ERR 102 mismatched input 'str' in rule when using DrlParser

I'm trying to use the example for the operator str[startsWith] according to the Drools documentation:

https://docs.jboss.org/drools/release/7.73.0.Final/drools-docs/html_single/#drl-operators-ref_drl-rules

But when I run the DrlParser on this simple example:

rule "message rule"
when
     Message( routingValue str[startsWith] "R1" )
then
end

I get the error:

[3,27]: [ERR 102] Line 3:27 mismatched input 'str' in rule "message rule"

I'm not sure why this happens as it works when using the rule in the KB.