diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
index a2e91e7..2a9b65c 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/Protobuf.xtext
@@ -103,7 +103,7 @@
   ('[' fieldOptions+=FieldOption (',' fieldOptions+=FieldOption)* ']')? (';')+;
 
 terminal HEX returns ecore::ELong:
-  ('-')? '0x' (NUMBER | 'a'..'f' | 'A'..'F')+;
+  ('-')? '0' ('x' | 'X') (NUMBER | 'a'..'f' | 'A'..'F')+;
 
 TypeExtension:
   =>'extend' type=ExtensibleTypeLink '{'
diff --git a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
index e553950..adb6208 100644
--- a/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
+++ b/com.google.eclipse.protobuf/src/com/google/eclipse/protobuf/conversion/HEXValueConverter.java
@@ -22,7 +22,7 @@
  * @author alruiz@google.com (Alex Ruiz)
  */
 public class HEXValueConverter extends AbstractLexerBasedConverter<Long> {
-  private static final String[] VALID_PREFIXES = { "0x", "-0x", "0X", "-0x" };
+  private static final String[] VALID_PREFIXES = { "0x", "-0x", "0X", "-0X" };
 
   /**
    * Creates an {@code int} from the given input, if the given input represents an hexadecimal number.