A question about the expressive power and limitations

As I'm just starting to read the latest WDs, I'd greatly appreciate it if somebody
could provide examples showing:

1. A problem, which cannot be easily solved by using "for", but which has a natural
recursive solution. Calling user-defined functions within an XPath expression must
be excluded, as we can do anything (e.g. recursion) within a user-defined function.

2. A (text processing), which cannot be solved (easily) by using regular
expressions. David already mentioned a string enclosed in balanced parenthesis.
Another example is a string consisting of equal number of 1-s and 0-s. It is known
that any language defined by a CFG but which cannot be defined by a RE. I just need
a small, and if possible meaningful, concrete example. 

Cheers,
Dimitre Novatchev.

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