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As Greg showed, you always need nested square brackets when you use character classes like [:alpha:]; the inner set is the syntax for the character class, but it only has meaning when you're defining a set of characters, which is indicated by the outer set.
Also, you can simply skip the 3rd argument to REGEXP_REPLACE whenever you're changing something to nothing:
SELECT REGEXP_REPLACE ( '305N'
) AS num
Thanks Greg and Frankulash.
Great it did work and came to know much about this function. Thanks a lot for helping me.
Frankulash, Your explanation is great.
Thanks once again.