non-a-slash:    # string that doesn't contain a non escaped slash
SUBSTITION:     non-eval-substition | eval-substitution
non-eval-substitution:
               's' '/' not-a-slash '/' not-a-slash '/' non-e-modifiers(*)
eval-substitution:
               's' '/' not-a-slash '/' slashless-code '/' e-modifiers(*)
non-e-modifiers: 'x' | 's' | 'm' | 'g' | 'x' | 'i'
e-modifiers:     'e' | non-e-modifiers