Bug: Changed doc-comment containing backslashes

[Julow]

Describe the bug

ocamlformat: Cannot process "src/lib/eliom_parameter.server.mli".
  Please report this bug at https://github.com/ocaml-ppx/ocamlformat/issues.
  BUG: comment changed.
File "_none_", line 1:
Error: formatting of doc-comment is unstable.
  before: (**  [regexp (Re.Pcre.regexp "\\[(.* )\\]")]  *)
   after: (**  [regexp (Re.Pcre.regexp "\[(.* )\]")]  *)

How to Reproduce

A small reproduction is:

(** [regexp (Re.Pcre.regexp "\\[(.* )\\]")] *)