Fixed regex bug in RetrievalQAWithSources in previous update (#9898)

- Description: In my previous PR, I had modified the code to catch all
kinds of [SOURCES, sources, Source, Sources]. However, this change
included checking for a colon or a white space which should actually
have been only checking for a colon.
  - Issue: the issue # it fixes (if applicable),
  - Dependencies: any dependencies required for this change,

libs/langchain/langchain/chains/qa_with_sources/base.py

@@ -120,9 +120,9 @@ class BaseQAWithSourcesChain(Chain, ABC):
 
     def _split_sources(self, answer: str) -> Tuple[str, str]:
         """Split sources from answer."""
-        if re.search(r"SOURCES?[:\s]", answer, re.IGNORECASE):
+        if re.search(r"SOURCES?:", answer, re.IGNORECASE):
             answer, sources = re.split(
-                r"SOURCES?[:\s]|QUESTION:\s", answer, flags=re.IGNORECASE
+                r"SOURCES?:|QUESTION:\s", answer, flags=re.IGNORECASE
             )[:2]
             sources = re.split(r"\n", sources)[0].strip()
         else:

libs/langchain/tests/unit_tests/chains/test_qa_with_sources.py

@@ -27,6 +27,12 @@ from tests.unit_tests.llms.fake_llm import FakeLLM
             "This Agreement is governed by English law.\n",
             "28-pl",
         ),
+        (
+            "According to the sources the agreement is governed by English law.\n"
+            "Source: 28-pl",
+            "According to the sources the agreement is governed by English law.\n",
+            "28-pl",
+        ),
         (
             "This Agreement is governed by English law.\n"
             "SOURCES: 28-pl\n\n"