regex, remove everything before first underscore

i have this string grafana-stack_alloy and require everything after the first underscore alloy. I need the result as a group $1. Tried this one, but it failed. (?:)?([^_]+)*$. Can anyone help me to solve this problem? I am testing this: rule { action = "replace" source_labels = [ "__meta_docker_container_label_com_docker_swarm_service_name", ] regex = "^(?:;*)?([^;]+).*$" work but wrong //regex = "[^_]+.$" ----> not work //regex = "([^_]+)$" ----> not work replacement = argument.namespace.value + "/$1" target_label = "job" } It is about the Grafana Agent with relabel-regex: Grafana prometheus.relabel which use [Google's RE2](https://github.com/google/re2/wiki/Syntax) as its regex engine. Thanks in advance