diff --git a/libs/core/langchain_core/runnables/graph_mermaid.py b/libs/core/langchain_core/runnables/graph_mermaid.py
index 0e500598335..2b8bf2526e3 100644
--- a/libs/core/langchain_core/runnables/graph_mermaid.py
+++ b/libs/core/langchain_core/runnables/graph_mermaid.py
@@ -41,8 +41,6 @@ except ImportError:
 
 MARKDOWN_SPECIAL_CHARS = "*_`"
 
-_HEX_COLOR_PATTERN = re.compile(r"^#(?:[0-9a-fA-F]{3}){1,2}$")
-
 
 def draw_mermaid(
     nodes: dict[str, Node],
@@ -432,8 +430,10 @@ def _render_mermaid_using_api(
     )
 
     # Check if the background color is a hexadecimal color code using regex
-    if background_color is not None and not _HEX_COLOR_PATTERN.match(background_color):
-        background_color = f"!{background_color}"
+    if background_color is not None:
+        hex_color_pattern = re.compile(r"^#(?:[0-9a-fA-F]{3}){1,2}$")
+        if not hex_color_pattern.match(background_color):
+            background_color = f"!{background_color}"
 
     # URL-encode the background_color to handle special characters like '!'
     encoded_bg_color = urllib.parse.quote(str(background_color), safe="")