Snippet that replace the whole text

Hi, first of all this plugin is great.
I do not have any idea of python, so I wonder if anybody could help me with this.

For me it is easy to write words with the keyboards, although writting numbers and simbols is different...
I would like to make a snippit like this:

You write the snippet tab trigger n
and now you write a mathematical expresion, but with letters, for example:
ob ontwthfofisiseeini cb di fo pl si ti on

where each 2 letters represetn a number or a symbol
on = 1
tw = 2
th = 3

ob = open brace (
cb = close brace )
di = /
pl = +

etc...

or it could be done with a differten mapping...
I ve do it with transformations, but the problem is that the first input remains

so you tipe n tab, and enter
ob ontwthfofisiseeini cb di fo pl si ti on

and you obtain

ob ontwthfofisiseeini cb di fo pl si ti on
( 123456789 ) / 4 + 6 * 1

How could I delet the first input and end only with the math

Thank you