(Math)Inset which only accepts a restricted set of characters

[Stefan Schimanski]

Hi!

I need to restrict the allowed characters a user can enter in a cell  
of a InsetMathNest class. More precisely I want to disallow entering  
anything which is not a valid tex identifier in the math macro  
definition inset. At the moment the user can enter a fraction or any  
other math. The only thing I do currently is to show everything in red  
to indicate the error.

Is there anything similar like that already anywhere? If not, how can  
one implement this easily? The only way I see is to tweak the  
behaviour of InsetMathNest a lot, i.e. by changing  
InsetMathNest::interpretChar. But I guess even then the user can paste  
invalid stuff into the cells. Another idea would be to tweak the  
MathData of the cell to skip everything that is no valid character.  
But I am not sure how easy this would be because a MathData is  
essentially an stl vector.

Stefan