pkg/utils: Add function to parse the user-specified release string

https://github.com/containers/toolbox/pull/318

src/pkg/utils/utils.go

@@ -24,6 +24,7 @@ import (
 	"os/user"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 	"syscall"
 
@@ -269,6 +270,27 @@ func ShortID(id string) string {
 	return id
 }
 
+func ParseRelease(str string) (string, error) {
+	var release string
+
+	if strings.HasPrefix(str, "F") || strings.HasPrefix(str, "f") {
+		release = str[1:]
+	} else {
+		release = str
+	}
+
+	releaseN, err := strconv.Atoi(release)
+	if err != nil {
+		return "", err
+	}
+
+	if releaseN <= 0 {
+		return "", errors.New("release must be a positive integer")
+	}
+
+	return release, nil
+}
+
 // PathExists wraps around os.Stat providing a nice interface for checking an existence of a path.
 func PathExists(path string) bool {
 	if _, err := os.Stat(path); !os.IsNotExist(err) {