Serverless functions uses MVFLEX Expression Language (MVEL) (https://en.wikipedia.org/wiki/MVEL). MVEL has largely been inspired by Java syntax, but has some fundamental differences aimed at making it more efficient as an expression language, such as operators that directly support collection, array and string matching, as well as regular expressions. MVEL is used to evaluate expressions written using Java syntax.
Please use this documentation for the functions development - http://mvel.documentnode.com/
Sample Scripts: https://github.com/mvel/mvel/tree/master/samples/scripts