How to check if a string parameter can be an IPv4 or an IPv6 or nothing in an iRule ?

string matching with ip addresses is a bit wonky but you could simply do this:

if { $string equals "" } {
# Empty string
} elseif { $string contains "." } {
# IPv4
} elseif { $string contains ":" } {
# IPv6
} else {
# error
}