Re: additional regular expression operators

Ralph Boland <rpboland@gmail.com> writes:


> On Mar 31, 5:30 am, torb...@pc-003.diku.dk (Torben Fgidius Mogensen)
> wrote:
>> Ralph Boland <rpbol...@gmail.com> writes:
>> > I plan to provide support for at least three additional
>> > unary operators to the standard three (?,*, and +)
>>
>> > R! :does not accept the empty string but otherwise
>> > accepts every expression that R does. Very useful.
>>
>> > R~ :accepts exactly the set of strings that R does not accept.
>> > note that R = R~~.
>>
>> > R% : equivalent to the string (R~)R
>>
>> I can see the usefulness of the first two, but not really the third.
>> Do you have an example?
>
> It's useful in pattern matching where we want to find the first
> occurrence of pattern R.


I can't see how, unless your general rule is that a regular expression
finds the shortest prefix of the string that matches the expression.
The usual rule is to find the longest prefix or to find the first
occurrence (i.e., not necessarily a prefix) that does.


> It is also useful in scanners where we have comments.
> If a comment starts with <: and ends with :> the
> definition of a comment would be:
> <:(:>)%.


I don't think this will work. Assume the string is


        <:abc:>:>


This will match the regexp, as abc:> matches (:>)~.


Torben

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