Apply agentic searching. (#5196)

### What problem does this PR solve?

#5173

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/nlp/__init__.py

@@ -17,6 +17,7 @@
 import logging
 import random
 from collections import Counter
+from typing import Optional
 
 from rag.utils import num_tokens_from_string
 from . import rag_tokenizer
@@ -601,3 +602,11 @@ def naive_merge_docx(sections, chunk_token_num=128, delimiter="\n。；！？"):
         add_chunk(sec, image, '')
 
     return cks, images
+
+
+def extract_between(text: str, start_tag: str, end_tag: str) -> Optional[str]:
+    pattern = re.escape(start_tag) + r"(.*?)" + re.escape(end_tag)
+    matches = re.findall(pattern, text, flags=re.DOTALL)
+    if matches:
+        return matches[-1].strip()
+    return None