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how can I check if group member exist ?

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  how can I check if group member exist ? Hans <hansyin@gmail.com> - 2013-06-21 02:07 -0700
    Re: how can I check if group member exist ? Peter Otten <__peter__@web.de> - 2013-06-21 11:25 +0200
    Re: how can I check if group member exist ? Ben Finney <ben+python@benfinney.id.au> - 2013-06-21 19:49 +1000

#48849 — how can I check if group member exist ?

Hi,

I'm doing a regular expression matching, let's say "a=re.search(re_str,match_str)", if matching, I don't know how many str/item will be extracted from re_str, maybe a.group(1), a.group(2) exist but a.group(3) does not. 

Can I somehow check it? something like:
if exist(a.group(1)): print a.group(1)
if exist(a.group(2)): print a.group(2)
if exist(a.group(3)): print a.group(3)


I don't want to be hit by "Indexerror":
>>>print a.group(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: no such group
>>> 

thanks!!!

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#48854

Hans wrote:

> Hi,
> 
> I'm doing a regular expression matching, let's say
> "a=re.search(re_str,match_str)", if matching, I don't know how many
> str/item will be extracted from re_str, maybe a.group(1), a.group(2) exist
> but a.group(3) does not.
> 
> Can I somehow check it? something like:
> if exist(a.group(1)): print a.group(1)
> if exist(a.group(2)): print a.group(2)
> if exist(a.group(3)): print a.group(3)
> 
> 
> I don't want to be hit by "Indexerror":
>>>>print a.group(3)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> IndexError: no such group
>>>> 
> 
> thanks!!!

You could catch the exception

for index in itertools.count(1):
    try:
        print a.group(index)
    except IndexError:
        break

but in this case there's the groups() method:

for g in a.groups():
    print g


The interactive interpreter is a good tool to find candidates for a solution 
yourself:

>>> a = re.compile("(.)(.)(.)").search("alpha")
>>> dir(a)
['__class__', '__copy__', '__deepcopy__', '__delattr__', '__doc__', 
'__format__', '__getattribute__', '__hash__', '__init__', '__new__', 
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', 
'__str__', '__subclasshook__', 'end', 'endpos', 'expand', 'group', 
'groupdict', 'groups', 'lastgroup', 'lastindex', 'pos', 're', 'regs', 
'span', 'start', 'string']

>From the above names lastindex looks promising, too. Can you find out how 
the output of

for i in range(a.lastindex):
    print a.group(i+1)

differs from that of looping over groups()?

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#48855

Hans <hansyin@gmail.com> writes:

> I'm doing a regular expression matching, let's say
> "a=re.search(re_str,match_str)", if matching, I don't know how many
> str/item will be extracted from re_str

Can you show an example of the pattern, and several examples of text you
want to match? This will help to understand your issue.

Also, your terminology is using some confusing names. Try:

    match = re.search(pattern, text)

That will use names which make it easier to think about the code.

-- 
 \     “Ours is a world where people don't know what they want and are |
  `\       willing to go through hell to get it.” —Donald Robert Perry |
_o__)                                                          Marquis |
Ben Finney

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