diff --git a/THIRD-PARTY-NOTICES b/THIRD-PARTY-NOTICES
index a307b30432..0b9fc738ed 100644
--- a/THIRD-PARTY-NOTICES
+++ b/THIRD-PARTY-NOTICES
@@ -71,26 +71,3 @@ OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-3) License Notice for encoding_helpers.c
-----------------------------------------
-
-Portions Copyright 2001 Unicode, Inc.
-
-Disclaimer
-
-This source code is provided as is by Unicode, Inc. No claims are
-made as to fitness for any particular purpose. No warranties of any
-kind are expressed or implied. The recipient agrees to determine
-applicability of information provided. If this file has been
-purchased on magnetic or optical media from Unicode, Inc., the
-sole remedy for any claim will be exchange of defective media
-within 90 days of receipt.
-
-Limitations on Rights to Redistribute This Code
-
-Unicode, Inc. hereby grants the right to freely use the information
-supplied in this file in the creation of products supporting the
-Unicode Standard, and to make copies of this file in any form
-for internal or external distribution as long as this notice
-remains attached.
diff --git a/bson/_cbsonmodule.c b/bson/_cbsonmodule.c
index 191ce9886f..da6a5cbda7 100644
--- a/bson/_cbsonmodule.c
+++ b/bson/_cbsonmodule.c
@@ -26,7 +26,6 @@
 
 #include "buffer.h"
 #include "time64.h"
-#include "encoding_helpers.h"
 
 #define _CBSON_MODULE
 #include "_cbsonmodule.h"
@@ -553,12 +552,12 @@ static int _write_regex_to_buffer(
     PyObject* py_flags;
     PyObject* py_pattern;
     PyObject* encoded_pattern;
+    PyObject* decoded_pattern;
     long int_flags;
     char flags[FLAGS_SIZE];
     char check_utf8 = 0;
     const char* pattern_data;
     int pattern_length, flags_length;
-    result_t status;
 
     /*
      * Both the builtin re type and our Regex class have attributes
@@ -597,18 +596,8 @@ static int _write_regex_to_buffer(
         Py_DECREF(encoded_pattern);
         return 0;
     }
-    status = cbson_check_string((const unsigned char*)pattern_data,
-                          pattern_length, check_utf8, 1);
-    if (status == NOT_UTF_8) {
-        PyObject* InvalidStringData = _error("InvalidStringData");
-        if (InvalidStringData) {
-            PyErr_SetString(InvalidStringData,
-                            "regex patterns must be valid UTF-8");
-            Py_DECREF(InvalidStringData);
-        }
-        Py_DECREF(encoded_pattern);
-        return 0;
-    } else if (status == HAS_NULL) {
+
+    if (strlen(pattern_data) != (size_t) pattern_length){
         PyObject* InvalidDocument = _error("InvalidDocument");
         if (InvalidDocument) {
             PyErr_SetString(InvalidDocument,
@@ -619,6 +608,22 @@ static int _write_regex_to_buffer(
         return 0;
     }
 
+    if (check_utf8) {
+        decoded_pattern = PyUnicode_DecodeUTF8(pattern_data, (Py_ssize_t) pattern_length, NULL);
+        if (decoded_pattern == NULL) {
+            PyErr_Clear();
+            PyObject* InvalidStringData = _error("InvalidStringData");
+            if (InvalidStringData) {
+                PyErr_SetString(InvalidStringData,
+                                "regex patterns must be valid UTF-8");
+                Py_DECREF(InvalidStringData);
+            }
+            Py_DECREF(encoded_pattern);
+            return 0;
+        }
+        Py_DECREF(decoded_pattern);
+    }
+
     if (!buffer_write_bytes(buffer, pattern_data, pattern_length + 1)) {
         Py_DECREF(encoded_pattern);
         return 0;
diff --git a/bson/encoding_helpers.c b/bson/encoding_helpers.c
deleted file mode 100644
index 187ce6f3bd..0000000000
--- a/bson/encoding_helpers.c
+++ /dev/null
@@ -1,118 +0,0 @@
-/*
- * Copyright 2009-2015 MongoDB, 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.
- */
-
-#include "encoding_helpers.h"
-
-/*
- * Portions Copyright 2001 Unicode, Inc.
- *
- * Disclaimer
- *
- * This source code is provided as is by Unicode, Inc. No claims are
- * made as to fitness for any particular purpose. No warranties of any
- * kind are expressed or implied. The recipient agrees to determine
- * applicability of information provided. If this file has been
- * purchased on magnetic or optical media from Unicode, Inc., the
- * sole remedy for any claim will be exchange of defective media
- * within 90 days of receipt.
- *
- * Limitations on Rights to Redistribute This Code
- *
- * Unicode, Inc. hereby grants the right to freely use the information
- * supplied in this file in the creation of products supporting the
- * Unicode Standard, and to make copies of this file in any form
- * for internal or external distribution as long as this notice
- * remains attached.
- */
-
-/*
- * Index into the table below with the first byte of a UTF-8 sequence to
- * get the number of trailing bytes that are supposed to follow it.
- */
-static const char trailingBytesForUTF8[256] = {
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
-    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
-    2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2, 3,3,3,3,3,3,3,3,4,4,4,4,5,5,5,5
-};
-
-/* --------------------------------------------------------------------- */
-
-/*
- * Utility routine to tell whether a sequence of bytes is legal UTF-8.
- * This must be called with the length pre-determined by the first byte.
- * The length can be set by:
- *  length = trailingBytesForUTF8[*source]+1;
- * and the sequence is illegal right away if there aren't that many bytes
- * available.
- * If presented with a length > 4, this returns 0.  The Unicode
- * definition of UTF-8 goes up to 4-byte sequences.
- */
-static unsigned char isLegalUTF8(const unsigned char* source, int length) {
-    unsigned char a;
-    const unsigned char* srcptr = source + length;
-    switch (length) {
-    default: return 0;
-        /* Everything else falls through when "true"... */
-    case 4: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return 0;
-    case 3: if ((a = (*--srcptr)) < 0x80 || a > 0xBF) return 0;
-    case 2: if ((a = (*--srcptr)) > 0xBF) return 0;
-        switch (*source) {
-            /* no fall-through in this inner switch */
-            case 0xE0: if (a < 0xA0) return 0; break;
-            case 0xF0: if (a < 0x90) return 0; break;
-            case 0xF4: if ((a > 0x8F) || (a < 0x80)) return 0; break;
-            default:  if (a < 0x80) return 0;
-        }
-        case 1: if (*source >= 0x80 && *source < 0xC2) return 0;
-        if (*source > 0xF4) return 0;
-    }
-    return 1;
-}
-
-result_t cbson_check_string(const unsigned char* string, const int length,
-                      const char check_utf8, const char check_null) {
-    int position = 0;
-    /* By default we go character by character. Will be different for checking
-     * UTF-8 */
-    int sequence_length = 1;
-
-    if (!check_utf8 && !check_null) {
-        return VALID;
-    }
-
-    while (position < length) {
-        if (check_null && *(string + position) == 0) {
-            return HAS_NULL;
-        }
-        if (check_utf8) {
-            sequence_length = trailingBytesForUTF8[*(string + position)] + 1;
-            if ((position + sequence_length) > length) {
-                return NOT_UTF_8;
-            }
-            if (!isLegalUTF8(string + position, sequence_length)) {
-                return NOT_UTF_8;
-            }
-        }
-        position += sequence_length;
-    }
-
-    return VALID;
-}
diff --git a/bson/encoding_helpers.h b/bson/encoding_helpers.h
deleted file mode 100644
index a5fb75860f..0000000000
--- a/bson/encoding_helpers.h
+++ /dev/null
@@ -1,29 +0,0 @@
-/*
- * Copyright 2009-2015 MongoDB, 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.
- */
-
-#ifndef ENCODING_HELPERS_H
-#define ENCODING_HELPERS_H
-
-typedef enum {
-    VALID,
-    NOT_UTF_8,
-    HAS_NULL
-} result_t;
-
-result_t cbson_check_string(const unsigned char* string, const int length,
-                      const char check_utf8, const char check_null);
-
-#endif
diff --git a/doc/changelog.rst b/doc/changelog.rst
index b2fcb7fa24..f074a9d464 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -19,6 +19,8 @@ Bug fixes
 
 - Fixed a bug where :meth:`~pymongo.collection.Collection.estimated_document_count`
   would fail with a "CommandNotSupportedOnView" error on views (`PYTHON-2885`_).
+- Fixed a bug where invalid UTF-8 strings could be passed as patterns for :class:`~bson.regex.Regex`
+  objects (`PYTHON-3048`_). :func:`bson.encode` now correctly raises :class:`bson.errors.InvalidStringData`.
 
 Unavoidable breaking changes
 ............................
@@ -38,6 +40,7 @@ Issues Resolved
 See the `PyMongo 4.2 release notes in JIRA`_ for the list of resolved issues
 in this release.
 
+.. _PYTHON-3048: https://jira.mongodb.org/browse/PYTHON-3048
 .. _PYTHON-2885: https://jira.mongodb.org/browse/PYTHON-2885
 .. _PYTHON-3167: https://jira.mongodb.org/browse/PYTHON-3167
 .. _PyMongo 4.2 release notes in JIRA: https://jira.mongodb.org/secure/ReleaseNote.jspa?projectId=10004&version=33196
diff --git a/setup.py b/setup.py
index a2df4fac67..c6b32b9fba 100755
--- a/setup.py
+++ b/setup.py
@@ -255,12 +255,7 @@ def build_extension(self, ext):
     Extension(
         "bson._cbson",
         include_dirs=["bson"],
-        sources=[
-            "bson/_cbsonmodule.c",
-            "bson/time64.c",
-            "bson/buffer.c",
-            "bson/encoding_helpers.c",
-        ],
+        sources=["bson/_cbsonmodule.c", "bson/time64.c", "bson/buffer.c"],
     ),
     Extension(
         "pymongo._cmessage",