fix variable lookup parse timing out with missing closing bracket

lib/liquid.rb

@@ -41,7 +41,7 @@ module Liquid
   AnyStartingTag              = /#{TagStart}|#{VariableStart}/o
   PartialTemplateParser       = /#{TagStart}.*?#{TagEnd}|#{VariableStart}.*?#{VariableIncompleteEnd}/om
   TemplateParser              = /(#{PartialTemplateParser}|#{AnyStartingTag})/om
-  VariableParser              = /\[(?:[^\[\]]+|\g<0>)*\]|#{VariableSegment}+\??/o
+  VariableParser              = /\[(?>[^\[\]]+|\g<0>)*\]|#{VariableSegment}+\??/o
 
   RAISE_EXCEPTION_LAMBDA = ->(_e) { raise }
 

test/integration/variable_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'test_helper'
+require 'timeout'
 
 class VariableTest < Minitest::Test
   include Liquid
@@ -169,4 +170,30 @@ class VariableTest < Minitest::Test
       }
     )
   end
+
+  def test_variable_lookup_should_not_hang_with_invalid_syntax
+    Timeout.timeout(1) do
+      assert_template_result(
+        'bar',
+        "{{['foo'}}",
+        {
+          'foo' => 'bar',
+        },
+        error_mode: :lax,
+      )
+    end
+
+    very_long_key = "1234567890" * 100
+
+    Timeout.timeout(1) do
+      assert_template_result(
+        'bar',
+        "{{['#{very_long_key}'}}",
+        {
+          very_long_key => 'bar',
+        },
+        error_mode: :lax,
+      )
+    end
+  end
 end