added detailed regular expression explanation to community.list

community.list

@@ -18,4 +18,20 @@ ntop[0-1][0-9]
 #
 #   *  Only fixed-name communities are supported for header encryption (-H)
 #
-
+#   *  Regular expression support the following placeholders
+#      '.'        Dot, matches any character                           (meaningless, as full matches only)
+#      '^'        Start anchor, matches beginning of string            (meaningless, as full matches only)
+#      '$'        End anchor, matches end of string
+#      '*'        Asterisk, match zero or more (greedy)
+#      '+'        Plus, match one or more (greedy)
+#      '?'        Question, match zero or one (non-greedy)
+#      '[abc]'    Character class, match if one of {'a', 'b', 'c'}
+#      '[^abc]'   Inverted class, match if NOT one of {'a', 'b', 'c'}  (feature is currently broken)
+#      '[a-zA-Z]' Character ranges, the character set of the ranges { a-z | A-Z }
+#      '\s'       Whitespace, \t \f \r \n \v and spaces
+#      '\S'       Non-whitespace
+#      '\w'       Alphanumeric, [a-zA-Z0-9_]
+#      '\W'       Non-alphanumeric
+#      '\d'       Digits, [0-9]
+#      '\D'       Non-digits
+#