex RE
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Xref: csiph.com comp.lang.python:108367

Sergio Spina wrote:

> In the following ipython session:
> 
>> Python 3.5.1+ (default, Feb 24 2016, 11:28:57)
>> Type "copyright", "credits" or "license" for more information.
>>
>> IPython 2.3.0 -- An enhanced Interactive Python.
>>
>> In [1]: import re
>>
>> In [2]: patt = r"""  # the match pattern is:
>> ...:     .+          # one or more characters
>> ...:     [ ]         # followed by a space
>> ...:     (?=[@#D]:)  # that is followed by one of the
>> ...:                 # chars "@#D" and a colon ":"
>> ...:    """
>> 
>> In [3]: pattern = re.compile(patt, re.VERBOSE)
>> 
>> In [4]: m = pattern.match("Jun@i Bun#i @:Janji")
>> 
>> In [5]: m.group()
>> Out[5]: 'Jun@i Bun#i '
>> 
>> In [6]: m = pattern.match("Jun@i Bun#i @:Janji D:Banji")
>> 
>> In [7]: m.group()
>> Out[7]: 'Jun@i Bun#i @:Janji '
>> 
>> In [8]: m = pattern.match("Jun@i Bun#i @:Janji D:Banji #:Junji")
>> 
>> In [9]: m.group()
>> Out[9]: 'Jun@i Bun#i @:Janji D:Banji '
> 
> Why the regex engine stops the search at last piece of string?
> Why not at the first match of the group "@:"?
> What can it be a regex pattern with the following result?
> 
>> In [1]: m = pattern.match("Jun@i Bun#i @:Janji D:Banji #:Junji")
>> 
>> In [2]: m.group()
>> Out[2]: 'Jun@i Bun#i '

Compare:

>>> re.compile("a+").match("aaaa").group()
'aaaa'
>>> re.compile("a+?").match("aaaa").group()
'a'

By default pattern matching is "greedy" -- the ".+" part of your regex 
matches as many characters as possible. Adding a ? like in ".+?" triggers 
non-greedy matching.