$> sed -r -e 's/(.*-)([0-9]+-[a-z]+)(.*)/ /' text
8-july <a href="arch/names-8-july--123123123">
12-february <a href="arch/seman-12-february-text-text--321321321">
17-june <a href="arch/nmaes-17-june--213213321">
Também parece ser apropriado para o seu exemplo mais recente:
$> sed -r 's/(.*-)([0-9]+-[a-z]+)(.*)/ /' text2
5-cervna <a href="/praha/jaktovidi/_zprava/jak-to-vidi-rektor-richard-hindls-5-cervna--1220102">
4-cervna <a href="/praha/jaktovidi/_zprava/jak-to-vidi-erik-best-4-cervna--1219526">
3-cervna <a href="/praha/jaktovidi/_zprava/jak-to-vidi-ivan-hoffman-3-cervna--1218826">
19-kvetna <a href="/praha/jaktovidi/_zprava/jak-to-vidi-pavel-kosatik-19-kvetna-verejne-chci-podekovat-statnim-zastupcum-a-policistum-kteri-se-nebali--1225675">