Cajole makefile into working with GNU grep

Author: i80and

Makefile

@@ -62,7 +62,7 @@ flit-publish: test ## Deploy the package to pypi
 package: dist/${PACKAGE_NAME}
 
 cut-release: ## Release a new version of snooty. Must provide BUMP_TO_VERSION
-	@if [ $$(echo "${BUMP_TO_VERSION}" | grep -cE '^\d+\.\d+\.\d+(-[[:alnum:]_-]+)?$$') -ne 1 ]; then \
+	@if [ $$(echo "${BUMP_TO_VERSION}" | grep -cE '^[0-9]+\.[0-9]+\.[0-9]+(-[[:alnum:]_-]+)?$$') -ne 1 ]; then \
 		echo "Must specify a valid BUMP_TO_VERSION (e.g. 'make cut-release BUMP_TO_VERSION=0.1.15')"; \
 		exit 1; \
 	fi