diff --git a/lark/lexer.py b/lark/lexer.py
index 1232c627..2fba894f 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -497,7 +497,24 @@ def _check_regex_collisions(terminal_to_regexp: Dict[TerminalDef, str], comparat
                 return
 
 
-class BasicLexer(Lexer):
+class AbstractBasicLexer(Lexer):
+    terminals_by_name: Dict[str, TerminalDef]
+
+    @abstractmethod
+    def __init__(self, conf: 'LexerConf', comparator=None) -> None:
+        ...
+
+    @abstractmethod
+    def next_token(self, lex_state: LexerState, parser_state: Any = None) -> Token:
+        ...
+
+    def lex(self, state: LexerState, parser_state: Any) -> Iterator[Token]:
+        with suppress(EOFError):
+            while True:
+                yield self.next_token(state, parser_state)
+
+
+class BasicLexer(AbstractBasicLexer):
     terminals: Collection[TerminalDef]
     ignore_types: FrozenSet[str]
     newline_types: FrozenSet[str]
@@ -569,11 +586,6 @@ def scanner(self):
     def match(self, text, pos):
         return self.scanner.match(text, pos)
 
-    def lex(self, state: LexerState, parser_state: Any) -> Iterator[Token]:
-        with suppress(EOFError):
-            while True:
-                yield self.next_token(state, parser_state)
-
     def next_token(self, lex_state: LexerState, parser_state: Any = None) -> Token:
         line_ctr = lex_state.line_ctr
         while line_ctr.char_pos < len(lex_state.text):
@@ -611,8 +623,10 @@ def next_token(self, lex_state: LexerState, parser_state: Any = None) -> Token:
 
 class ContextualLexer(Lexer):
 
-    lexers: Dict[str, BasicLexer]
-    root_lexer: BasicLexer
+    lexers: Dict[str, AbstractBasicLexer]
+    root_lexer: AbstractBasicLexer
+
+    BasicLexer: Type[AbstractBasicLexer] = BasicLexer
 
     def __init__(self, conf: 'LexerConf', states: Dict[str, Collection[str]], always_accept: Collection[str]=()) -> None:
         terminals = list(conf.terminals)
@@ -625,7 +639,7 @@ def __init__(self, conf: 'LexerConf', states: Dict[str, Collection[str]], always
             comparator = interegular.Comparator.from_regexes({t: t.pattern.to_regexp() for t in terminals})
         else:
             comparator = None
-        lexer_by_tokens: Dict[FrozenSet[str], BasicLexer] = {}
+        lexer_by_tokens: Dict[FrozenSet[str], AbstractBasicLexer] = {}
         self.lexers = {}
         for state, accepts in states.items():
             key = frozenset(accepts)
@@ -635,14 +649,14 @@ def __init__(self, conf: 'LexerConf', states: Dict[str, Collection[str]], always
                 accepts = set(accepts) | set(conf.ignore) | set(always_accept)
                 lexer_conf = copy(trad_conf)
                 lexer_conf.terminals = [terminals_by_name[n] for n in accepts if n in terminals_by_name]
-                lexer = BasicLexer(lexer_conf, comparator)
+                lexer = self.BasicLexer(lexer_conf, comparator)
                 lexer_by_tokens[key] = lexer
 
             self.lexers[state] = lexer
 
         assert trad_conf.terminals is terminals
         trad_conf.skip_validation = True  # We don't need to verify all terminals again
-        self.root_lexer = BasicLexer(trad_conf, comparator)
+        self.root_lexer = self.BasicLexer(trad_conf, comparator)
 
     def lex(self, lexer_state: LexerState, parser_state: Any) -> Iterator[Token]:
         try: