Define common regexps used across registry application

This commit adds regular expression definitions for several string identifiers
used througout the registry. The repository name regex supports up to five path
path components and restricts repeated periods, dashes and underscores. The tag
regex simply validates the length of the tag and that printable characters are
required.

Though we define a new package common, these definition should land in docker
core.

common/names.go

@@ -0,0 +1,19 @@
+package common
+
+import (
+	"regexp"
+)
+
+// RepositoryNameComponentRegexp restricts registtry path components names to
+// start with at least two letters or numbers, with following parts able to
+// separated by one period, dash or underscore.
+var RepositoryNameComponentRegexp = regexp.MustCompile(`[a-z0-9](?:[a-z0-9]+[._-]?)*[a-z0-9]`)
+
+// RepositoryNameRegexp builds on RepositoryNameComponentRegexp to allow 2 to
+// 5 path components, separated by a forward slash.
+var RepositoryNameRegexp = regexp.MustCompile(`(?:` + RepositoryNameComponentRegexp.String() + `/){1,4}` + RepositoryNameComponentRegexp.String())
+
+// TagNameRegexp matches valid tag names. From docker/docker:graph/tags.go.
+var TagNameRegexp = regexp.MustCompile(`[\w][\w.-]{0,127}`)
+
+// TODO(stevvooe): Contribute these exports back to core, so they are shared.