Merge pull request opencontainers#421 from TrumanLing/RefsRegularExp

Add regular expression for object names in 'refs'

Author: jonboulle

specs-go/v1/layout.go

@@ -14,8 +14,15 @@
 
 package v1
 
+import "regexp"
+
 // ImageLayout is the structure in the "oci-layout" file, found in the root
 // of an OCI Image-layout directory.
 type ImageLayout struct {
 	Version string `json:"imageLayoutVersion"`
 }
+
+var (
+	// RefsRegexp matches requirement of image-layout 'refs' charset.
+	RefsRegexp = regexp.MustCompile(`^[a-zA-Z0-9-._]+$`)
+)