Don’t know how to write a specific regular expression

I have a string like this:

“56×74 x567 x7 7889 x97x89xx”.

I need to leave everything except the letter x, which has a left empty space involved. It has to be done by the Match method.
I was trying to do it, but it has not worked at all.
Please, help me.
The result should look like this:

“56×745677 788997x89xx”.