I tried searching for this, but can't find anything.

I have a really simple snippet:

<snippet>
    <content><![CDATA[
% _______________ want to repeat underscore... x{100-TM_LINE_INDEX} times
%% ${1:DESCRIPTION}
]]></content>
    <!-- Optional: Set a tabTrigger to define how to trigger the snippet -->
    <tabTrigger>:%%</tabTrigger>
    <!-- Optional: Set a scope to limit where the snippet will trigger -->
    <scope>source.matlab,source.octave</scope>
    <description>Cell block</description>
</snippet>

Basically when I type ":%%" I want the top line to add underscores from the caret position until column 100. So the output will look something like this:

    %_________________  ...  __________________________ 
    %% DESCRIPTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . column 100 here ↑

I figured out how to calculate the number of underscores, which is what it prints currently, with this command:

{100-TM_LINE_INDEX}

But I cannot figure out how to do iterated replacement/while loop/for loop/whatever. I'm sure there must be a simple way of doing this!