Empty string is not equivalent to :VOID, signals error in CREATE-SCANNER in some cases

[tshatrov]

I often have to generate regexes dynamically. For this the tree syntax of CL-PPCRE is the most optimal solution. However I have discovered a strange corner case. In the documentation it says that

The symbol :VOID is equivalent to the empty string.

However in practice the empty string "" and :VOID have different behavior in some cases.

Consider two parse trees: (:sequence "" "a") and (:sequence :void "a"). Logically both are supposed to match "a". However

(ppcre:create-scanner '(:sequence "" "a"))

results in an error:

Invalid index 0 for (SIMPLE-ARRAY CHARACTER (0)), should be a non-negative integer below 0.
   [Condition of type SB-INT:INVALID-ARRAY-INDEX-ERROR]

The version with :void works just fine. Also if the empty string is not the first item of a sequence, it also works.

However if :sequence is replaced by :alternation, empty string causes an error in any position!

This behavior makes it harder to create dynamic regex trees, since I need to check if a string is empty and replace it with :void to avoid the error.