One thing you can do without writing a script and using script triggers, is select the "Unique Value" under the Validation option for your "f_text" field in Manage Database.
Under this options tab you can also set the field to be validated always and not allow the user to override the validation. You can also set your custome message dialog, such as "sorry, this text already exists...", etc.
If "fun" already exists in this field, you can still enter "f", "fu", "n", and so on, as allowable entries. But when "fun" is entered again in another record, the validation will be triggered.
Hello Rdowler, Thanks for your sugestion. It is working,great.