[ref] Factor out __remove_backslash_line_continuation_chars(...)

Author: bbugyi200

black.py

@@ -2580,13 +2580,9 @@ def _my_regexp(self) -> str:
 
     def _do_transform(self, line: Line, _string_idx: Optional[int]) -> Iterator[Line]:
         # Merge strings that were split across multiple lines using backslashes.
-        for leaf in line.leaves:
-            if leaf.type == token.STRING and leaf.value.lstrip(STRING_PREFIX_CHARS)[
-                :3
-            ] not in {'"""', "'''"}:
-                leaf.value = leaf.value.replace("\\\n", "")
+        new_line = self.__remove_backslash_line_continuation_chars(line)
 
-        (new_line, line_was_changed) = self.__merge_first_string_group(line)
+        (new_line, line_was_changed) = self.__merge_first_string_group(new_line)
         while line_was_changed:
             (new_line, line_was_changed) = self.__merge_first_string_group(new_line)
 
@@ -2732,6 +2728,29 @@ def __get_string_group_index(line: Line) -> Optional[int]:
 
         return None
 
+    @staticmethod
+    def __remove_backslash_line_continuation_chars(line: Line) -> Line:
+        for leaf in line.leaves:
+            if (
+                leaf.type == token.STRING
+                and "\\\n" in leaf.value
+                and leaf.value.lstrip(STRING_PREFIX_CHARS)[:3] not in {'"""', "'''"}
+            ):
+                break
+        else:
+            return line
+
+        new_line = line.clone()
+        new_line.comments = line.comments
+        append_leaves(new_line, line, line.leaves)
+        for leaf in new_line.leaves:
+            if leaf.type == token.STRING and leaf.value.lstrip(STRING_PREFIX_CHARS)[
+                :3
+            ] not in {'"""', "'''"}:
+                leaf.value = leaf.value.replace("\\\n", "")
+
+        return new_line
+
     @staticmethod
     def __remove_bad_trailing_commas(line: Line) -> Line:
         line_str = line_to_string(line)