Sorry I didn't mean to suggest this approach. In general you cannot put search related operators into a constraint. In fact I think we should replace any instances of bind* and bind in constraint logic as they really are a part of the search infrastructure and I'd like them to become implementation details of a particular search approach.

Instead I meant to suggest that the disjunction expressed by your example

(run 1 [re]
  (forall [s o]
    (conde
      [(== s '(foo bar bar bar))
       (== o regex-BLANK)]
      [(!= s '(foo bar bar bar))
       (== o regex-NULL)])
    (regex-matcho re s o)))

Is perhaps best described as an explicit constraint something more like:

(defn regexc [s o]
  (reify
    IFn
    (invoke [c a]
      (let [s (walk* a)[
        (if (= s '(foo bar bar bar))
          ((== o regex-BLANK) a)
          ((== o regex-NULL) a)))))
    ...
    IRunnable
    (runnable? [c a]
      (ground-terms? x))
    ...))

Of course someone might want to write constraint like this quite often, then I think it's question of what the best way to support that might be.

No plans to add support for search operators in constraints in the near future.