get PG sql funnctions name from given query

Question

our goal is to get pg sql inbuilt functions present in a query given by an user.

TablesNamesFinder tablesNamesFinder = new TablesNamesFinder(){
@Override
public void visit(Function function) {
    functionNames.add(function.getName().toLowerCase());
    super.visit(function);
}
@Override
public void visit(TableFunction valuesList) {
    functionNames.add(valuesList.getFunction().getName().toLowerCase());
    super.visit(valuesList);
}
};

We have tried the above code but this will not give functions like

select current_user;
select current_schema;

SO kindly help us to get all functions in given sql query