I am dealing with a String,

```
30-Nov-2012 30-Nov-2012 United Kingdom, 31-Oct-2012 31-Oct-2012 United Arab Emirates, 29-Oct-2012 31-Oct-2012 India 

```

I need to add spaces every time a space appears after a four digit numbers, i.e.:

```
30-Nov-2012@30-Nov-2012@United Kingdom, 31-Oct-2012@31-Oct-2012@United Arab Emirates, 29-Oct-2012@31-Oct-2012@India 

```

I don't know how to write Regular Expressions, any help please?