I'm afraid RadExpressionEditor does not provide such functionality out of the box. However, you can download the source code of the control from your account and modify it to suit your needs.
You can also use the new TryParse extension method to parse a string expression programmatically, which you can then compile to a delegate and invoke manually.
RadExpressionEditor is contained in the Telerik.Windows.Controls.Expressions assembly, in the Telerik.Windows.Controls namespace. Please make sure you have the right references and namespace imports, as outlined in our documentation
All the best,
the Telerik team
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