Ingo incorrectly sends commas in regex expression to sieve backend. 
For example I wanted to create rule that filters all letters with more 
than 10 recipients. Regex for such rule looks like this: (@.*){11,0}. 
When I click "Save" sieve throws an error: {39}script errors: line 71: 
Unmatched \{.

This is because Ingo somehow thinks that this is two blocks: (@.*){11 
and 0}. Escaping comma with \ solves the problem.