Use format itself to see if dsetname is OK

concert/storage.py

@@ -243,13 +243,19 @@ def _write_coroutine(self, dsetname=None):
 
     def _dset_exists(self, dsetname):
         """Check if *dsetname* exists on the current level."""
-        match = re.search(r'(?P<prefix>.*){.*}.*', dsetname)
-        if not match:
+        bad = '{' not in dsetname
+
+        try:
+            dsetname.format(0)
+        except ValueError:
+            bad = True
+
+        if bad:
             raise ValueError('dsetname `{}\' has wrong format'.format(dsetname))
 
         filenames = os.listdir(self._current)
         for name in filenames:
-            if name.startswith(match.group('prefix')):
+            if name.startswith(dsetname.split('{')[0]):
                 return True
 
         return False