Try this:

    String s = ' this is a    multiple spaces sample   ';
    System.debug(s.replaceAll('\\s+','|'));

gives you this result: `DEBUG||this|is|a|multiple|spaces|sample|`

To replace ONLY when you have more than one space use this:

    System.debug(s.replaceAll('[ ]{2,}','|'));

this returns : 

    DEBUG| this is a|multiple spaces sample|
    12:08:18.174 (37342000)|CUMULATIVE_LIMIT_USAGE