# Ruby, 44 characters

<!-- language: lang-ruby -->

    ->a,b{i=0;i+=1while a[i]&&a[i]==b[i];a[0,i]}

Sample run:

    2.1.5 :001 > ->a,b{i=0;i+=1while a[i]&&a[i]==b[i];a[0,i]}["global , "glossary"]
     => "glo"

    2.1.5 :002 > ->a,b{i=0;i+=1while a[i]&&a[i]==b[i];a[0,i]}["department", "depart"]
     => "depart"

    2.1.5 :003 > ->a,b{i=0;i+=1while a[i]&&a[i]==b[i];a[0,i]}["glove", "dove"]
     => ""