Re: Aw: Re: regexp: matching expressions b4 and after ....

[Ben Abbott]

On Tuesday, September 09, 2008, at 09:41AM, "David Bateman" <address@hidden> 
wrote:
>
>Grrrr, its more annoying than I thought. PCRE CAN do arbitrary length 
>lookahead, but not arbitrary length lookbehind. Thus "(?[a-z]*)" is ok 
>but "(?<[a-z]*)" isn't. I'd hoped to replace this with 
>"(?<[a-z]{0,MAXLENGTH})" but the variable but not arbitrary length is 
>not ok either. What I'd have to do is replace it with
>
>((?<[a-z]{0})(?<[a-z]{1})...(?<[a-z]{MAXLENGTH}))
>
>which used the alternate operator and MALENGTH+1 copies of the 
>lookbehind expression to get the effect. This seems to be a ridiculous 
>amount of extra crap in the pattern space to get this functionality. Is 
>it worth supporting arbitrary length lookbehind expressions like 
>"(?<[a-z]*)" if this is what is needed to get it to work with PCRE? Is 
>it worth supporting it but limits max_length, and print a warning? If so 
>what value should be the limit?
>
>Frankly I wonder how mathworks got this to work as they appear to be 
>using the Boost regex library which also doesn't support arbitrary 
>length lookbehind expressions....
>
>D.

David,

Have you tried the example in Matlab?

Using 2007b, It does *not* work for me. My 2008a/b is busy running some 
simulations, so I can't try it there until later.

>> g='x^(-1)+y(-1)+z(-1)=0';
>> regexprep(g,'(?<=[a-z]*)\(\-[1-9]*\)','\_minus1')
ans =
x^_minus1+y_minus1+z_minus1=0

If I understand correctly the result should be 

ans =
x^(-1)+y_minus1+z_minus1=0

Correct?

Ben