split a string of expression into elements

Hello !

I'd like to split a string of expressions into several
elements.
For example:
<string>
        x := b + c   
</string>

transform to sth like

<assignment>
       <left> x </left> 
       <right>  
             <operator> plus </operator>     
             <aleft> b </aleft>
             <aright> c </aright>
       </right>
</assignment>

The expression may be not simple like that, meaning
that it contains more operations x:= b + c - d ... 

Can you give me some directions how to transform like
that ? Can xslt match the string according to a
regular expression and give the substring? 

Thanks for your attention.
ha.

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