mboxgrep is a small utility that scans a mailbox for messages matching a
regular expression. Found messages can be either displayed on standard output,
counted, deleted, piped to a shell command or written to another mailbox.

Its features include:

* ability to limit the search to message body or headers (although the whole
message is scanned by default)
* message counting
* ability to invert the sense of matching
* ability to write found messages to another mailbox
* support for mbox (either plain or compressed), MH, nnmh, nnml and maildir
folders
* support for basic and extended POSIX regular expressions, and, optionally,
Perl-complatible regular expressions (if linked with the PCRE library)

WWW: http://mboxgrep.sourceforge.net/