Generally you just try to find something that matches any of the possible prompts. The Expect documentation uses qr'[#>:] $'. When dealing with interactive shells, I usually either set the prompt myself to something that is easier to match on, or (more commonly) wrap the command being sent to the other side in something easily identifiable such as:

# Instead of this:
# $exp->send( "hostname\n" );
# $exp->expect( $timeout, $shell_prompt );
#
# Try something like this
$exp->send( "echo START_COMMAND; hostname; echo END_COMMAND" );
$exp->expect( $timout,
    [ qr/START_COMMAND\n(.*)END_COMMAND.*?$shell_prompt/,
      sub {}, # do whatever with the data...
    ],
);
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'/[\$%#>] $/'
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