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The observed behavior is due to the Internet Explorer's rich text editing engine that the RadEditor's content area uses to provide rich text capabilities. When Internet Explorer finds a string starting with "www." it assumes that this is a URL and converts it to <a> tag automatically when the user presses the interval key.
The link autocomplete is offered as a built-in feature of the editable iframe/div elements by Internet Explorer only. The behavior is not supported by Firefox, Chrome/Safari and Opera.
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