diff --git a/lib/rdoc/parser/ruby.rb b/lib/rdoc/parser/ruby.rb
index 0323e4de413eb8..2c8a19f6088507 100644
--- a/lib/rdoc/parser/ruby.rb
+++ b/lib/rdoc/parser/ruby.rb
@@ -2134,7 +2134,7 @@ def read_directive allowed
       if :on_nl == tk[:kind] or (:on_kw == tk[:kind] && 'def' == tk[:text]) then
         return
       elsif :on_comment == tk[:kind] or :on_embdoc == tk[:kind] then
-        return unless tk[:text] =~ /\s*:?([\w-]+):\s*(.*)/
+        return unless tk[:text] =~ /:?\b([\w-]+):\s*(.*)/
 
         directive = $1.downcase
 
diff --git a/test/rdoc/test_rdoc_parser_ruby.rb b/test/rdoc/test_rdoc_parser_ruby.rb
index 76fb28808c6c52..aea923765ce791 100644
--- a/test/rdoc/test_rdoc_parser_ruby.rb
+++ b/test/rdoc/test_rdoc_parser_ruby.rb
@@ -3350,6 +3350,13 @@ def test_read_directive_one_liner
     assert_equal :on_const, parser.get_tk[:kind]
   end
 
+  def test_read_directive_linear_performance
+    pre = ->(i) {util_parser '# ' + '0'*i + '=000:'}
+    assert_linear_performance((1..5).map{|i|10**i}, pre: pre) do |parser|
+      assert_nil parser.read_directive []
+    end
+  end
+
   def test_read_documentation_modifiers
     c = RDoc::Context.new