From: Robert Klemme <bob.news@...>
Date: 2006-03-12T21:13:44+09:00
Subject: Re: Regular Expression Intersection

Benjohn Barnes <benjohn@fysh.org> wrote:
> On 12 Mar 2006, at 09:58, Dan Kohn wrote:
>
>> You'll generally get a better answer if you include an example in
>> your question.  Also, that link didn't work.
>>
>> If you're asking how to apply two regexps, you can use the scan
>> method to get the results in arrays, and then intersect them with &.

This does not help at all as Benjon explains.  The question is not whether 
there are some strings in a set of strings that are matched by both RX but 
whether there is at least one string among *all* possible strings that is 
matched by both.

>> string ="hello world"; puts string.scan(/.../) & string.scan(/..l/)
>
> I think he's trying to do this...
>
> If you have a regular expression, R, then there is a (potentially
> infinite) set S(R) of input strings that it will match.
>
> Given two regular expressions, R1 and R2, you can find the strings
> that match either regular expression:
> S(R1) & S(R2)
>
> He now wants to find a regular expression Ri such that:
> S(Ri) = S(R1) & S(R2)

No.  He wants to know whether S(R1) & S(R2) is not empty.  At least that's 
what he stated.  You might be able to solve the problem by finding Ri but 
that was not the original problem stated.

Kind regards

    robert