The syntax for [?prompt] is
[?<prompt>;<mask>;<default value>;<flags>;<keycodes>]
<mask> part evaluates as a regular expression. As the error message says there is a syntax error in regular expression, probably part of your CALL result reads as mask.
Edit: Only the first item of the list can be selected as default. To do that you can start value list with a * character like *Option1|Option2|Option3. Selecting values other than first value by default not implemented yet.