diff --git a/docker/utils/build.py b/docker/utils/build.py
index a5c4b0c2..81a19225 100644
--- a/docker/utils/build.py
+++ b/docker/utils/build.py
@@ -10,8 +10,9 @@ from ..constants import IS_WINDOWS_PLATFORM
 
 _SEP = re.compile('/|\\\\') if IS_WINDOWS_PLATFORM else re.compile('/')
 _TAG = re.compile(
-    r"^[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*(\/[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*)*" \
-        + "(:[a-zA-Z0-9_][a-zA-Z0-9._-]{0,127})?$"
+    r"^[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*" \
+    + "(?::[0-9]+)?(/[a-z0-9]+((\.|_|__|-+)[a-z0-9]+)*)*" \
+    + "(:[a-zA-Z0-9_][a-zA-Z0-9._-]{0,127})?$"
 )