Config: fix native variable scan to ignore invalid identifiers (e.g. $1)

CHANGES.txt

@@ -13,6 +13,7 @@ Config
 ------
 
 * Fixed error due to variables at namelist level
+* Fixed recognition of native variables to ignore invalid variable names
 
 Tools
 -----

mkexp

@@ -226,8 +226,11 @@ def format_namelist(section, group=None, default_value=''):
 def format_vars(section, key, log, fmt):
 
     def replace(match):
-        log.add(match.group(1))
-        return fmt%match.group(1)
+        result = match.group(1)
+        if result.isidentifier():
+            log.add(result)
+            return fmt%result
+        return result
     def transform(string):
         return re.sub(r'\$\{(\w+)\}', replace, string)