All text patterns for searching and matching in Mutt must be specified as regular expressions (regexp) in the ``POSIX extended'' syntax (which is more or less the syntax used by egrep and GNU awk). For your convenience, we have included below a brief description of this syntax.
The search is case sensitive if the pattern contains at least one upper case letter, and case insensitive otherwise. Note that ``\'' must be quoted if used for a regular expression in an initialization command: ``\\''. For more information, see the section on searching below.