provide way to specify class starting with !

lib/capybara/selector/builders/css_builder.rb

@@ -37,8 +37,8 @@ module Capybara
           when Regexp
             attribute_conditions(class: classes)
           else
-            cls = Array(classes).group_by { |cl| cl.start_with? '!' }
-            (cls[false].to_a.map { |cl| ".#{Capybara::Selector::CSS.escape(cl)}" } +
+            cls = Array(classes).group_by { |cl| cl.start_with?('!') && !cl.start_with?('!!!') }
+            (cls[false].to_a.map { |cl| ".#{Capybara::Selector::CSS.escape(cl.sub(/^!!/,''))}" } +
             cls[true].to_a.map { |cl| ":not(.#{Capybara::Selector::CSS.escape(cl.slice(1..-1))})" }).join
           end
         end

lib/capybara/selector/builders/xpath_builder.rb

@@ -30,10 +30,10 @@ module Capybara
             attribute_conditions(class: classes)
           else
             Array(classes).map do |klass|
-              if klass.start_with?('!')
+              if klass.start_with?('!') && !klass.start_with?('!!!')
                 !XPath.attr(:class).contains_word(klass.slice(1..-1))
               else
-                XPath.attr(:class).contains_word(klass)
+                XPath.attr(:class).contains_word(klass.sub(/^!!/,''))
               end
             end.reduce(:&)
           end

spec/selector_spec.rb

@@ -17,7 +17,7 @@ RSpec.describe Capybara do
                 <p>Yes it is</p>
               </div>
               <p class="bb cc">Some Content</p>
-              <p class="bb dd"></p>
+              <p class="bb dd !mine"></p>
             </div>
             <div id="#special">
             </div>
@@ -253,6 +253,11 @@ RSpec.describe Capybara do
           expect(string.all(:custom_xpath_selector, XPath.descendant(:div, :p), class: ['!cc', '!dd', 'bb']).size).to eq 1
         end
 
+        it 'handles classes starting with ! by requiring negated negated first', :focus_ do
+          expect(string.all(:custom_css_selector, 'div, p', class: ['!!!mine']).size).to eq 1
+          expect(string.all(:custom_xpath_selector, XPath.descendant(:div, :p), class: ['!!!mine']).size).to eq 1
+        end
+
         it "works with 'special' characters" do
           expect(string.find(:custom_css_selector, 'input', class: '.special')[:id]).to eq 'file'
           expect(string.find(:custom_css_selector, 'input', class: '2checkbox')[:id]).to eq '2checkbox'