Fix envsubst parse of empty replace pattern

envsubst/eval_test.go

@@ -126,6 +126,18 @@ func TestExpand(t *testing.T) {
 			input:  "${stringZ/#abc/XYZ}",
 			output: "XYZABC123ABCabc",
 		},
+		// replace prefix with empty pattern (prepend)
+		{
+			params: map[string]string{"stringZ": "somevalue"},
+			input:  "${stringZ/#/prefix-}",
+			output: "prefix-somevalue",
+		},
+		// replace suffix with empty pattern (append)
+		{
+			params: map[string]string{"stringZ": "somevalue"},
+			input:  "${stringZ/%/-suffix}",
+			output: "somevalue-suffix",
+		},
 		// replace all
 		{
 			params: map[string]string{"stringZ": "abcABC123ABCabc"},

envsubst/parse/parse.go

@@ -283,11 +283,18 @@ func (t *Tree) parseReplaceFunc(name string) (Node, error) {
 
 	// scan arg[1]
 	{
-		param, err := t.parseParam(acceptNotSlash, scanIdent|scanEscape)
-		if err != nil {
-			return nil, err
+		// An empty pattern (e.g. ${param/#/string}) means the next rune is the
+		// delimiter, so there is nothing to scan. Record an empty pattern node
+		// to keep the two-argument shape the eval side expects.
+		if t.scanner.peek() == '/' {
+			node.Args = append(node.Args, newTextNode(""))
+		} else {
+			param, err := t.parseParam(acceptNotSlash, scanIdent|scanEscape)
+			if err != nil {
+				return nil, err
+			}
+			node.Args = append(node.Args, param)
 		}
-		node.Args = append(node.Args, param)
 	}
 
 	// expect delimiter

envsubst/parse/parse_test.go

@@ -219,6 +219,28 @@ var tests = []struct {
 			},
 		},
 	},
+	{
+		Text: "${string/#/replacement}",
+		Node: &FuncNode{
+			Param: "string",
+			Name:  "/#",
+			Args: []Node{
+				&TextNode{Value: ""},
+				&TextNode{Value: "replacement"},
+			},
+		},
+	},
+	{
+		Text: "${string/%/replacement}",
+		Node: &FuncNode{
+			Param: "string",
+			Name:  "/%",
+			Args: []Node{
+				&TextNode{Value: ""},
+				&TextNode{Value: "replacement"},
+			},
+		},
+	},
 
 	//
 	// default value functions