'Use regexp inSQL/PL for no repetitions in digits

I have a table below and need to use a regular expression to display the correct course digits ( 3 digits being the max # of digits) and also max sure no number appears twice.

enter image description here

the following code I have created is this:

select regexp_substr(course_number,'([0-9][0-9][0-9])\1') as CourseNumber

from courses /

this is correct showing me an output of this:

enter image description here

How can I sure the regexp in sql/pl to query the digits with no numbers repeating?

the table I created looks like this:

enter image description here

sqlsqlplusregexp-substrregexp-likexregexp


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