[Python-checkins] CVS: python/dist/src/Doc/lib libre.tex,1.77,1.78

[Sjoerd Mullender]

I don't think the first example will actually work: there is a "c" missing in
the to-be-matched string.

"Fred L. Drake" wrote:
> 
> Update of /cvsroot/python/python/dist/src/Doc/lib
> In directory usw-pr-cvs1:/tmp/cvs-serv24044/lib
> 
> Modified Files:
>         libre.tex
> Log Message:
> Clarify the descriptions of the positive and negative lookbehind assertions.
> Added examples of positive lookbehind assertions.
> This closes SF bug #529708.
> 
> Index: libre.tex
> ===================================================================
> RCS file: /cvsroot/python/python/dist/src/Doc/lib/libre.tex,v
> retrieving revision 1.77
> retrieving revision 1.78
> diff -C2 -d -r1.77 -r1.78
> *** libre.tex   5 Mar 2002 04:02:39 -0000       1.77
> --- libre.tex   16 Mar 2002 05:58:12 -0000      1.78
> ***************
> *** 273,288 ****
>   \item[\code{(?<=...)}] Matches if the current position in the string
>   is preceded by a match for \regexp{...} that ends at the current
> ! position.  This is called a positive lookbehind assertion.
> ! \regexp{(?<=abc)def} will match \samp{abcdef}, since the lookbehind
> ! will back up 3 characters and check if the contained pattern matches.
> ! The contained pattern must only match strings of some fixed length,
> ! meaning that \regexp{abc} or \regexp{a|b} are allowed, but \regexp{a*}
> ! isn't.
> 
>   \item[\code{(?<!...)}] Matches if the current position in the string
> ! is not preceded by a match for \regexp{...}.  This
> ! is called a negative lookbehind assertion.  Similar to positive lookbehind
>   assertions, the contained pattern must only match strings of some
> ! fixed length.
> 
>   \end{list}
> --- 273,309 ----
>   \item[\code{(?<=...)}] Matches if the current position in the string
>   is preceded by a match for \regexp{...} that ends at the current
> ! position.  This is called a \dfn{positive lookbehind assertion}.
> ! \regexp{(?<=abc)def} will find a match in \samp{abcdef}, since the
> ! lookbehind will back up 3 characters and check if the contained
> ! pattern matches.  The contained pattern must only match strings of
> ! some fixed length, meaning that \regexp{abc} or \regexp{a|b} are
> ! allowed, but \regexp{a*} and \regexp{a\{3,4\}} are not.  Note that
> ! patterns which start with positive lookbehind assertions will never
> ! match at the beginning of the string being searched; you will most
> ! likely want to use the \function{search()} function rather than the
> ! \function{match()} function:
> !
> ! \begin{verbatim}
> ! >>> import re
> ! >>> m = re.search('(?<=abc)def', 'abdef')
> ! >>> m.group(0)
> ! 'def'
> ! \end{verbatim}
> !
> ! This example looks for a word following a hyphen:
> !
> ! \begin{verbatim}
> ! >>> m = re.search('(?<=-)\w+', 'spam-egg')
> ! >>> m.group(0)
> ! 'egg'
> ! \end{verbatim}
> 
>   \item[\code{(?<!...)}] Matches if the current position in the string
> ! is not preceded by a match for \regexp{...}.  This is called a
> ! \dfn{negative lookbehind assertion}.  Similar to positive lookbehind
>   assertions, the contained pattern must only match strings of some
> ! fixed length.  Patterns which start with negative lookbehind
> ! assertions will may match at the beginning of the string being
> ! searched.
> 
>   \end{list}
> 
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