diff --git a/ivlpp/lexor.lex b/ivlpp/lexor.lex
index d64e33866..18f1d9764 100644
--- a/ivlpp/lexor.lex
+++ b/ivlpp/lexor.lex
@@ -393,7 +393,7 @@ W [ \t\b\f]+
   }
 
   /* This pattern notices macros and arranges for them to be replaced. */
-`[a-zA-Z][a-zA-Z0-9_$]* { def_match(); }
+`[a-zA-Z_][a-zA-Z0-9_$]* { def_match(); }
 
   /* Any text that is not a directive just gets passed through to the
      output. Very easy. */