Exception expected not empty string

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Have an invalid pattern (e.g. (.{12})&(a{3}).
2. Ensure assertions are disabled.
3. Generate pattern based on invalid pattern.

What is the expected output? What do you see instead?
I expect an exception.
I get the empty string ("").
The empty string is however valid for some patterns (such as a*).

What version of the product are you using? On what operating system?
Latest version on windows.

Please provide any additional information below.
The patterns are not under my control: I get them at run-time.
I would like to inform the users of their incorrect pattern, but I can't check 
that! 

Original issue reported on code.google.com by [email protected] on 20 Dec 2013 at 9:51