[Changeset] Re: Aw: Re: regexp: matching expressions b4 and after ....

[David Bateman]

David Bateman wrote:
> address@hidden wrote:
> >  
> >
> > Hi,
> >
> > thanks for the reply Ben. As you see in your attempt, also the first 
> > "(-1)" gets replaced, which is exactly what the regexprep example 
> > command (in MATLAB) avoids.
> >
> > In practice I'm looking for the look-around operators for OCTAVE as 
> > described in MATLAB.
> >   
>
> Where exactly is "look-around" explained?
>
> D.
Ok, forget it.. I figured it out.. The issue is that matlab uses a 
different syntax for named tokens than PCRE, so we are obliged to look 
for named tokens like "(?<name>)" and replace them with the PCRE 
compatible "(?P<name>)". The test in Octave to do this was trapping 
"(?<=...)" and "(?<!...") as a syntax error for a matlab named token. 
The other lookaround operator "(?=...)" and "(?!...)" seem to work as 
pretty much as expected.

One issue is that PCRE does not accept arbitrary length lookaround 
expressions and so  "(?<=[a-z]*)" is not legal with PCRE. Though maximum 
length lookarounds are acceptable, so you can write instead 
"(?<=[a-z]{10})" for example.

I have a changeset to address this, but wonder if I should look for 
lookaround operators with "*" or "+" and replace with "{MAX_LENGTH}" and 
"{1:MAX_LENGTH}" respectively, with a warning about this limitation. 
Should I do this before submitting the changeset?

D.

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