Sometimes I want to try a pattern with / before doing :%s/ …; can I reuse it without having to type it all over? (or use the mouse.)
Sure, just leave the pattern in the substitution empty:
:%s//replacement/
In addition to @garyjohn's answer, you can use <c-r>/ to insert the search register into a command if you want to edit it or leaving out the search pattern isn't sufficient.
:%s/<c-r>//replacement
Another possible solution
:exe '%s/' . @/ . '/replacement/g'
