diff --git a/protovalidate/internal/extra_func.py b/protovalidate/internal/extra_func.py
index ae2b02c0..442cdb77 100644
--- a/protovalidate/internal/extra_func.py
+++ b/protovalidate/internal/extra_func.py
@@ -21,6 +21,7 @@
 from celpy import celtypes
 
 from protovalidate.internal import string_format
+from protovalidate.internal.matches import cel_matches
 from protovalidate.internal.rules import MessageType, field_to_cel
 
 # See https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address
@@ -1554,12 +1555,13 @@ def __peek(self, char: str) -> bool:
 
 
 def make_extra_funcs(locale: str) -> dict[str, celpy.CELFunction]:
-    # TODO(#257): Fix types and add tests for StringFormat.
     # For now, ignoring the type.
     string_fmt = string_format.StringFormat(locale)  # type: ignore
     return {
         # Missing standard functions
         "format": string_fmt.format,
+        # Overridden standard functions
+        "matches": cel_matches,
         # protovalidate specific functions
         "getField": cel_get_field,
         "isNan": cel_is_nan,
diff --git a/protovalidate/internal/matches.py b/protovalidate/internal/matches.py
new file mode 100644
index 00000000..72728582
--- /dev/null
+++ b/protovalidate/internal/matches.py
@@ -0,0 +1,66 @@
+# Copyright 2023-2025 Buf Technologies, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+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
+invalid_patterns = [
+    r"\\[1-9]",  # backreference
+    r"\\k<\w+>",  # backreference
+    r"\(\?\=",  # lookahead
+    r"\(\?\!",  # negative lookahead
+    r"\(\?\<\=",  # lookbehind
+    r"\(\?\<\!",  # negative lookbehind
+    r"\\c[A-Z]",  # control character
+    r"\\u[0-9a-fA-F]{4}",  # UTF-16 code-unit
+    r"\\0(?!\d)",  # NUL
+    r"\[\\b.*\]",  # Backspace eg: [\b]
+]
+
+
+def cel_matches(text: celtypes.Value, pattern: celtypes.Value) -> celpy.Result:
+    """Return True if the given pattern matches text. False otherwise.
+
+    CEL uses RE2 syntax which diverges from Python re in various ways. Ideally, we
+    would use the google-re2 package, which is an extra dep in celpy, but at press
+    time it does not provide a pre-built binary for the latest version of Python (3.13)
+    which means those using this version will run into many issues.
+
+    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)
+
+    # Simulate re2 by failing on any patterns not compatible with re2 syntax
+    for invalid_pattern in invalid_patterns:
+        r = re.search(invalid_pattern, pattern)
+        if r is not None:
+            msg = f"error evaluating pattern {pattern}, invalid RE2 syntax"
+            raise celpy.CELEvalError(msg)
+
+    try:
+        m = re.search(pattern, text)
+    except re.error as ex:
+        return celpy.CELEvalError("match error", ex.__class__, ex.args)
+
+    return celtypes.BoolType(m is not None)
diff --git a/tests/matches_test.py b/tests/matches_test.py
new file mode 100644
index 00000000..7e3a91aa
--- /dev/null
+++ b/tests/matches_test.py
@@ -0,0 +1,44 @@
+# Copyright 2023-2025 Buf Technologies, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+import celpy
+from celpy import celtypes
+
+from protovalidate.internal import extra_func
+
+invalid_patterns = [
+    r"\1",
+    r"\k<name>",
+    r"Jack(?=Sprat)",
+    "Jack(?!Sprat)",
+    "(?<=Sprat)Jack",
+    "(?<!Sprat)Jack",
+    r"\cM\cJ",
+    r"\u0041",
+    r"\0 \01 \0a \012",
+    r"[\b]",
+]
+
+
+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}")
+            except celpy.CELEvalError as e:
+                self.assertEqual(str(e), f"error evaluating pattern {cel_pattern}, invalid RE2 syntax")