Re: Overlapping Regexps

[Kim Hansen]

On Sun, Mar 30, 2008 at 6:26 PM, Bill Denney <address@hidden> wrote:
> When running the following,
>
>  frag = {"MGTGGR" "R" "GAAAAPLLVAVAALLLGAAGHLYPGEVCPGMDIR" "NNLTR" \
>         "LHELENCSVIEGHLQILLMFK" "TRPEDFR" "DLSFPK" "LIMITDYLLLFR" \
>         "VYGLESLK" "DLFPNLTVIR"};
>  seq = strcat (frag{:});
>  cuts = regexp (seq, '[KR][^P]');
>
>  the result is
>  cuts = [6 41 46 67 74 80 92 100],
>  but I expect for cuts to also find 7.  In other words, I expected
>  cuts = [6 7 41 46 67 74 80 92 100].
>
>  On a related note, if there is overlap in matches, is there a way to
>  make regexp return the overlapping matches?  For example:
>
>  a = "ababababab"
>  b = regexp (a, "aba")
>
>  returns b = [1 5] when I would like for it to return b = [1 3 5 7].
>
>  Is this a bug in my understanding of regexp or in regexp?

What you need is the "zero-width positive look-ahead assertion", it is
documented for Perl in "man perlre". I have just tested it in Octave
and it works there too (octave uses libpcre for regexpes).

Your first regexp should be: "[KR](?=[^P])"   or "[KR](?!P)"

The second: "a(?=ba)"

-- 
Kim Hansen
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