While GNU APL has, as its primary focus, adherence to the APL2 ISO 13751 standard, it is open to enhancements which do not violate compatibility with APL/2.

So, as of SVN 1847M:

              s ← 'Mississippi'
          ≠s    
    1 1 1 0 0 0 0 0 1 0 0
          (≠s)/s
    Misp