[Exim] Need a little filter syntax help - lower case

Here's part of a filter expression I'm working on - it just shows the
string I'm working with to do a substitution:

${sg{$h_subject:${substr_0_100:$message_body}}\

but - what I really want is the low case of the string

$h_subject:${substr_0_100:$message_body}

What is the syntax for doing that?