How to use regex query for filter in Kibana

Hello community! First post, new to Kibana.

I am trying to use a custom Query DSL to filter Kibana for a field called extra.monitor_value_name that contains the string pH at the front. The field is called extra.monitor_value_name . Examples of potential values are Temperature_ABC01 , DO_ABC01 , or pH_ABC01 . The "pH" always comes first.

Kibana's Elasticsearch Query DSL does not seem to have a "contains string" so I think the best path forward is to use regex.

My current starting point is:
"query": {
"regexp": {
"extra.monitor_value_name": "pH.*"

However, this doesn't work, nothing comes up after this filter. What am I missing?

Well, my answer ended up being that I needed extra.monitor_value_name.keyword
All syntax was correct!