Re: regexp strangeness

[Daniel J Sebald]

On 2/8/20 3:32 AM, Kay Nick wrote:
> Hey all,
>
> the documentation to regexp says:
>
> '\w'
>            Match any word character
>
> what exactly is a word character (maybe even more important what isn't)?
> Am I right in assuming its
> [abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ]? What about non
> english characters like öäßłńŚ?

https://en.wikipedia.org/wiki/Regular_expression#Character_classes

lists \w as the equivalent to [A-Za-z0-9_]

Probably non-english won't handle this, but maybe you could try [ä-Ś] or 
whatever makes sense for the alphabet of interest.

> And here some other strange (to me) behavior:
>
> > > regexp("#w#","#\w#")
> ans =  1                        <- seems to work in general...

As you point out two examples later, there is need of an escape.  That 
has nothing to do with the regexp() programming, but generally in Octave 
double quotes are like the C printf syntax, i.e., escapes.  Matlab 
doesn't interpret double quotes.  On the other hand, Octave treats 
single quotes just the way that Matlab does.

So, in the above \w is an escape sequence, but probably one that isn't 
defined so that \w ends up the same as w.  So what you've done is 
regexp("#w#","#w#"), which matches.


> > > regexp("#d#","#\w#")
> ans = [](1x0)                    <- why?

Because by the same logic as above, you've done regexp("#d#","#w#"), 
which doesn't match.

> > > regexp("#d#","#\\w#")       <- so we need to double escape these
> special characters... no mention of that in the help... :-(
> ans =  1
> > > regexp("#j#","#\\w#")
> ans =  1                        <- ok
> > > regexp("#E#","#\\w#")
> ans =  1                        <- ok
> > > regexp("#E#","#\\w*#")
> ans =  1                        <- ok
> > > regexp("##","#\\w*#")
> ans =  1                        <- ok
> > > regexp("#.#","#\\w*#")
> ans = [](1x0)                    <- why?

Because . is not in [A-Za-z0-9_]

Dan


> Especially the last one >> regexp("#.#","#\\w*#") ans = [](1x0) looks
> like a bug to me. Or am I getting something wrong here?
>
> Thanks
>
>
> Kay