Regular expression to select symbol with only digits in word

Can't create a regular expression that would select the "X" symbol that is between digits and a word does not contain other letters or "X" is on the first or last place of a word. Examples:

1. test123X123test - should not find X because there are letters in word
2. test 123X123 test - should find X because there is no letters (except X) in word
3. 123X123 test - should find X
4. test 123x123 - should find X
5. 123X - should not find X because X is on the last place
6. X123 - should not find X because X is on the first place

I was wondering if anyone could help me. Btw I use ruby, so the regular expression must work with it.