Github user sebastian-nagel commented on a diff in the pull request:
https://github.com/apache/nutch/pull/89#discussion_r52833909
--- Diff: conf/db-ignore-external-exemptions.txt ---
@@ -0,0 +1,37 @@
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements. See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+#
+# Exemption rules to db.ignore.external.links
+#
+# Format :
+#--------
+# UrlRegex1
+# UrlRegex2
+# UrlRegex3
+
+
+# NOTE ::
+# 1. When the url matches any of the regex then that url is exempted.
+# 2. # in the beginning makes it a comment line
+# 3. To Test the regex, update this file and use the below command
+# bin/nutch plugin urlfilter-ignoreexempt
org.apache.nutch.urlfilter.ignoreexempt.ExemptionUrlFilter <URL>
+# 4. Dont forget to enable this plugin in nutch-site.xml
+
+
+# Example 1:
+#----------
+# To exempt urls ending with image extensions, uncomment the below line
+#.*\.(jpg|JPG|png$|PNG|gif|GIF)$
--- End diff --
Regex could be simplified to `#(?i).*\.(?:jpg|png|gif)` (or
`#(?i)\.(?:jpg|png|gif)$` if Pattern.find() is used). `(?i)` makes the pattern
case insensitive, cf.
[NUTCH-2035](https://issues.apache.org/jira/browse/NUTCH-2035)
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