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I fixed it, because it was breaking with more levels:
\newcommand{\underb}[2]{\tikz[baseline=(text.base)]{
\node[inner sep=0pt,outer sep=0pt,text height = 2ex] (text) {\ensuremath{#1}};
\draw[thick] ($(text.base east)-(0,0.1)-#2*(0,1ex)$) -- ++(0,-0.1) -| ($(text.base west)-(0,0.1)-(0,{#2*1ex ...

Thanks, it's almost it, but there is one problem. I should have mentioned it before, that I need to use those commands in math mode - some commands need to be nested. That is, although this works:
\underb{$\pi^l \pi$}
I have to use it like that:
$\underb{\pi^l \pi}$
I don't know Tikz at all, so ...

I use underbrackets to show that two elements can be reduced to, let's call it, an empty string. My problem is that in some examples I need to show that particular elements don't match - string can't be reduced by matching these two elements.

To make an underbracket, I simply write:
\documentclass ...