diff --git a/bigquery/google/cloud/bigquery/_helpers.py b/bigquery/google/cloud/bigquery/_helpers.py
index bb3998732a5a..eb5161c9fe71 100644
--- a/bigquery/google/cloud/bigquery/_helpers.py
+++ b/bigquery/google/cloud/bigquery/_helpers.py
@@ -18,6 +18,7 @@
 import copy
 import datetime
 import decimal
+import re
 
 from google.cloud._helpers import UTC
 from google.cloud._helpers import _date_from_iso8601_date
@@ -29,6 +30,12 @@
 _RFC3339_MICROS_NO_ZULU = "%Y-%m-%dT%H:%M:%S.%f"
 _TIMEONLY_WO_MICROS = "%H:%M:%S"
 _TIMEONLY_W_MICROS = "%H:%M:%S.%f"
+_PROJECT_PREFIX_PATTERN = re.compile(
+    r"""
+    (?P<project_id>\S+\:[^.]+)\.(?P<dataset_id>[^.]+)(?:$|\.(?P<custom_id>[^.]+)$)
+""",
+    re.VERBOSE,
+)
 
 
 def _not_null(value, field):
@@ -586,24 +593,42 @@ def _str_or_none(value):
         return str(value)
 
 
+def _split_id(full_id):
+    """Helper: split full_id into composite parts.
+
+    Args:
+        full_id (str): Fully-qualified ID in standard SQL format.
+
+    Returns:
+        List[str]: ID's parts separated into components.
+    """
+    with_prefix = _PROJECT_PREFIX_PATTERN.match(full_id)
+    if with_prefix is None:
+        parts = full_id.split(".")
+    else:
+        parts = with_prefix.groups()
+        parts = [part for part in parts if part]
+    return parts
+
+
 def _parse_3_part_id(full_id, default_project=None, property_name="table_id"):
     output_project_id = default_project
     output_dataset_id = None
     output_resource_id = None
-    parts = full_id.split(".")
+    parts = _split_id(full_id)
 
     if len(parts) != 2 and len(parts) != 3:
         raise ValueError(
             "{property_name} must be a fully-qualified ID in "
-            'standard SQL format. e.g. "project.dataset.{property_name}", '
+            'standard SQL format, e.g., "project.dataset.{property_name}", '
             "got {}".format(full_id, property_name=property_name)
         )
 
     if len(parts) == 2 and not default_project:
         raise ValueError(
             "When default_project is not set, {property_name} must be a "
-            "fully-qualified ID in standard SQL format. "
-            'e.g. "project.dataset_id.{property_name}", got {}'.format(
+            "fully-qualified ID in standard SQL format, "
+            'e.g., "project.dataset_id.{property_name}", got {}'.format(
                 full_id, property_name=property_name
             )
         )
diff --git a/bigquery/google/cloud/bigquery/dataset.py b/bigquery/google/cloud/bigquery/dataset.py
index 494c219d4f67..67a7353f94e7 100644
--- a/bigquery/google/cloud/bigquery/dataset.py
+++ b/bigquery/google/cloud/bigquery/dataset.py
@@ -18,7 +18,6 @@
 
 import six
 import copy
-import re
 
 import google.cloud._helpers
 from google.cloud.bigquery import _helpers
@@ -27,14 +26,6 @@
 from google.cloud.bigquery.table import TableReference
 
 
-_PROJECT_PREFIX_PATTERN = re.compile(
-    r"""
-    (?P<project_id>\S+\:[^.]+)\.(?P<dataset_id>[^.]+)$
-""",
-    re.VERBOSE,
-)
-
-
 def _get_table_reference(self, table_id):
     """Constructs a TableReference.
 
@@ -299,13 +290,7 @@ def from_string(cls, dataset_id, default_project=None):
         """
         output_dataset_id = dataset_id
         output_project_id = default_project
-        with_prefix = _PROJECT_PREFIX_PATTERN.match(dataset_id)
-        if with_prefix is None:
-            parts = dataset_id.split(".")
-        else:
-            project_id = with_prefix.group("project_id")
-            dataset_id = with_prefix.group("dataset_id")
-            parts = [project_id, dataset_id]
+        parts = _helpers._split_id(dataset_id)
 
         if len(parts) == 1 and not default_project:
             raise ValueError(
diff --git a/bigquery/tests/unit/test_table.py b/bigquery/tests/unit/test_table.py
index 8ba7fee892e5..2f72dd3b542e 100644
--- a/bigquery/tests/unit/test_table.py
+++ b/bigquery/tests/unit/test_table.py
@@ -215,11 +215,23 @@ def test_from_string(self):
         self.assertEqual(got.dataset_id, "string_dataset")
         self.assertEqual(got.table_id, "string_table")
 
+    def test_from_string_w_prefix(self):
+        cls = self._get_target_class()
+        got = cls.from_string("google.com:string-project.string_dataset.string_table")
+        self.assertEqual(got.project, "google.com:string-project")
+        self.assertEqual(got.dataset_id, "string_dataset")
+        self.assertEqual(got.table_id, "string_table")
+
     def test_from_string_legacy_string(self):
         cls = self._get_target_class()
         with self.assertRaises(ValueError):
             cls.from_string("string-project:string_dataset.string_table")
 
+    def test_from_string_w_incorrect_prefix(self):
+        cls = self._get_target_class()
+        with self.assertRaises(ValueError):
+            cls.from_string("google.com.string-project.string_dataset.string_table")
+
     def test_from_string_not_fully_qualified(self):
         cls = self._get_target_class()
         with self.assertRaises(ValueError):