Interestingly, I was informed some time ago that modern Perl regular
expressions CAN do so... suggesting that perhaps in the name of
correctness, Perl programmers ought to stop calling them "regular
expressions" and start calling them "context-free expressions".  Java,
though, does not include the Perl madness that makes this possible.  To
quote from the API docs:

   Perl constructs not supported by this class:
       [...]
       The embedded code constructs (?{code}) and (??{code}),

   I believe that in formal language theory, they define "match" as
accepting all the strings from a certain set of strings, and not accepting
that set's complement with respect to a fixed alphabet.

   (Since this is posted on a Java newsgroup, we can assume that the
alphabet is Unicode, and the implementation of regexp used is Sun's
implementation).

   So actually ".*" does not "match" the set of "mathematical expressions"
because "Hello World!" is not a mathematical expression, and yet it is
accepted by the provided regular expression.

See: http://en.wikipedia.org/wiki/Regular_expression

   - Oliver