Many of Mutt's commands allow you to specify a pattern to match (limit, tag-pattern, delete-pattern, etc.). There are several ways to select messages:
~A all messages
~b EXPR messages which contain EXPR in the message body
~B EXPR messages which contain EXPR in the whole message
~c USER messages carbon-copied to USER
~C EXPR message is either to: or cc: EXPR
~D deleted messages
~d [MIN]-[MAX] messages with ``date-sent'' in a Date range
~E expired messages
~e EXPR message which contains EXPR in the ``Sender'' field
~F flagged messages
~f USER messages originating from USER
~g cryptographically signed messages
~G cryptographically encrypted messages
~h EXPR messages which contain EXPR in the message header
~k message contains PGP key material
~i ID message which match ID in the ``Message-ID'' field
~L EXPR message is either originated or received by EXPR
~l message is addressed to a known mailing list
~m [MIN]-[MAX] message in the range MIN to MAX *)
~n [MIN]-[MAX] messages with a score in the range MIN to MAX *)
~N new messages
~O old messages
~p message is addressed to you (consults $alternates)
~P message is from you (consults $alternates)
~Q messages which have been replied to
~R read messages
~r [MIN]-[MAX] messages with ``date-received'' in a Date range
~S superseded messages
~s SUBJECT messages having SUBJECT in the ``Subject'' field.
~T tagged messages
~t USER messages addressed to USER
~U unread messages
~v message is part of a collapsed thread.
~V cryptographically verified messages
~x EXPR messages which contain EXPR in the `References' field
~y EXPR messages which contain EXPR in the `X-Label' field
~z [MIN]-[MAX] messages with a size in the range MIN to MAX *)
~= duplicated messages (see $duplicate_threads)
Where EXPR, USER, ID, and SUBJECT are regular expressions. Special attention has to be made when using regular expressions inside of patterns. Specifically, Mutt's parser for these patterns will strip one level of backslash (\), which is normally used for quoting. If it is your intention to use a backslash in the regular expression, you will need to use two backslashes instead (\\).
*) The forms <[MAX], >[MIN],
[MIN]- and -[MAX]
are allowed, too.