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Python re.search simple question

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  Python re.search simple question Ganesh Pal <ganesh1pal@gmail.com> - 2014-12-08 12:22 +0530
    Re: Python re.search simple question Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-12-08 09:08 +0200
      Re: Python re.search simple question Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-12-08 09:13 +0200
    Re: Python re.search simple question Denis McMahon <denismfmcmahon@gmail.com> - 2014-12-09 00:31 +0000

#82109 — Python re.search simple question

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Hi Folks ,

This  might seem to be very trivial question but iam breaking my head over
it for a while .

 My understanding is that re.search should search for the match anywhere in
the string .


why is re.search failing in the below case  ??

>>> pattern
'Token-based migrations cannot be mixed with level-based: [prev 0 , now 1]'
>>> text
' LogMessage {type NDMP_LOG_DBG} {msg_id 0} {msg The process id for NDMP
service is 0x9c216370} {associated_msg 0} {associated_msg_seq 0} Source
filer:DartOS  Error: Token-based migrations cannot be mixed with
level-based: [prev 0 , now 1]'

>>> if (re.search(pattern,text)):
...    print "Hi"
... else:
...     print "BYE"
...
BYE

Regards,
Ganesh

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#82110

Ganesh Pal writes:

> why is re.search failing in the below case  ??

Your pattern, '... level-based: [prev 0 , now 1]', matches a literal
string '--- level-based: ' followed by 'p', 'r', 'e', 'v', ' ', '0',
..., or '1', none of which is the '[' found in your text at that
position.

Are you sure you need a regexp?

If you are, make your pattern r'... \[prev 0 , now 1\]'. The 'r' is
not a typo.

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#82113

Jussi Piitulainen writes:

> Ganesh Pal writes:
> 
> > why is re.search failing in the below case  ??
> 
> Your pattern, '... level-based: [prev 0 , now 1]', matches a literal
> string '--- level-based: ' followed by 'p', 'r', 'e', 'v', ' ', '0',
> ..., or '1', none of which is the '[' found in your text at that
> position.
> 
> Are you sure you need a regexp?
> 
> If you are, make your pattern r'... \[prev 0 , now 1\]'. The 'r' is
> not a typo.

On the other hand, the '---' *is* a typo, meant to be an ellipsis
where I shortened your text. The shortened pattern would still match
it, but that's an accident: '.' has a special meaning in a regexp.

Sorry about that.

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#82164

On Mon, 08 Dec 2014 12:22:37 +0530, Ganesh Pal wrote:

>>>> pattern
> 'Token-based migrations cannot be mixed with level-based: [prev 0 , now
> 1]'

Because [] defines a character class in a regex, if you want literal 
[ and ] you need to escape them with backslash.

[prev 0 , now 1] -> match any single character from the set "prev0,now1 "

\[prev 0 , now 1\] -> match the actual text [prev 0 , now 1]

Try these:

re.search('[prev 0 , now 1]','p') # matches (p in "prev0,now1 ")

re.search('[prev 0 , now 1]','x') # doesn't match (x not in "prev0,now1 ")

re.search('\[prev 0 , now 1\]','p') # doesn't match

re.search('\[prev 0 , now 1\]','[prev 0 , now 1]') # matches

-- 
Denis McMahon, denismfmcmahon@gmail.com

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