Fix #883: JSON braces no longer treated as blackboard variables

Enhanced isBlackboardPointer() to validate that content between braces
is a valid variable name (matching [a-zA-Z_@][a-zA-Z0-9_./]*) rather
than just checking for surrounding braces. This prevents JSON strings
like {"key": "value"} from being misidentified as blackboard references.

Also centralized the check in xml_parsing.cpp to use
TreeNode::isBlackboardPointer() instead of an inline brace check.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: facontidavide

src/tree_node.cpp

@@ -383,11 +383,62 @@ bool TreeNode::isBlackboardPointer(StringView str, StringView* stripped_pointer)
   }
   const auto size = (last_index - front_index) + 1;
   auto valid = size >= 3 && str[front_index] == '{' && str[last_index] == '}';
-  if(valid && stripped_pointer != nullptr)
+
+  if(!valid)
+  {
+    return false;
+  }
+
+  // Extract content between braces
+  StringView content(&str[front_index + 1], size - 2);
+
+  // Special case: {=} is valid (auto-remapping syntax)
+  if(content == "=")
+  {
+    if(stripped_pointer != nullptr)
+    {
+      *stripped_pointer = content;
+    }
+    return true;
+  }
+
+  // Validate that the content looks like a blackboard variable name:
+  // must match [@]?[a-zA-Z_][a-zA-Z0-9_./]*
+  // The optional '@' prefix refers to the root blackboard.
+  // This rejects JSON strings like {"key": "value"}.
+  if(content.empty())
+  {
+    return false;
+  }
+  size_t start = 0;
+  if(content.front() == '@')
+  {
+    start = 1;
+    if(start >= content.size())
+    {
+      return false;
+    }
+  }
+  const char first_ch = content[start];
+  if(!std::isalpha(static_cast<unsigned char>(first_ch)) && first_ch != '_')
+  {
+    return false;
+  }
+  for(size_t i = start + 1; i < content.size(); i++)
+  {
+    const char ch = content[i];
+    if(!std::isalnum(static_cast<unsigned char>(ch)) && ch != '_' && ch != '.' &&
+       ch != '/')
+    {
+      return false;
+    }
+  }
+
+  if(stripped_pointer != nullptr)
   {
-    *stripped_pointer = StringView(&str[front_index + 1], size - 2);
+    *stripped_pointer = content;
   }
-  return valid;
+  return true;
 }
 
 StringView TreeNode::stripBlackboardPointer(StringView str)

src/xml_parsing.cpp

@@ -801,9 +801,7 @@ TreeNode::Ptr XMLParser::PImpl::createNodeFromXML(const XMLElement* element,
         else
         {
           const auto& port_model = port_model_it->second;
-          const bool is_blackboard = port_value.size() >= 3 &&
-                                     port_value.front() == '{' &&
-                                     port_value.back() == '}';
+          const bool is_blackboard = TreeNode::isBlackboardPointer(port_value);
           // let's test already if conversion is possible
           if(!is_blackboard && port_model.converter() && port_model.isStronglyTyped())
           {

tests/gtest_ports.cpp

@@ -1,6 +1,7 @@
 #include "behaviortree_cpp/basic_types.h"
 #include "behaviortree_cpp/bt_factory.h"
 #include "behaviortree_cpp/json_export.h"
+#include "behaviortree_cpp/tree_node.h"
 #include "behaviortree_cpp/xml_parsing.h"
 
 #include <gtest/gtest.h>
@@ -815,3 +816,36 @@ TEST(PortTest, SubtreeStringLiteralToLoopDouble_Issue1065)
   EXPECT_DOUBLE_EQ(collected[1], 2.0);
   EXPECT_DOUBLE_EQ(collected[2], 3.0);
 }
+
+// Issue #883: JSON braces should not be treated as blackboard pointers
+TEST(PortTest, IsBlackboardPointer_Issue883)
+{
+  using BT::TreeNode;
+
+  // Valid blackboard pointers
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{my_var}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{my.var}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{my/var}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{_private}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{=}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{a}"));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("  {my_var}  "));
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{@root_var}"));
+
+  // Invalid: JSON strings should NOT be treated as blackboard pointers
+  EXPECT_FALSE(TreeNode::isBlackboardPointer(R"({"key": "value"})"));
+  EXPECT_FALSE(TreeNode::isBlackboardPointer(R"({"config": {"nested": true}})"));
+
+  // Invalid: other non-variable content
+  EXPECT_FALSE(TreeNode::isBlackboardPointer("{123}"));
+  EXPECT_FALSE(TreeNode::isBlackboardPointer("{}"));
+  EXPECT_FALSE(TreeNode::isBlackboardPointer("ab"));
+
+  // Verify stripped_pointer output
+  BT::StringView stripped;
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{foo_bar}", &stripped));
+  EXPECT_EQ(stripped, "foo_bar");
+
+  EXPECT_TRUE(TreeNode::isBlackboardPointer("{=}", &stripped));
+  EXPECT_EQ(stripped, "=");
+}