RE: <restriction> - Bad Kleene Notation

Hello,

     I wrote:
>>>>>>>
     For example, consider a regular set of strings consisting of any 
number of the letters a and b, but at most one instance of each of the 
letters c and d.  One way of specifying this is as follows:

  (a|b)*((c?(a|b)*d?)|(d?(a|b)*c?))(a|b)*

It's much easier to express by saying it has the following form:

  (a|b|c|d)*

with the added restriction that that there is at most one instance of each 
of c and d.
<<<<<<<

     Another way of describing this language would be as the intersection 
of the languages generated by the following regular expression:

  (a|b|c)*d?(a|b|c)*  and (a|b|d)*c?(a|b|d)*

Of course, I still thinks that's not as clear as (a|b|c|d)* with the 
additional prose restriction.

     If someone has a more concise way of describing this using only 
regular expressions, please jump in.

Thanks,

Henry
------------------------------------------------------------------------
Henry Zongaro      XML Parsers development
IBM SWS Toronto Lab   Tie Line 969-6044;  Phone (905) 413-6044
mailto:zongaro@ca.ibm.com