diff -Naur lua-5.1.4/src/llex.c my-lua-changes/src/llex.c
--- lua-5.1.4/src/llex.c	2007-12-27 07:02:25.000000000 -0600
+++ my-lua-changes/src/llex.c	2009-04-06 12:37:07.496444467 -0500
@@ -190,10 +190,14 @@
 
 /* LUA_NUMBER */
 static void read_numeral (LexState *ls, SemInfo *seminfo) {
-  lua_assert(isdigit(ls->current));
+  lua_assert(isdigit(ls->current) || ls->current == '_');
   do {
-    save_and_next(ls);
-  } while (isdigit(ls->current) || ls->current == '.');
+    if(ls->current == '_') {
+      next(ls);
+    } else {
+      save_and_next(ls);
+    }
+  } while (isdigit(ls->current) || ls->current == '.' || ls->current == '_');
   if (check_next(ls, "Ee"))  /* `E'? */
     check_next(ls, "+-");  /* optional exponent sign */
   while (isalnum(ls->current) || ls->current == '_')