fix regex parser (#446)

langchain/prompts/base.py

@@ -69,14 +69,21 @@ class RegexParser(BaseOutputParser, BaseModel):
 
     regex: str
     output_keys: List[str]
+    default_output_key: Optional[str] = None
 
     def parse(self, text: str) -> Dict[str, str]:
         """Parse the output of an LLM call."""
         match = re.search(self.regex, text)
         if match:
-            return {key: match.group(i) for i, key in enumerate(self.output_keys)}
+            return {key: match.group(i + 1) for i, key in enumerate(self.output_keys)}
         else:
-            raise ValueError(f"Could not parse output: {text}")
+            if self.default_output_key is None:
+                raise ValueError(f"Could not parse output: {text}")
+            else:
+                return {
+                    key: text if key == self.default_output_key else ""
+                    for key in self.output_keys
+                }
 
 
 class BasePromptTemplate(BaseModel, ABC):