diff --git a/android/expand.go b/android/expand.go
new file mode 100644
index 0000000..dafb2b6
--- /dev/null
+++ b/android/expand.go
@@ -0,0 +1,72 @@
+// Copyright 2016 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package android
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+)
+
+// Expand substitutes $() variables in a string
+// $(var) is passed to Expander(var)
+// $$ is converted to $
+func Expand(s string, mapping func(string) (string, error)) (string, error) {
+	// based on os.Expand
+	buf := make([]byte, 0, 2*len(s))
+	i := 0
+	for j := 0; j < len(s); j++ {
+		if s[j] == '$' {
+			if j+1 >= len(s) {
+				return "", fmt.Errorf("expected character after '$'")
+			}
+			buf = append(buf, s[i:j]...)
+			value, w, err := getMapping(s[j+1:], mapping)
+			if err != nil {
+				return "", err
+			}
+			buf = append(buf, value...)
+			j += w
+			i = j + 1
+		}
+	}
+	return string(buf) + s[i:], nil
+}
+
+func getMapping(s string, mapping func(string) (string, error)) (string, int, error) {
+	switch s[0] {
+	case '(':
+		// Scan to closing brace
+		for i := 1; i < len(s); i++ {
+			if s[i] == ')' {
+				ret, err := mapping(strings.TrimSpace(s[1:i]))
+				return ret, i + 1, err
+			}
+		}
+		return "", len(s), fmt.Errorf("missing )")
+	case '$':
+		return s[0:1], 1, nil
+	default:
+		i := strings.IndexFunc(s, func(c rune) bool {
+			return !(unicode.IsLetter(c) || unicode.IsNumber(c) || c == '_' || c == '.' || c == '-')
+		})
+		if i == 0 {
+			return "", 0, fmt.Errorf("unexpected character '%c' after '$'", s[0])
+		} else if i == -1 {
+			i = len(s)
+		}
+		return "", 0, fmt.Errorf("expected '(' after '$', did you mean $(%s)?", s[:i])
+	}
+}