Yes it is underspecified. The deletion example shows that an empty string is possibly a REGEX. So you can essentially treat any position as containing as many empty string regexes as you want. So there are indeed infinite number of parses.
If we instead require regex to be non-empty (breaking the deletion examples), then the ambiguity becomes that of concatenation: whether it's '(((c:d)(a:o))(t:g))' or '((c:d)((a:o)(d:g)))'. Assuming associativity, this would not matter.
If we instead require regex to be non-empty (breaking the deletion examples), then the ambiguity becomes that of concatenation: whether it's '(((c:d)(a:o))(t:g))' or '((c:d)((a:o)(d:g)))'. Assuming associativity, this would not matter.