move cmd/pruning to pruning, as it's independent of the command implementation

pruning/keep_prefix.go

@@ -0,0 +1,33 @@
+package pruning
+
+import (
+	"regexp"
+)
+
+type KeepRegex struct {
+	expr *regexp.Regexp
+}
+
+var _ KeepRule = &KeepRegex{}
+
+func NewKeepRegex(expr string) (*KeepRegex, error) {
+	re, err := regexp.Compile(expr)
+	if err != nil {
+		return nil, err
+	}
+	return &KeepRegex{re}, nil
+}
+
+func MustKeepRegex(expr string) *KeepRegex {
+	k, err := NewKeepRegex(expr)
+	if err != nil {
+		panic(err)
+	}
+	return k
+}
+
+func (k *KeepRegex) KeepRule(snaps []Snapshot) []Snapshot {
+	return filterSnapList(snaps, func(s Snapshot) bool {
+		return k.expr.FindStringIndex(s.Name()) == nil
+	})
+}