Conditional Data Validation through List

[caabdul]

Hello experts,

I have applied data validation through list in excel 2013.
That list contains some cells in which text does not start with two minus signs i.e. "--".

I want my data validation to expand in such a way that only cells started with "--" are picked and other are not allowed.

EXAMPLE
=======
List
A1 [Services]
A2 [--Teaching]
A3 [Jobs]
A4 [--Management]

I want to allow only A2 and A4 type of data (that starts with "--") through data validation.

Please note that drop down list is a must have.

I hope my situation is well described, however anything else you want to know, please let me know.

Thanks to all!

 

[caabdul]

Somebody help please!