diff --git a/src/encoding/xml/marshal.go b/src/encoding/xml/marshal.go
index d8a04a9..0554b0d 100644
--- a/src/encoding/xml/marshal.go
+++ b/src/encoding/xml/marshal.go
@@ -345,11 +345,8 @@
 	if prefix == "" || !isName([]byte(prefix)) || strings.Contains(prefix, ":") {
 		prefix = "_"
 	}
-	// xmlanything is reserved and any variant of it regardless of
-	// case should be matched, so:
-	//    (('X'|'x') ('M'|'m') ('L'|'l'))
-	// See Section 2.3 of https://www.w3.org/TR/REC-xml/
-	if len(prefix) >= 3 && strings.EqualFold(prefix[:3], "xml") {
+	if strings.HasPrefix(prefix, "xml") {
+		// xmlanything is reserved.
 		prefix = "_" + prefix
 	}
 	if p.attrNS[prefix] != "" {
diff --git a/src/encoding/xml/marshal_test.go b/src/encoding/xml/marshal_test.go
index d2e5137..31309ef 100644
--- a/src/encoding/xml/marshal_test.go
+++ b/src/encoding/xml/marshal_test.go
@@ -2283,30 +2283,6 @@
 		}},
 	},
 	want: `<foo xmlns="space"><bar xmlns="space" xmlns:space="space" space:attr="value">`,
-}, {
-	desc: "reserved namespace prefix -- all lower case",
-	toks: []Token{
-		StartElement{Name{"", "foo"}, []Attr{
-			{Name{"http://www.w3.org/2001/xmlSchema-instance", "nil"}, "true"},
-		}},
-	},
-	want: `<foo xmlns:_xmlSchema-instance="http://www.w3.org/2001/xmlSchema-instance" _xmlSchema-instance:nil="true">`,
-}, {
-	desc: "reserved namespace prefix -- all upper case",
-	toks: []Token{
-		StartElement{Name{"", "foo"}, []Attr{
-			{Name{"http://www.w3.org/2001/XMLSchema-instance", "nil"}, "true"},
-		}},
-	},
-	want: `<foo xmlns:_XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" _XMLSchema-instance:nil="true">`,
-}, {
-	desc: "reserved namespace prefix -- all mixed case",
-	toks: []Token{
-		StartElement{Name{"", "foo"}, []Attr{
-			{Name{"http://www.w3.org/2001/XmLSchema-instance", "nil"}, "true"},
-		}},
-	},
-	want: `<foo xmlns:_XmLSchema-instance="http://www.w3.org/2001/XmLSchema-instance" _XmLSchema-instance:nil="true">`,
 }}
 
 func TestEncodeToken(t *testing.T) {