[TxMt] LaTeX sub/superscript snippet

[Guido Governatori]

I'm trying to define a "smart" snippet for LaTeX sub/superscript.  
What I would like to achieve is a snippet

(1) triggered by _ (or ^ for superscripts)
(2) curly braces are inserted only when there are least two  
characters after _

Thus, if I type

_a, then I get _a

if I type

_ab then I get _{ab}

My current attempt is as follows:

_${1/(.{2,})|./(?1:{)/}$1${1/(.{2,})|./(?1:})/}$0

However, the above has a major shortcoming:

When I type _ as the second character after the first _, then the  
caret is in the wrong place. Thus,

_a_

produces

-{a|_}

and the caret is between a and _ instead of being after the second _

Any suggestions?

Guido
--
Dr Guido Governatori
School of Information Technology and Electrical Engineering
The University of Queensland
Brisbane, Queensland, 4072, Australia
Phone: +61-(0)7-336 52907
Fax: +61-(0)7-336 54999
http://www.governatori.net/TextMate/
http://www.governatori.net/TextMate/LaTeXExtension.tmbundle.zip
http://www.defeasible.org