Read rex. stats command doesn't have a function to do extraction.
Meanwhile, your sample code suggests that Splunk gives you a field named Subject and you are trying to get some info from this field. If this is the case, there is a slightly more efficient way using split function:
my search
| Subject = mvindex(split(Subject, "|"), 0)
| stats count by Subject
Another way equivalent to rex is to use replace function.
