SNMP text values contain surrounding quotes when processed with *.regexp()

I use a global regular expression in a trigger for an SNMP item (example value: "54 C"):

=0

The regexp only works if I include the surrounding quotes:
This works: ^"[1-5][0-9] C"$
This doesn't: ^[1-5][0-9] C$

I guess the intended behaviour should be to strip off the quotes from the SNMP text values before they are further processed by Zabbix.