mu4e: handle conversion from pcre regexps

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.

1 files changed, 9 insertions, 0 deletions

diff --git a/man/mu-init.1.org b/man/mu-init.1.org
index d790f2f..b98cdb5 100644
--- a/man/mu-init.1.org
+++ b/man/mu-init.1.org
@@ -39,6 +39,15 @@ _email-address-or-regex_ can be either a plain e-mail address (such as
 for details), wrapped in */* (such as =/foo-.*@example\\.com/=). Depending on your
 shell, the argument may need to be quoted.
 
+A note of warning for *mu4e* users: any regular-expressions used with
+=--with-address= are also used by *mu4e*. Unfortunately, PCRE regular expressions
+are *not* generally compatible with Emacs regular expressions. For instance,
+*(foo|bar)* in PCRE syntax has *\(foo\|bar\)* as its Emacs equivalent.
+
+The good news is that *mu4e* can automatically translate the regular-expressions,
+if you allow it to, by installing the *pcre2el* Elisp package. See the *mu4e*
+documentation for further details.
+
 ** --ignored-address _email-address-or-regex_
 Specifies that some e-mail address is to be ignored from the contacts-cache (the
 option can be used multiple times). Such addresses then cannot be found with