a less stringent approach could be this: You set up a sequence for the incrementing numeric portion, then in the default value for the text field concatenate 'Emp' to the sequence nextvalue.
This does not enforce the format, nor does it require that the sequence be used. But, if you don't supply a pk value when you insert, pg will provide it for you.
HOWEVER, I agree with everyone else that you would be better served by using only a serial(sequence) for the key field, and provide a view containing 'Emp' concatenated with the key field for all users/reports.
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if you are happy with varchar daat type instead of text datatype then i can provide you the solution. it can be done by auto computing formula. it will automatically count and give you the required result and automaticall it will take your column as varchar datatype by default.
if you are fine with varchar datatype, let me know. i will provide you the solution.