diff --git a/lib/lexer.rb b/lib/lexer.rb
index 80cb51f..aa87d79 100644
--- a/lib/lexer.rb
+++ b/lib/lexer.rb
@@ -66,13 +66,13 @@ class Query < Racc::Parser
                   when (text = @ss.scan(/\)/))
                      action { return [:R_BRACKET, text] }
 
-                  when (text = @ss.scan(/(?i)or/))
+                  when (text = @ss.scan(/(?i)\bor\b/))
                      action { return [:OPERATOR_OR, text] }
 
-                  when (text = @ss.scan(/(?i)and/))
+                  when (text = @ss.scan(/(?i)\band\b/))
                      action { return [:OPERATOR_AND, text] }
 
-                  when (text = @ss.scan(/(?i)not/))
+                  when (text = @ss.scan(/(?i)\bnot\b/))
                      action { return [:OPERATOR_NOT, text] }
 
                   when (text = @ss.scan(/"([^"]*)"/))