How do I return a substring of a value

[ickerh]

Hello
I got a json response like this : {"System":{"state":"GREEN","memory-usage":{"used":"28 %","total":"8192 MB"}}}
I am trying to extract the 'used' value. My config.xml looks like

modules:
default:
headers:
X-Dummy: My-Json-Test
metrics:
- name: memory_used
path: "{$.System.memory-usage}"
type: object
values:
used: "{.used}"
help: Memory used

when I load this into json_exporter I get a parse exception that the value cannot be parsed to float64, which makes sense, because it contains a '%'.
I tried several ways to get a substring from the used field but nothing was working.

Is there a way in json_exporter or jsonpath that I can substring a value?

Thanks a lot