Move SemVerTag to its own package

internal/pkg/semver/parsing.go

@@ -0,0 +1,38 @@
+package semver
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/coreos/go-semver/semver"
+)
+
+var (
+	rxSemVerPrefix = regexp.MustCompile(`^[^\d]*`)
+)
+
+// Returns nil if the tag did not parse as semver
+func ParseTagAsSemVer(dockerTag string) *Tag {
+	var prefix string
+	loc := rxSemVerPrefix.FindStringIndex(dockerTag)
+	if loc != nil {
+		prefix = dockerTag[:loc[1]]
+		dockerTag = dockerTag[loc[1]:]
+	}
+
+	version, err := semver.NewVersion(dockerTag)
+	if err != nil {
+		return nil
+	}
+
+	svt := Tag{
+		prefix,
+		*version,
+	}
+
+	return &svt
+}
+
+func (svt Tag) String() string {
+	return fmt.Sprintf("%s%s", svt.Prefix, svt.Version.String())
+}

internal/pkg/semver/types.go

@@ -0,0 +1,8 @@
+package semver
+
+import "github.com/coreos/go-semver/semver"
+
+type Tag struct {
+	Prefix  string
+	Version semver.Version
+}