mu server uses PCRE-compatible regular expressions, but mu4e needs
Emacs-style. In simple cases, the two are the same, but when it gets
slightly more complicated they do not.
E.g.,
(foo|bar) => \(foo\|bar\)
Mu4e can do the conversion automatically, but for this the pcre2el
package (available in MELPA) is required, and is used if user installs
is. Anyway, this code implements the automatic conversion and adds some
documentation.
Fixes#2816.
3dbd8b9763