diff --git a/gen/tests/example/v1/validations_pb2.py b/gen/tests/example/v1/validations_pb2.py
index 6c88b1ca..dc397ffa 100644
--- a/gen/tests/example/v1/validations_pb2.py
+++ b/gen/tests/example/v1/validations_pb2.py
@@ -40,7 +40,7 @@
 from google.protobuf import timestamp_pb2 as google_dot_protobuf_dot_timestamp__pb2
 
 
-DESCRIPTOR = _descriptor_pool.Default().AddSerializedFile(b'\n\"tests/example/v1/validations.proto\x12\x10tests.example.v1\x1a\x1b\x62uf/validate/validate.proto\x1a\x1fgoogle/protobuf/timestamp.proto\"T\n\x13MultipleValidations\x12 \n\x05title\x18\x01 \x01(\tB\n\xbaH\x07r\x05:\x03\x66ooR\x05title\x12\x1b\n\x04name\x18\x02 \x01(\tB\x07\xbaH\x04r\x02\x10\x05R\x04name\")\n\x0c\x44oubleFinite\x12\x19\n\x03val\x18\x01 \x01(\x01\x42\x07\xbaH\x04\x12\x02@\x01R\x03val\";\n\x0eSFixed64ExLTGT\x12)\n\x03val\x18\x01 \x01(\x10\x42\x17\xbaH\x14\x62\x12\x11\x00\x00\x00\x00\x00\x00\x00\x00!\n\x00\x00\x00\x00\x00\x00\x00R\x03val\")\n\x0cTestOneofMsg\x12\x19\n\x03val\x18\x01 \x01(\x08\x42\x07\xbaH\x04j\x02\x08\x01R\x03val\"q\n\x05Oneof\x12\x1a\n\x01x\x18\x01 \x01(\tB\n\xbaH\x07r\x05:\x03\x66ooH\x00R\x01x\x12\x17\n\x01y\x18\x02 \x01(\x05\x42\x07\xbaH\x04\x1a\x02 \x00H\x00R\x01y\x12.\n\x01z\x18\x03 \x01(\x0b\x32\x1e.tests.example.v1.TestOneofMsgH\x00R\x01zB\x03\n\x01o\"[\n\x12ProtovalidateOneof\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\x0b\xbaH\x08\"\x06\n\x01\x61\n\x01\x62\"e\n\x1aProtovalidateOneofRequired\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\r\xbaH\n\"\x08\n\x01\x61\n\x01\x62\x10\x01\"p\n\"ProtovalidateOneofUnknownFieldName\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\x10\xbaH\r\"\x0b\n\x01\x61\n\x01\x62\n\x03xxx\"H\n\x0eTimestampGTNow\x12\x36\n\x03val\x18\x01 \x01(\x0b\x32\x1a.google.protobuf.TimestampB\x08\xbaH\x05\xb2\x01\x02@\x01R\x03val\"\x87\x01\n\tMapMinMax\x12\x42\n\x03val\x18\x01 \x03(\x0b\x32$.tests.example.v1.MapMinMax.ValEntryB\n\xbaH\x07\x9a\x01\x04\x08\x02\x10\x04R\x03val\x1a\x36\n\x08ValEntry\x12\x10\n\x03key\x18\x01 \x01(\tR\x03key\x12\x14\n\x05value\x18\x02 \x01(\x08R\x05value:\x02\x38\x01\"\x85\x01\n\x07MapKeys\x12\x42\n\x03val\x18\x01 \x03(\x0b\x32\".tests.example.v1.MapKeys.ValEntryB\x0c\xbaH\t\x9a\x01\x06\"\x04\x42\x02\x10\x00R\x03val\x1a\x36\n\x08ValEntry\x12\x10\n\x03key\x18\x01 \x01(\x12R\x03key\x12\x14\n\x05value\x18\x02 \x01(\tR\x05value:\x02\x38\x01\"\"\n\x05\x45mbed\x12\x19\n\x03val\x18\x01 \x01(\x03\x42\x07\xbaH\x04\"\x02 \x00R\x03val\"K\n\x11RepeatedEmbedSkip\x12\x36\n\x03val\x18\x01 \x03(\x0b\x32\x17.tests.example.v1.EmbedB\x0b\xbaH\x08\x92\x01\x05\"\x03\xd8\x01\x03R\x03valB\x8a\x01\n\x14\x63om.tests.example.v1B\x10ValidationsProtoP\x01\xa2\x02\x03TEX\xaa\x02\x10Tests.Example.V1\xca\x02\x10Tests\\Example\\V1\xe2\x02\x1cTests\\Example\\V1\\GPBMetadata\xea\x02\x12Tests::Example::V1b\x06proto3')
+DESCRIPTOR = _descriptor_pool.Default().AddSerializedFile(b'\n\"tests/example/v1/validations.proto\x12\x10tests.example.v1\x1a\x1b\x62uf/validate/validate.proto\x1a\x1fgoogle/protobuf/timestamp.proto\"T\n\x13MultipleValidations\x12 \n\x05title\x18\x01 \x01(\tB\n\xbaH\x07r\x05:\x03\x66ooR\x05title\x12\x1b\n\x04name\x18\x02 \x01(\tB\x07\xbaH\x04r\x02\x10\x05R\x04name\")\n\x0c\x44oubleFinite\x12\x19\n\x03val\x18\x01 \x01(\x01\x42\x07\xbaH\x04\x12\x02@\x01R\x03val\";\n\x0eSFixed64ExLTGT\x12)\n\x03val\x18\x01 \x01(\x10\x42\x17\xbaH\x14\x62\x12\x11\x00\x00\x00\x00\x00\x00\x00\x00!\n\x00\x00\x00\x00\x00\x00\x00R\x03val\")\n\x0cTestOneofMsg\x12\x19\n\x03val\x18\x01 \x01(\x08\x42\x07\xbaH\x04j\x02\x08\x01R\x03val\"q\n\x05Oneof\x12\x1a\n\x01x\x18\x01 \x01(\tB\n\xbaH\x07r\x05:\x03\x66ooH\x00R\x01x\x12\x17\n\x01y\x18\x02 \x01(\x05\x42\x07\xbaH\x04\x1a\x02 \x00H\x00R\x01y\x12.\n\x01z\x18\x03 \x01(\x0b\x32\x1e.tests.example.v1.TestOneofMsgH\x00R\x01zB\x03\n\x01o\"[\n\x12ProtovalidateOneof\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\x0b\xbaH\x08\"\x06\n\x01\x61\n\x01\x62\"e\n\x1aProtovalidateOneofRequired\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\r\xbaH\n\"\x08\n\x01\x61\n\x01\x62\x10\x01\"p\n\"ProtovalidateOneofUnknownFieldName\x12\x0c\n\x01\x61\x18\x01 \x01(\tR\x01\x61\x12\x0c\n\x01\x62\x18\x02 \x01(\tR\x01\x62\x12\x1c\n\tunrelated\x18\x03 \x01(\x08R\tunrelated:\x10\xbaH\r\"\x0b\n\x01\x61\n\x01\x62\n\x03xxx\"H\n\x0eTimestampGTNow\x12\x36\n\x03val\x18\x01 \x01(\x0b\x32\x1a.google.protobuf.TimestampB\x08\xbaH\x05\xb2\x01\x02@\x01R\x03val\"\x87\x01\n\tMapMinMax\x12\x42\n\x03val\x18\x01 \x03(\x0b\x32$.tests.example.v1.MapMinMax.ValEntryB\n\xbaH\x07\x9a\x01\x04\x08\x02\x10\x04R\x03val\x1a\x36\n\x08ValEntry\x12\x10\n\x03key\x18\x01 \x01(\tR\x03key\x12\x14\n\x05value\x18\x02 \x01(\x08R\x05value:\x02\x38\x01\"\x85\x01\n\x07MapKeys\x12\x42\n\x03val\x18\x01 \x03(\x0b\x32\".tests.example.v1.MapKeys.ValEntryB\x0c\xbaH\t\x9a\x01\x06\"\x04\x42\x02\x10\x00R\x03val\x1a\x36\n\x08ValEntry\x12\x10\n\x03key\x18\x01 \x01(\x12R\x03key\x12\x14\n\x05value\x18\x02 \x01(\tR\x05value:\x02\x38\x01\"\"\n\x05\x45mbed\x12\x19\n\x03val\x18\x01 \x01(\x03\x42\x07\xbaH\x04\"\x02 \x00R\x03val\"K\n\x11RepeatedEmbedSkip\x12\x36\n\x03val\x18\x01 \x03(\x0b\x32\x17.tests.example.v1.EmbedB\x0b\xbaH\x08\x92\x01\x05\"\x03\xd8\x01\x03R\x03val\"3\n\x0fInvalidRESyntax\x12 \n\x05value\x18\x01 \x01(\tB\n\xbaH\x07r\x05\x32\x03^\\zR\x05valueB\x8a\x01\n\x14\x63om.tests.example.v1B\x10ValidationsProtoP\x01\xa2\x02\x03TEX\xaa\x02\x10Tests.Example.V1\xca\x02\x10Tests\\Example\\V1\xe2\x02\x1cTests\\Example\\V1\\GPBMetadata\xea\x02\x12Tests::Example::V1b\x06proto3')
 
 _globals = globals()
 _builder.BuildMessageAndEnumDescriptors(DESCRIPTOR, _globals)
@@ -82,6 +82,8 @@
   _globals['_EMBED'].fields_by_name['val']._serialized_options = b'\272H\004\"\002 \000'
   _globals['_REPEATEDEMBEDSKIP'].fields_by_name['val']._loaded_options = None
   _globals['_REPEATEDEMBEDSKIP'].fields_by_name['val']._serialized_options = b'\272H\010\222\001\005\"\003\330\001\003'
+  _globals['_INVALIDRESYNTAX'].fields_by_name['value']._loaded_options = None
+  _globals['_INVALIDRESYNTAX'].fields_by_name['value']._serialized_options = b'\272H\007r\0052\003^\\z'
   _globals['_MULTIPLEVALIDATIONS']._serialized_start=118
   _globals['_MULTIPLEVALIDATIONS']._serialized_end=202
   _globals['_DOUBLEFINITE']._serialized_start=204
@@ -112,4 +114,6 @@
   _globals['_EMBED']._serialized_end=1158
   _globals['_REPEATEDEMBEDSKIP']._serialized_start=1160
   _globals['_REPEATEDEMBEDSKIP']._serialized_end=1235
+  _globals['_INVALIDRESYNTAX']._serialized_start=1237
+  _globals['_INVALIDRESYNTAX']._serialized_end=1288
 # @@protoc_insertion_point(module_scope)
diff --git a/gen/tests/example/v1/validations_pb2.pyi b/gen/tests/example/v1/validations_pb2.pyi
index e255c703..88aec111 100644
--- a/gen/tests/example/v1/validations_pb2.pyi
+++ b/gen/tests/example/v1/validations_pb2.pyi
@@ -131,3 +131,9 @@ class RepeatedEmbedSkip(_message.Message):
     VAL_FIELD_NUMBER: _ClassVar[int]
     val: _containers.RepeatedCompositeFieldContainer[Embed]
     def __init__(self, val: _Optional[_Iterable[_Union[Embed, _Mapping]]] = ...) -> None: ...
+
+class InvalidRESyntax(_message.Message):
+    __slots__ = ("value",)
+    VALUE_FIELD_NUMBER: _ClassVar[int]
+    value: str
+    def __init__(self, value: _Optional[str] = ...) -> None: ...
diff --git a/proto/tests/example/v1/validations.proto b/proto/tests/example/v1/validations.proto
index 7e51bdee..fbf0467b 100644
--- a/proto/tests/example/v1/validations.proto
+++ b/proto/tests/example/v1/validations.proto
@@ -93,6 +93,11 @@ message MapKeys {
 message Embed {
   int64 val = 1 [(buf.validate.field).int64.gt = 0];
 }
+
 message RepeatedEmbedSkip {
   repeated Embed val = 1 [(buf.validate.field).repeated.items.ignore = IGNORE_ALWAYS];
 }
+
+message InvalidRESyntax {
+  string value = 1 [(buf.validate.field).string.pattern = "^\\z"];
+}
diff --git a/protovalidate/config.py b/protovalidate/config.py
index 1e21683b..d376a49c 100644
--- a/protovalidate/config.py
+++ b/protovalidate/config.py
@@ -12,7 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from collections.abc import Callable
 from dataclasses import dataclass
+from typing import Optional
 
 
 @dataclass
@@ -21,6 +23,10 @@ class Config:
 
     Attributes:
         fail_fast (bool): If true, validation will stop after the first violation. Defaults to False.
+        regex_matches_func: An optional regex matcher to use. If specified, this will be used to match
+                            on regex expressions instead of this library's `matches` logic.
     """
 
     fail_fast: bool = False
+
+    regex_matches_func: Optional[Callable[[str, str], bool]] = None
diff --git a/protovalidate/internal/extra_func.py b/protovalidate/internal/extra_func.py
index 442cdb77..5f233105 100644
--- a/protovalidate/internal/extra_func.py
+++ b/protovalidate/internal/extra_func.py
@@ -15,13 +15,15 @@
 import math
 import re
 import typing
+from collections.abc import Callable
 from urllib import parse as urlparse
 
 import celpy
 from celpy import celtypes
 
+from protovalidate.config import Config
 from protovalidate.internal import string_format
-from protovalidate.internal.matches import cel_matches
+from protovalidate.internal.matches import matches as protovalidate_matches
 from protovalidate.internal.rules import MessageType, field_to_cel
 
 # See https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address
@@ -1554,14 +1556,31 @@ def __peek(self, char: str) -> bool:
         return self._index < len(self._string) and self._string[self._index] == char
 
 
-def make_extra_funcs(locale: str) -> dict[str, celpy.CELFunction]:
-    # For now, ignoring the type.
-    string_fmt = string_format.StringFormat(locale)  # type: ignore
+def get_matches_func(matcher: typing.Optional[Callable[[str, str], bool]]):
+    if matcher is None:
+        matcher = protovalidate_matches
+
+    def cel_matches(text: celtypes.Value, pattern: celtypes.Value) -> celpy.Result:
+        if not isinstance(text, celtypes.StringType):
+            msg = "invalid argument for text, expected string"
+            raise celpy.CELEvalError(msg)
+        if not isinstance(pattern, celtypes.StringType):
+            msg = "invalid argument for pattern, expected string"
+            raise celpy.CELEvalError(msg)
+
+        b = matcher(text, pattern)
+        return celtypes.BoolType(b)
+
+    return cel_matches
+
+
+def make_extra_funcs(config: Config) -> dict[str, celpy.CELFunction]:
+    string_fmt = string_format.StringFormat()
     return {
         # Missing standard functions
         "format": string_fmt.format,
         # Overridden standard functions
-        "matches": cel_matches,
+        "matches": get_matches_func(config.regex_matches_func),
         # protovalidate specific functions
         "getField": cel_get_field,
         "isNan": cel_is_nan,
@@ -1575,6 +1594,3 @@ def make_extra_funcs(locale: str) -> dict[str, celpy.CELFunction]:
         "isHostAndPort": cel_is_host_and_port,
         "unique": cel_unique,
     }
-
-
-EXTRA_FUNCS = make_extra_funcs("en_US")
diff --git a/protovalidate/internal/matches.py b/protovalidate/internal/matches.py
index 72728582..27e29f9e 100644
--- a/protovalidate/internal/matches.py
+++ b/protovalidate/internal/matches.py
@@ -15,7 +15,6 @@
 import re
 
 import celpy
-from celpy import celtypes
 
 # Patterns that are supported in Python's re package and not in re2.
 # RE2: https://github.com/google/re2/wiki/syntax
@@ -30,10 +29,11 @@
     r"\\u[0-9a-fA-F]{4}",  # UTF-16 code-unit
     r"\\0(?!\d)",  # NUL
     r"\[\\b.*\]",  # Backspace eg: [\b]
+    r"\\Z",  # End of text (only lowercase z is supported in re2)
 ]
 
 
-def cel_matches(text: celtypes.Value, pattern: celtypes.Value) -> celpy.Result:
+def matches(text: str, pattern: str) -> bool:
     """Return True if the given pattern matches text. False otherwise.
 
     CEL uses RE2 syntax which diverges from Python re in various ways. Ideally, we
@@ -43,14 +43,13 @@ def cel_matches(text: celtypes.Value, pattern: celtypes.Value) -> celpy.Result:
 
     Instead of foisting this issue on users, we instead mimic re2 syntax by failing
     to compile the regex for patterns not compatible with re2.
-    """
-    if not isinstance(text, celtypes.StringType):
-        msg = "invalid argument for text, expected string"
-        raise celpy.CELEvalError(msg)
-    if not isinstance(pattern, celtypes.StringType):
-        msg = "invalid argument for pattern, expected string"
-        raise celpy.CELEvalError(msg)
 
+    Users can choose to override this behavior by providing their own custom matches
+    function via the Config.
+
+    Raises:
+        celpy.CELEvalError: If pattern contains invalid re2 syntax or if an re.error is raised during matching.
+    """
     # Simulate re2 by failing on any patterns not compatible with re2 syntax
     for invalid_pattern in invalid_patterns:
         r = re.search(invalid_pattern, pattern)
@@ -61,6 +60,7 @@ def cel_matches(text: celtypes.Value, pattern: celtypes.Value) -> celpy.Result:
     try:
         m = re.search(pattern, text)
     except re.error as ex:
-        return celpy.CELEvalError("match error", ex.__class__, ex.args)
+        msg = "match error"
+        raise celpy.CELEvalError(msg, ex.__class__, ex.args) from ex
 
-    return celtypes.BoolType(m is not None)
+    return m is not None
diff --git a/protovalidate/internal/rules.py b/protovalidate/internal/rules.py
index 989abf96..7a6758c0 100644
--- a/protovalidate/internal/rules.py
+++ b/protovalidate/internal/rules.py
@@ -15,6 +15,7 @@
 import dataclasses
 import datetime
 import typing
+from collections.abc import Callable
 
 import celpy
 from celpy import celtypes
@@ -44,7 +45,7 @@ def unwrap(msg: message.Message) -> celtypes.Value:
     return field_to_cel(msg, msg.DESCRIPTOR.fields_by_name["value"])
 
 
-_MSG_TYPE_URL_TO_CTOR: dict[str, typing.Callable[..., celtypes.Value]] = {
+_MSG_TYPE_URL_TO_CTOR: dict[str, Callable[..., celtypes.Value]] = {
     "google.protobuf.Duration": make_duration,
     "google.protobuf.Timestamp": make_timestamp,
     "google.protobuf.StringValue": unwrap,
diff --git a/protovalidate/internal/string_format.py b/protovalidate/internal/string_format.py
index 922aff2e..0755d7bf 100644
--- a/protovalidate/internal/string_format.py
+++ b/protovalidate/internal/string_format.py
@@ -24,8 +24,7 @@
 class StringFormat:
     """An implementation of string.format() in CEL."""
 
-    def __init__(self, locale: str):
-        self.locale = locale
+    def __init__(self):
         self.fmt = None
 
     def format(self, fmt: celtypes.Value, args: celtypes.Value) -> celpy.Result:
diff --git a/protovalidate/validator.py b/protovalidate/validator.py
index 30d3fd2d..5c1850ba 100644
--- a/protovalidate/validator.py
+++ b/protovalidate/validator.py
@@ -39,8 +39,9 @@ class Validator:
     _cfg: Config
 
     def __init__(self, config=None):
-        self._factory = _rules.RuleFactory(extra_func.EXTRA_FUNCS)
         self._cfg = config if config is not None else Config()
+        funcs = extra_func.make_extra_funcs(self._cfg)
+        self._factory = _rules.RuleFactory(funcs)
 
     def validate(
         self,
diff --git a/tests/config_test.py b/tests/config_test.py
index 71f33af7..16d7eca3 100644
--- a/tests/config_test.py
+++ b/tests/config_test.py
@@ -21,3 +21,4 @@ class TestConfig(unittest.TestCase):
     def test_defaults(self):
         cfg = Config()
         self.assertFalse(cfg.fail_fast)
+        self.assertIsNone(cfg.regex_matches_func)
diff --git a/tests/format_test.py b/tests/format_test.py
index 353a8478..e8609656 100644
--- a/tests/format_test.py
+++ b/tests/format_test.py
@@ -23,6 +23,7 @@
 
 from gen.cel.expr import eval_pb2
 from gen.cel.expr.conformance.test import simple_pb2
+from protovalidate.config import Config
 from protovalidate.internal import extra_func
 from protovalidate.internal.cel_field_presence import InterpretedRunner
 
@@ -108,7 +109,7 @@ def test_format_successes(self):
             if test.name in skipped_tests:
                 continue
             ast = self._env.compile(test.expr)
-            prog = self._env.program(ast, functions=extra_func.EXTRA_FUNCS)
+            prog = self._env.program(ast, functions=extra_func.make_extra_funcs(Config()))
 
             bindings = build_variables(test.bindings)
             # Ideally we should use pytest parametrize instead of subtests, but
@@ -132,7 +133,7 @@ def test_format_errors(self):
             if test.name in skipped_error_tests:
                 continue
             ast = self._env.compile(test.expr)
-            prog = self._env.program(ast, functions=extra_func.EXTRA_FUNCS)
+            prog = self._env.program(ast, functions=extra_func.make_extra_funcs(Config()))
 
             bindings = build_variables(test.bindings)
             # Ideally we should use pytest parametrize instead of subtests, but
diff --git a/tests/matches_test.py b/tests/matches_test.py
index 7e3a91aa..7d730852 100644
--- a/tests/matches_test.py
+++ b/tests/matches_test.py
@@ -15,9 +15,8 @@
 import unittest
 
 import celpy
-from celpy import celtypes
 
-from protovalidate.internal import extra_func
+from protovalidate.internal.matches import matches
 
 invalid_patterns = [
     r"\1",
@@ -30,15 +29,15 @@
     r"\u0041",
     r"\0 \01 \0a \012",
     r"[\b]",
+    r"^\Z",
 ]
 
 
 class TestMatches(unittest.TestCase):
     def test_invalid_re2_syntax(self):
         for pattern in invalid_patterns:
-            cel_pattern = celtypes.StringType(pattern)
             try:
-                extra_func.cel_matches(celtypes.StringType("test"), cel_pattern)
-                self.fail(f"expected an error on pattern {cel_pattern}")
+                matches("test", pattern)
+                self.fail(f"expected an error on pattern {pattern}")
             except celpy.CELEvalError as e:
-                self.assertEqual(str(e), f"error evaluating pattern {cel_pattern}, invalid RE2 syntax")
+                self.assertEqual(str(e), f"error evaluating pattern {pattern}, invalid RE2 syntax")
diff --git a/tests/validate_test.py b/tests/validate_test.py
index 792aeb08..c1761f99 100644
--- a/tests/validate_test.py
+++ b/tests/validate_test.py
@@ -12,8 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import re
 import unittest
 
+import celpy
 from google.protobuf import message
 
 import protovalidate
@@ -209,6 +211,42 @@ def test_fail_fast(self):
         violations = validator.collect_violations(msg)
         self._compare_violations(violations, [expected_violation])
 
+    def test_custom_matcher(self):
+        r"""Tests usage of the custom regex_matches_func in the config
+
+        A bit of a contrived example, but this exercises the code path
+        for specifying a custom regex matches function when writing regex rules.
+
+        Usage of the pattern \z is not supported in Python's re engine, only \Z is supported.
+        However, the inverse is true with re2 (\Z is _not_ supported and \z is supported).
+
+        This test shows using a custom matcher that converts any re2-compliant usage of \z
+        to \Z so that Python's re engine can execute it.
+        """
+        msg = validations_pb2.InvalidRESyntax()
+
+        def matcher(text: str, pattern: str) -> bool:
+            pattern = pattern.replace("z", "Z")
+            try:
+                m = re.search(pattern, text)
+            except re.error as ex:
+                msg = "match error"
+                raise celpy.CELEvalError(msg, ex.__class__, ex.args) from ex
+            return m is not None
+
+        cfg = Config(regex_matches_func=matcher)
+        validator = protovalidate.Validator(config=cfg)
+
+        # Test validate
+        try:
+            validator.validate(msg)
+        except Exception:
+            self.fail("unexpected validation failure")
+
+        # Test collect_violations
+        violations = validator.collect_violations(msg)
+        self.assertEqual(len(violations), 0)
+
     def _run_valid_tests(self, msg: message.Message):
         """A helper function for testing successful validation on a given message
 
@@ -257,12 +295,12 @@ def _run_compilation_error_tests(self, msg: message.Message, expected: str):
                 # Test validate
                 with self.assertRaises(protovalidate.CompilationError) as vce:
                     v.validate(msg)
-                    self.assertEqual(str(vce.exception), expected)
+                self.assertEqual(str(vce.exception), expected)
 
                 # Test collect_violations
                 with self.assertRaises(protovalidate.CompilationError) as cvce:
                     v.collect_violations(msg)
-                    self.assertEqual(str(cvce.exception), expected)
+                self.assertEqual(str(cvce.exception), expected)
 
     def _compare_violations(self, actual: list[rules.Violation], expected: list[rules.Violation]) -> None:
         """Compares two lists of violations. The violations are expected to be in the expected order also."""