diff --git a/rich/highlighter.py b/rich/highlighter.py
index 32bf79e0..57600e48 100644
--- a/rich/highlighter.py
+++ b/rich/highlighter.py
@@ -85,8 +85,8 @@ class ReprHighlighter(RegexHighlighter):
         _combine_regex(
             r"(?P<ipv4>[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})",
             r"(?P<ipv6>([A-Fa-f0-9]{1,4}::?){1,7}[A-Fa-f0-9]{1,4})",
-            r"(?P<eui64>([0-9A-Fa-f]{1,2}-){7}[0-9A-Fa-f]{1,2}|([0-9A-Fa-f]{1,2}:){7}[0-9A-Fa-f]{1,2}|([0-9A-Fa-f]{4}\.){3}[0-9A-Fa-f]{4})",
-            r"(?P<eui48>([0-9A-Fa-f]{1,2}-){5}[0-9A-Fa-f]{1,2}|([0-9A-Fa-f]{1,2}:){5}[0-9A-Fa-f]{1,2}|([0-9A-Fa-f]{4}\.){2}[0-9A-Fa-f]{4})",
+            r"(?P<eui64>(?:[0-9A-Fa-f]{1,2}-){7}[0-9A-Fa-f]{1,2}|(?:[0-9A-Fa-f]{1,2}:){7}[0-9A-Fa-f]{1,2}|(?:[0-9A-Fa-f]{4}\.){3}[0-9A-Fa-f]{4})",
+            r"(?P<eui48>(?:[0-9A-Fa-f]{1,2}-){5}[0-9A-Fa-f]{1,2}|(?:[0-9A-Fa-f]{1,2}:){5}[0-9A-Fa-f]{1,2}|(?:[0-9A-Fa-f]{4}\.){2}[0-9A-Fa-f]{4})",
             r"(?P<brace>[\{\[\(\)\]\}])",
             r"(?P<attrib_name>\w+?)=(?P<attrib_value>\"?[\w_]+\"?)?",
             r"(?P<bool_true>True)|(?P<bool_false>False)|(?P<none>None)",
diff --git a/rich/rule.py b/rich/rule.py
index 2844ec17..ad393d20 100644
--- a/rich/rule.py
+++ b/rich/rule.py
@@ -42,9 +42,13 @@ class Rule(JupyterMixin):
     ) -> RenderResult:
         width = options.max_width
 
+        # Python3.6 doesn't have an isascii method on str
+        isascii = getattr(str, "isascii", None) or (
+            lambda s: all(ord(c) < 128 for c in s)
+        )
         characters = (
             "-"
-            if (options.ascii_only and not self.characters.isascii())
+            if (options.ascii_only and not isascii(self.characters))
             else self.characters
         )