Added string case operators

pkg/yqlib/operator_strings.go

@@ -9,6 +9,37 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
+type changeCasePrefs struct {
+	ToUpperCase bool
+}
+
+func changeCaseOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	results := list.New()
+	prefs := expressionNode.Operation.Preferences.(changeCasePrefs)
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+
+		node := unwrapDoc(candidate.Node)
+
+		if guessTagFromCustomType(node) != "!!str" {
+			return Context{}, fmt.Errorf("cannot change case with %v, can only operate on strings. ", node.Tag)
+		}
+
+		newStringNode := &yaml.Node{Kind: yaml.ScalarNode, Tag: node.Tag, Style: node.Style}
+		if prefs.ToUpperCase {
+			newStringNode.Value = strings.ToUpper(node.Value)
+		} else {
+			newStringNode.Value = strings.ToLower(node.Value)
+		}
+		results.PushBack(candidate.CreateReplacement(newStringNode))
+
+	}
+
+	return context.ChildContext(results), nil
+
+}
+
 func getSubstituteParameters(d *dataTreeNavigator, block *ExpressionNode, context Context) (string, string, error) {
 	regEx := ""
 	replacementText := ""