Ok, I've updated after more reading--also, I've switched to the "ends_with" operator. Will this work?
when HTTP_REQUEST {
Check for given string and redirect if found
if {[HTTP::uri] ends_with "alumni/giving/gift/"}{
HTTP::respond 301 Location "https://[getfield [HTTP::host] : 1][HTTP::uri]"
} elseif { [HTTP::uri] ends_with "alumni/giving/pledge/" } {
HTTP::respond 301 Location "https://[getfield [HTTP::host] : 1][HTTP::uri]"
} elseif { [HTTP::uri] ends_with "alumni/directory/search.aspx" } {
HTTP::respond 301 Location "https://[getfield [HTTP::host] : 1][HTTP::uri]"
} elseif { [HTTP::uri] ends_with "alumni/directory/update.aspx" } {
HTTP::respond 301 Location "https://[getfield [HTTP::host] : 1][HTTP::uri]"
}