Mirror vertex selection

Let's say I have selected some vertexes:

How do I mirror this selection (in x axis) so I can select the other side (same vertices assuming my mesh is symmetric) I just want to select the other side not do any operations.

• @Arijoon Extend option is in Last Operator panel, either open Toolshelf or press F6. Although it should work without that the option - it will only add mirrored selection to your original one. Check that you don't use Mirror modifier – Mr Zak Sep 26 '16 at 9:39