Support invalid regex blacklist from config

freqtrade/plugins/pairlist/pairlist_helpers.py

@@ -4,7 +4,12 @@ from typing import List
 
 def expand_pairlist(wildcardpl: List[str], available_pairs: List[str]) -> List[str]:
     """
-    TODO: Add docstring here
+    Expand pairlist potentially containing wildcards based on available markets.
+    This will implicitly filter all pairs in the wildcard-list which are not in available_pairs.
+    :param wildcardpl: List of Pairlists, which may contain regex
+    :param available_pairs: List of all available pairs, usually with `exchange.get_markets().keys()`
+    :return expanded pairlist, with Regexes from wildcardpl applied to match all available pairs.
+    :raises: ValueError if a wildcard is invalid (like '*/BTC' - which should be `.*/BTC`)
     """
     result = []
     for pair_wc in wildcardpl: