diff --git a/dnstool.py b/dnstool.py
index e0cea8c..428a08c 100644
--- a/dnstool.py
+++ b/dnstool.py
@@ -7,8 +7,6 @@ class DnsCase:
         import os
         import re
 
-        p = re.compile("fort[.][0-9]{4,}")
-
         # Case name
         if case_name is None:
             self.name = os.path.basename(case_root)
@@ -19,6 +17,8 @@ class DnsCase:
         self.case_root = os.path.abspath(case_root)
 
         # List of data files. fort.xxxx[x*]
+        # matches fort.? files with more than 4 digits
+        p = re.compile("fort[.][0-9]{4,}")
         self.data_files = (sorted(filter(p.match, os.listdir(case_root))))
 
         # Read meta data from the first data file