Replace all occurrences of dash to the left of a colon using sed

I have the following values in a config.yml file: scratch-org-def: config/project-scratch-def.json assign-permset: false permset-name: run-apex-tests: true apex-test-format: tap delete-scratch-org: false show-scratch-org-url: true I need to replace all occurrences of dashes (-) to the left of the colon (:) with underscores (_). I've tried sed '/.*: /s/-/_/g' config.yml but it replaces all dashes (including the config/project-scratch-def.json). I've tried sed '/^[^:]*:/s/-/_/g' config.yml, hoping to stop at the colon, but it does the same.