config: interpolationWalker detects functions

config/interpolate.go

@@ -98,6 +98,12 @@ func NewInterpolation(v string) (Interpolation, error) {
 
 		args := make([]InterpolatedVariable, 0, len(match)-2)
 		for i := 2; i < len(match); i++ {
+			// This can be empty if we have a single argument
+			// due to the format of the regexp.
+			if match[i] == "" {
+				continue
+			}
+
 			v, err := NewInterpolatedVariable(match[i])
 			if err != nil {
 				return nil, err
@@ -196,6 +202,12 @@ func (i *VariableInterpolation) Variables() map[string]InterpolatedVariable {
 
 func NewResourceVariable(key string) (*ResourceVariable, error) {
 	parts := strings.SplitN(key, ".", 3)
+	if len(parts) < 3 {
+		return nil, fmt.Errorf(
+			"%s: resource variables must be three parts: type.name.attr",
+			key)
+	}
+
 	field := parts[2]
 	multi := false
 	var index int

config/interpolate_walk.go

@@ -10,7 +10,7 @@ import (
 
 // interpRegexp is a regexp that matches interpolations such as ${foo.bar}
 var interpRegexp *regexp.Regexp = regexp.MustCompile(
-	`(?i)(\$+)\{([*-.a-z0-9_]+)\}`)
+	`(?i)(\$+)\{([\s*-.,\(\)a-z0-9_]+)\}`)
 
 // interpolationWalker implements interfaces for the reflectwalk package
 // (github.com/mitchellh/reflectwalk) that can be used to automatically

config/interpolate_walk_test.go

@@ -33,6 +33,24 @@ func TestInterpolationWalker_detect(t *testing.T) {
 				},
 			},
 		},
+
+		{
+			Input: map[string]interface{}{
+				"foo": "${lookup(var.foo)}",
+			},
+			Result: []Interpolation{
+				&FunctionInterpolation{
+					Func: nil,
+					Args: []InterpolatedVariable{
+						&UserVariable{
+							Name: "foo",
+							key:  "var.foo",
+						},
+					},
+					key: "lookup(var.foo)",
+				},
+			},
+		},
 	}
 
 	for i, tc := range cases {
@@ -48,6 +66,14 @@ func TestInterpolationWalker_detect(t *testing.T) {
 			t.Fatalf("err: %s", err)
 		}
 
+		for _, a := range actual {
+			// This is jank, but reflect.DeepEqual never has functions
+			// being the same.
+			if f, ok := a.(*FunctionInterpolation); ok {
+				f.Func = nil
+			}
+		}
+
 		if !reflect.DeepEqual(actual, tc.Result) {
 			t.Fatalf("%d: bad:\n\n%#v", i, actual)
 		}