Possible sollution: delete or comment this line https://code.google.com/p/vim/source/browse/runtime/syntax/groovy.vim?r=22fa3049e9348a8ce556a3e5ba65c6f0a7aa6b4a#260
a slightly better (but incomplete) solution would be to change the line (260)
from
start='/[^/]'
to
start='/[^ /]'
this would treat something like (x / y) as simple division ... but would not treat / xyzz/ as a groovy reg expr string ... That is, groovy reg expr strings that do not start with a space are interpreted correctly, but ones that start with a space will not be highlighted