ORA-FO-STARTINGATZERO 15.1.6

*Functions and Operators, Section 15.1.6*
"If the value of $posParam is ... equal to zero (0), then an error is 
raised ".
This is inconsistent with the way the substring functions work.

Suggest: each of the string functions should define what they do with 
zero and negative values for position and length, and they should be 
consistent.