regexp: try again to fix the issue with backslashes.

regexp: try again to fix the issue with backslashes.

the new simple rule is inside R/ syntax all backslashes should be
escaped to allow easy regexp literal tokenization:

R/ [\/]/

but in the constructor <regexp> they shouldn't be:

"[/]" <regexp>

When the regexp is prettyprinted we make sure to visually escape the
backslashes in the raw regexp.