Fix validation for serivce bus topic name

azurerm/helpers/azure/servicebus.go

@@ -28,8 +28,8 @@ func ValidateServiceBusSubscriptionName() schema.SchemaValidateFunc {
 
 func ValidateServiceBusTopicName() schema.SchemaValidateFunc {
 	return validation.StringMatch(
-		regexp.MustCompile("^[a-zA-Z][-._~a-zA-Z0-9]{0,258}([a-zA-Z0-9])?$"),
-		"The topic name can contain only letters, numbers, periods, hyphens, tildas and underscores. The namespace must start with a letter, and it must end with a letter or number and be less then 260 characters long.",
+		regexp.MustCompile("^[a-zA-Z0-9]([-._~a-zA-Z0-9]{0,258}[a-zA-Z0-9])?$"),
+		"The topic name can contain only letters, numbers, periods, hyphens, tildas and underscores. The namespace must start with a letter or number, and it must end with a letter or number and be less then 260 characters long.",
 	)
 }
 

azurerm/helpers/azure/servicebus_test.go

@@ -0,0 +1,76 @@
+package azure
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestValidateServiceBusTopicName(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		valid bool
+	}{
+		{
+			name:  "Empty value",
+			input: "",
+			valid: false,
+		},
+		{
+			name:  "Invalid name with only 1 letter",
+			input: "a",
+			valid: true,
+		},
+		{
+			name:  "Invalid name starts with underscore",
+			input: "_a",
+			valid: false,
+		},
+		{
+			name:  "Invalid name ends with period",
+			input: "a.",
+			valid: false,
+		},
+		{
+			name:  "Valid name with numbers",
+			input: "12345",
+			valid: true,
+		},
+		{
+			name:  "Valid name with only 1 number",
+			input: "1",
+			valid: true,
+		},
+		{
+			name:  "Valid name with hyphens",
+			input: "malcolm-in-the-middle",
+			valid: true,
+		},
+		{
+			name:  "Valid name with 259 characters",
+			input: strings.Repeat("w", 259),
+			valid: true,
+		},
+		{
+			name:  "Valid name with 260 characters",
+			input: strings.Repeat("w", 260),
+			valid: true,
+		},
+		{
+			name:  "Invalid name with 261 characters",
+			input: strings.Repeat("w", 261),
+			valid: false,
+		},
+	}
+
+	var validationFunction = ValidateServiceBusTopicName()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := validationFunction(tt.input, "name")
+			valid := err == nil
+			if valid != tt.valid {
+				t.Errorf("Expected valid status %t but got %t for input %s", tt.valid, valid, tt.input)
+			}
+		})
+	}
+}