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

[David Bateman]

David Bateman wrote:
> 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?
>

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.


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