Re: regexp question

[Philip Nienhuis]

Sergei, Wiliam,

2 answers in one post:

Sergei Steshenko wrote:
> I guess you need 'aa' surrounded by not 'a'. Octave uses PCRE; I am not 
> familiar with nuances of Octave PCRE usage; in Perl I would write the regular 
> expression this way:
>
> [^a]aa[^a]
>
> and if/when it matches, it returns pointer to the character preceding the 'aa' 
> substring, i.e. in case of 'baab' it should return pointer to the first 'b'.

Thanks, Sergei. I already tried this and found it'll work, but 
unfortunately not in a more complicated situation:

octave:35> tststr3 = 'aa aaaaa baa'     ## Patterns at start & end
tststr3 = aa aaaaa baa
octave:36> regexp (tststr3, "[^a]aa[^a]")
ans = [](1x0)                           ## Hey......

but
octave:41> tststr4 = ' aa aaaaa baa '   ## Note spaces at start and end
tststr4 =  aa aaaaa baa
octave:42> regexp (tststr4, "[^a]aa[^a]")
ans =
    1   11

... so it doesn't catch the pattern at start and end of line.


William Krekeler wrote:
> Sorry I responded too soon. To only get the second shorter set of aa the 
> following works.
>
> indexes = regexp (tststr, "a{2,}");
> indexes2 = regexp (tststr, "a{3,}");
> desiredIndex = setxor( indexes, indexes2 )

Thanks.

octave:37> indexes = regexp (tststr3, "a{2,}")
indexes =
    1    4   11

octave:38> indexes2 = regexp (tststr3, "a{3,}")
indexes2 =  4
octave:39> desiredIndex = setxor( indexes, indexes2 )
desiredIndex =
    1   11

... so your trick seems to catch the right ones.


I'm a bit concerned that this solution will be too time-consuming.
What I actually want to do is check format strings from a spreadsheet 
which potentially may involve several tens of thousands strings (which 
are not much more complicated than the example above).
The following script:

tic
tststr3 = 'aa aaaaa baa';
for ii=1:50000
  indexes = regexp (tststr3, "a{2,}");
  indexes2 = regexp (tststr3, "a{3,}");
  desiredIndex = setxor( indexes, indexes2 );
  endfor
toc

...says:
"Elapsed time is 33.1 seconds."

A script using strfind:

tic
tststr3 = 'aa aaaaa baa';
for ii=1:50000
   idx1 = strfind (tststr3, 'aa');
   idx2 = strfind (tststr3, 'aaa');
   idx2 = strfind (tststr3, 'aaaa');
   idx2 = strfind (tststr3, 'aaaaa');
endfor
toc

...gives:
"Elapsed time is 2.27 seconds."

so a solution based on strfind() seems more preferrable.
(findstr() takes about 68 secs, I suspect it is based on regexp()).


Thanks again,

Philip