[Python-bugs-list] [ python-Bugs-529708 ] error in re docs or in sre

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Bugs item #529708, was opened at 2002-03-13 20:39
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Category: Documentation
Group: Python 2.2
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Ben Wolfson (rumjuggler)
Assigned to: Fred L. Drake, Jr. (fdrake)
Summary: error in re docs or in sre

Initial Comment:
The docs for positive lookbehind assertions state that
"(?<=abc)def will match "abcdef", since the lookbehind
will back up 3 characters and check if the contained
pattern matches", but that doesn't gibe with experience:

>>> import re
>>> f = re.compile('(?<=abc)def')
>>> print f.match('abcdef')
None
>>> 

I don't know enough about REs to know if this is a
documentation error or an sre error, though.

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>Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2002-03-16 01:26

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Added clarifications and examples in Doc/lib/libre.tex
revisions 1.78, 1.73.6.3, and 1.60.2.4.

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Comment By: Fredrik Lundh (effbot)
Date: 2002-03-14 01:50

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the documentation is confused.  <= means that the
pattern following the assertion ("def") will only
match if preceeded by the assertion ("abc").

>>> re.search("(?<=abc)def", "abcdef").group(0)
'def'

>>> re.search("(?<=abc)...", "abcdef").group(0)
'def'

>>> re.search("(?<=c)\w", "abcdef").group(0)
'd'


here's a slighty better example: match words, but
only if they're preceeded with a hyphen:

>>> re.search("(?<=-)\w+", "spam -egg").group(0)
'egg'


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Comment By: Tim Peters (tim_one)
Date: 2002-03-14 01:42

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Changed to Docs and reassigned to Fred.  Reassign to /F if 
it will help.

The example works fine (and matches 'def') if .search() is 
used instead of .match().  It shouldn't match if .match() 
is used (and doesn't).  This is a confusion due to the 
reader mistaking the word "match" used to describe that the 
regexp, well, *matches* <wink>, with the method named "match
()" that merely constrains the match to begin at the start 
of the string.

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