diff --git a/docs/features.md b/docs/features.md
index 121089e6..5249693f 100644
--- a/docs/features.md
+++ b/docs/features.md
@@ -11,6 +11,7 @@
  - Flexible error handling by using an interactive parser interface (LALR only)
  - Automatic line & column tracking (for both tokens and matched rules)
  - Automatic terminal collision resolution
+ - Optional Warnings for the cases where automatic collision resolution isn't possible. ([see here](how_to_use.html#regex-collisions))
  - Grammar composition - Import terminals and rules from other grammars
  - Standard library of terminals (strings, numbers, names, etc.)
  - Unicode fully supported
diff --git a/docs/how_to_use.md b/docs/how_to_use.md
index 08547829..9b770089 100644
--- a/docs/how_to_use.md
+++ b/docs/how_to_use.md
@@ -42,6 +42,27 @@ Usually, by the time you get to a minimal grammar, the problem becomes clear.
 
 But if it doesn't, feel free to ask us on gitter, or even open an issue. Post a reproducing code, with the minimal grammar and input, and we'll do our best to help.
 
+### Regex collisions
+
+Especially if you have multiple complex Regular Expressions, the problem can occur that you have collisions between two Terminals that aren't obvious and therefore hard to notice. If you install `interegular`, an extra library, `lark` will check for collisions and warns about any conflicts it can find:
+
+```
+import logging
+from lark import Lark, logger
+
+logger.setLevel(logging.WARN)
+
+collision_grammar = '''
+start: A | B
+A: /a+/
+B: /[ab]+/
+'''
+p = Lark(collision_grammar, parser='lalr')
+```
+If unresolved, these conflicts can result in hard to find and reproduce bugs, since which one gets chosen is arbitrary and potentially changes with each interpreter restart.
+Note that this only works when the `lexer` is not `dynamic` or `dynamic_complete`.
+
+
 ### LALR
 
 By default Lark silently resolves Shift/Reduce conflicts as Shift. To enable warnings pass `debug=True`. To get the messages printed you have to configure the `logger` beforehand. For example:
diff --git a/lark/lexer.py b/lark/lexer.py
index 4291f2d8..4d119d2c 100644
--- a/lark/lexer.py
+++ b/lark/lexer.py
@@ -9,16 +9,25 @@
 )
 from types import ModuleType
 import warnings
+try:
+    import interegular
+except ImportError:
+    pass
 if TYPE_CHECKING:
     from .common import LexerConf
 
-from .utils import classify, get_regexp_width, Serialize
+from .utils import classify, get_regexp_width, Serialize, logger
 from .exceptions import UnexpectedCharacters, LexError, UnexpectedToken
 from .grammar import TOKEN_DEFAULT_PRIORITY
 
+
 ###{standalone
 from copy import copy
 
+try:  # For the standalone parser, we need to make sure that has_interegular is False to avoid NameErrors later on
+    has_interegular = bool(interegular)
+except NameError:
+    has_interegular = False
 
 class Pattern(Serialize, ABC):
 
@@ -27,7 +36,7 @@ class Pattern(Serialize, ABC):
     raw: Optional[str]
     type: ClassVar[str]
 
-    def __init__(self, value: str, flags: Collection[str]=(), raw: Optional[str]=None) -> None:
+    def __init__(self, value: str, flags: Collection[str] = (), raw: Optional[str] = None) -> None:
         self.value = value
         self.flags = frozenset(flags)
         self.raw = raw
@@ -110,7 +119,7 @@ class TerminalDef(Serialize):
     pattern: Pattern
     priority: int
 
-    def __init__(self, name: str, pattern: Pattern, priority: int=TOKEN_DEFAULT_PRIORITY) -> None:
+    def __init__(self, name: str, pattern: Pattern, priority: int = TOKEN_DEFAULT_PRIORITY) -> None:
         assert isinstance(pattern, Pattern), pattern
         self.name = name
         self.pattern = pattern
@@ -120,7 +129,7 @@ def __repr__(self):
         return '%s(%r, %r)' % (type(self).__name__, self.name, self.pattern)
 
     def user_repr(self) -> str:
-        if self.name.startswith('__'): # We represent a generated terminal
+        if self.name.startswith('__'):  # We represent a generated terminal
             return self.pattern.raw or self.name
         else:
             return self.name
@@ -162,29 +171,29 @@ class Token(str):
 
     @overload
     def __new__(
-        cls,
-        type: str,
-        value: Any,
-        start_pos: Optional[int]=None,
-        line: Optional[int]=None,
-        column: Optional[int]=None,
-        end_line: Optional[int]=None,
-        end_column: Optional[int]=None,
-        end_pos: Optional[int]=None
+            cls,
+            type: str,
+            value: Any,
+            start_pos: Optional[int] = None,
+            line: Optional[int] = None,
+            column: Optional[int] = None,
+            end_line: Optional[int] = None,
+            end_column: Optional[int] = None,
+            end_pos: Optional[int] = None
     ) -> 'Token':
         ...
 
     @overload
     def __new__(
-        cls,
-        type_: str,
-        value: Any,
-        start_pos: Optional[int]=None,
-        line: Optional[int]=None,
-        column: Optional[int]=None,
-        end_line: Optional[int]=None,
-        end_column: Optional[int]=None,
-        end_pos: Optional[int]=None
+            cls,
+            type_: str,
+            value: Any,
+            start_pos: Optional[int] = None,
+            line: Optional[int] = None,
+            column: Optional[int] = None,
+            end_line: Optional[int] = None,
+            end_column: Optional[int] = None,
+            end_pos: Optional[int] = None
     ) -> 'Token':        ...
 
     def __new__(cls, *args, **kwargs):
@@ -213,11 +222,11 @@ def _future_new(cls, type, value, start_pos=None, line=None, column=None, end_li
         return inst
 
     @overload
-    def update(self, type: Optional[str]=None, value: Optional[Any]=None) -> 'Token':
+    def update(self, type: Optional[str] = None, value: Optional[Any] = None) -> 'Token':
         ...
 
     @overload
-    def update(self, type_: Optional[str]=None, value: Optional[Any]=None) -> 'Token':
+    def update(self, type_: Optional[str] = None, value: Optional[Any] = None) -> 'Token':
         ...
 
     def update(self, *args, **kwargs):
@@ -230,7 +239,7 @@ def update(self, *args, **kwargs):
 
         return self._future_update(*args, **kwargs)
 
-    def _future_update(self, type: Optional[str]=None, value: Optional[Any]=None) -> 'Token':
+    def _future_update(self, type: Optional[str] = None, value: Optional[Any] = None) -> 'Token':
         return Token.new_borrow_pos(
             type if type is not None else self.type,
             value if value is not None else self.value,
@@ -364,7 +373,7 @@ def _build_mres(self, terminals, max_size):
             try:
                 mre = self.re_.compile(pattern, self.g_regex_flags)
             except AssertionError:  # Yes, this is what Python provides us.. :/
-                return self._build_mres(terminals, max_size//2)
+                return self._build_mres(terminals, max_size // 2)
 
             mres.append(mre)
             terminals = terminals[max_size:]
@@ -457,7 +466,7 @@ class BasicLexer(Lexer):
     callback: Dict[str, _Callback]
     re: ModuleType
 
-    def __init__(self, conf: 'LexerConf') -> None:
+    def __init__(self, conf: 'LexerConf', comparator=None) -> None:
         terminals = list(conf.terminals)
         assert all(isinstance(t, TerminalDef) for t in terminals), terminals
 
@@ -465,18 +474,37 @@ def __init__(self, conf: 'LexerConf') -> None:
 
         if not conf.skip_validation:
             # Sanitization
+            terminal_to_regexp = {}
             for t in terminals:
+                regexp = t.pattern.to_regexp()
                 try:
-                    self.re.compile(t.pattern.to_regexp(), conf.g_regex_flags)
+                    self.re.compile(regexp, conf.g_regex_flags)
                 except self.re.error:
                     raise LexError("Cannot compile token %s: %s" % (t.name, t.pattern))
 
                 if t.pattern.min_width == 0:
                     raise LexError("Lexer does not allow zero-width terminals. (%s: %s)" % (t.name, t.pattern))
+                if t.pattern.type == "re":
+                    terminal_to_regexp[t] = regexp
 
             if not (set(conf.ignore) <= {t.name for t in terminals}):
                 raise LexError("Ignore terminals are not defined: %s" % (set(conf.ignore) - {t.name for t in terminals}))
 
+            if has_interegular:
+                if not comparator:
+                    comparator = interegular.Comparator.from_regexes(terminal_to_regexp)
+                for group in classify(terminal_to_regexp, lambda t: t.priority).values():
+                    for a, b in comparator.check(group, skip_marked=True):
+                        assert a.priority == b.priority
+                        # Mark this pair to not repeat warnings when multiple different BasicLexers see the same collision
+                        comparator.mark(a, b)
+
+                        # leave it as a warning for the moment
+                        # raise LexError("Collision between Terminals %s and %s" % (a.name, b.name))
+                        example = comparator.get_example_overlap(a, b).format_multiline()
+                        logger.warning(f"Collision between Terminals {a.name} and {b.name}. "
+                                       f"The lexer will choose between them arbitrarily\n" + example)
+
         # Init
         self.newline_types = frozenset(t.name for t in terminals if _regexp_has_newline(t.pattern.to_regexp()))
         self.ignore_types = frozenset(conf.ignore)
@@ -517,7 +545,7 @@ def lex(self, state: LexerState, parser_state: Any) -> Iterator[Token]:
             while True:
                 yield self.next_token(state, parser_state)
 
-    def next_token(self, lex_state: LexerState, parser_state: Any=None) -> Token:
+    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):
             res = self.match(lex_state.text, line_ctr.char_pos)
@@ -565,6 +593,10 @@ def __init__(self, conf: 'LexerConf', states: Dict[str, Collection[str]], always
         trad_conf = copy(conf)
         trad_conf.terminals = terminals
 
+        if has_interegular and not conf.skip_validation:
+            comparator = interegular.Comparator.from_regexes({t: t.pattern.to_regexp() for t in terminals})
+        else:
+            comparator = None
         lexer_by_tokens: Dict[FrozenSet[str], BasicLexer] = {}
         self.lexers = {}
         for state, accepts in states.items():
@@ -575,13 +607,14 @@ 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)
+                lexer = BasicLexer(lexer_conf, comparator)
                 lexer_by_tokens[key] = lexer
 
             self.lexers[state] = lexer
 
         assert trad_conf.terminals is terminals
-        self.root_lexer = BasicLexer(trad_conf)
+        trad_conf.skip_validation = True  # We don't need to verify all terminals again
+        self.root_lexer = BasicLexer(trad_conf, comparator)
 
     def lex(self, lexer_state: LexerState, parser_state: Any) -> Iterator[Token]:
         try:
diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index ce295e03..3018133a 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -79,7 +79,7 @@
     '_RBRA': r'\]',
     '_LBRACE': r'\{',
     '_RBRACE': r'\}',
-    'OP': '[+*]|[?](?![a-z])',
+    'OP': '[+*]|[?](?![a-z_])',
     '_COLON': ':',
     '_COMMA': ',',
     '_OR': r'\|',
diff --git a/lark/tools/__init__.py b/lark/tools/__init__.py
index 391f991f..c6995c69 100644
--- a/lark/tools/__init__.py
+++ b/lark/tools/__init__.py
@@ -6,6 +6,11 @@
 import warnings
 
 from lark import Lark, logger
+try:
+    from interegular import logger as interegular_logger
+    has_interegular = True
+except ImportError:
+    has_interegular = False
 
 lalr_argparser = ArgumentParser(add_help=False, epilog='Look at the Lark documentation for more info on the options')
 
@@ -40,6 +45,8 @@
 
 def build_lalr(namespace):
     logger.setLevel((ERROR, WARN, INFO, DEBUG)[min(namespace.verbose, 3)])
+    if has_interegular:
+        interegular_logger.setLevel(logger.getEffectiveLevel())
     if len(namespace.start) == 0:
         namespace.start.append('start')
     kwargs = {n: getattr(namespace, n) for n in options}
diff --git a/lark/utils.py b/lark/utils.py
index 198c3520..753610a3 100644
--- a/lark/utils.py
+++ b/lark/utils.py
@@ -3,7 +3,7 @@
 from functools import reduce
 from itertools import product
 from collections import deque
-from typing import Callable, Iterator, List, Optional, Tuple, Type, TypeVar, Union, Dict, Any, Sequence
+from typing import Callable, Iterator, List, Optional, Tuple, Type, TypeVar, Union, Dict, Any, Sequence, Iterable
 
 ###{standalone
 import sys, re
@@ -21,7 +21,7 @@
 T = TypeVar("T")
 
 
-def classify(seq: Sequence, key: Optional[Callable] = None, value: Optional[Callable] = None) -> Dict:
+def classify(seq: Iterable, key: Optional[Callable] = None, value: Optional[Callable] = None) -> Dict:
     d: Dict[Any, Any] = {}
     for item in seq:
         k = key(item) if (key is not None) else item
diff --git a/setup.py b/setup.py
index 6c4a3676..57f9db47 100644
--- a/setup.py
+++ b/setup.py
@@ -15,6 +15,7 @@
         "regex": ["regex"],
         "nearley": ["js2py"],
         "atomic_cache": ["atomicwrites"],
+        "interegular": ["interegular>=0.2.4"],
     },
 
     package_data = {'': ['*.md', '*.lark'], 'lark': ['py.typed']},
diff --git a/test-requirements.txt b/test-requirements.txt
index 43014967..d11b671f 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -1,2 +1,3 @@
+interegular>=0.2.4
 Js2Py==0.68
 regex
diff --git a/tests/test_logger.py b/tests/test_logger.py
index cbe7f668..b52cd841 100644
--- a/tests/test_logger.py
+++ b/tests/test_logger.py
@@ -1,13 +1,18 @@
 import logging
 from contextlib import contextmanager
 from lark import Lark, logger
-from unittest import TestCase, main
+from unittest import TestCase, main, skipIf
 
 try:
     from StringIO import StringIO
 except ImportError:
     from io import StringIO
 
+try:
+    import interegular
+except ImportError:
+    interegular = None
+
 @contextmanager
 def capture_log():
     stream = StringIO()
@@ -46,7 +51,7 @@ def test_non_debug(self):
             Lark(collision_grammar, parser='lalr', debug=False)
         log = log.getvalue()
         # no log message
-        self.assertEqual(len(log), 0)
+        self.assertEqual(log, "")
 
     def test_loglevel_higher(self):
         logger.setLevel(logging.ERROR)
@@ -61,5 +66,37 @@ def test_loglevel_higher(self):
         # no log message
         self.assertEqual(len(log), 0)
 
+    @skipIf(interegular is None, "interegular is not installed, can't test regex collisions")
+    def test_regex_collision(self):
+        logger.setLevel(logging.WARNING)
+        collision_grammar = '''
+        start: A | B
+        A: /a+/
+        B: /(a|b)+/
+        '''
+        with capture_log() as log:
+            Lark(collision_grammar, parser='lalr')
+
+        log = log.getvalue()
+        # since there are conflicts between A and B
+        # symbols A and B should appear in the log message
+        self.assertIn("A", log)
+        self.assertIn("B", log)
+
+    @skipIf(interegular is None, "interegular is not installed, can't test regex collisions")
+    def test_regex_no_collision(self):
+        logger.setLevel(logging.WARNING)
+        collision_grammar = '''
+        start: A " " B
+        A: /a+/
+        B: /(a|b)+/
+        '''
+        with capture_log() as log:
+            Lark(collision_grammar, parser='lalr')
+
+        log = log.getvalue()
+        self.assertEqual(log, "")
+
+
 if __name__ == '__main__':
     main()
diff --git a/tox.ini b/tox.ini
index 0b1426dc..034e7a0d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -12,6 +12,9 @@ passenv =
 # to always force recreation and avoid unexpected side effects
 recreate = True
 
+# Require since the commands use `git`
+allowlist_externals = git
+
 commands =
     git submodule sync -q
     git submodule update --init
@@ -23,12 +26,14 @@ skip_install = true
 recreate = false
 deps =
     mypy==0.950
+    interegular>=0.2.4
     types-atomicwrites
     types-regex
     rich
 commands =
     mypy
 
+
 [testenv:lint]
 description = run linters on code base
 skip_install = true