diff --git a/CHANGES b/CHANGES
index 40af0eb..988aa6b 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,9 @@
+Version 0.3
+-----------
+1/16/2018  Usability improvement on Lexer class.  Regular expression rules
+           specified as strings that don't match any name in tokens are
+           now reported as errors.
+
 Version 0.2
 -----------
 
diff --git a/sly/lex.py b/sly/lex.py
index 9ac6575..82b5a24 100644
--- a/sly/lex.py
+++ b/sly/lex.py
@@ -141,6 +141,8 @@ class Lexer(metaclass=LexerMeta):
         for key, value in definitions:
             if (key in cls.tokens) or key.startswith('ignore_') or hasattr(value, 'pattern'):
                 rules.append((key, value))
+            elif isinstance(value, str) and not key.startswith('_') and key not in {'ignore'}:
+                raise LexerBuildError(f'{key} does not match a name in tokens')
         return rules
 
     @classmethod