<p class="txt" xml:lang="es"><span class="lookforthistag">Change These Words</span><span class="txt">, Roland (2005). </span><span class="crva">El grado cero de la escritura. </span><span class="txt">Seguido de </span><span class="crva">Ensayos crÃticos. </span><span class="txt">Siglo XXI. Madrid. </span></p>
You're asking two different questions and I can only help with the first one.
To select the "lookforthistag" span element, you can use: <span class="lookforthistag">[^<]*</span>
The .* expression is greedy, so it matches as much text as it can.
However, this doesn't actually help with your second question, which is to add the "txt" and "small-caps" tags into the content of the element. I'll leave that for someone else to tackle...