dix-yas.el

(defvar-local dix-yas-key-rex ""
  "Used by `dix-yas-update-key-rex' for caching the regex-opt of
  possible snippet keys.")
(defvar-local dix-yas-key-rex-tables nil
  "Used by `dix-yas-update-key-rex' for checking if we need to
  update `dix-yas-key-rex'.")

(defun dix-yas-update-key-rex ()
  "Update `dix-yas-key-rex', used by `dix-yas-skip-backwards-to-key'."
  (let ((tables (yas--get-snippet-tables)))
    (unless (equal tables dix-yas-key-rex-tables)
      ;; Only update key-rex if tables changed (but is this equal test slow?):
      (setq dix-yas-key-rex-tables tables)
      (setq dix-yas-key-rex
	    (let (keys) (mapc
			 (lambda (table) 
			   (let* ((keyhash (yas--table-hash table)))
			     (when keyhash
			       (maphash (lambda (k v) (push k keys)) keyhash))))
			 dix-yas-key-rex-tables)
		 (concat (regexp-opt keys) "$"))))))

(defun dix-yas-skip-backwards-to-key ()
  "Skip backwards to the first possible yasnippet key. 

This is meant to be used in `yas-key-syntaxes', since the
defaults don't let you expand e.g. \"<s>\" without having
whitespace before it. To use this function, put the following in
your init file:

    (eval-after-load 'yasnippet
      '(add-to-list 'yas-key-syntaxes 'dix-yas-skip-backwards-to-key))

Only has an effect in `dix-mode' so the above shouldn't change
how yasnippet expansion works in other modes."
  (when dix-mode
    (dix-yas-update-key-rex)
    (let ((haystack (buffer-substring-no-properties (line-beginning-position) (point))))
      (when (string-match dix-yas-key-rex haystack)
	(goto-char (+ (line-beginning-position) (match-beginning 0)))))))

;; Only update key-rex if tables changed (but is this equal test slow?):

I think equal is equivalent to eq for hashtables, so it will be fast, but it won't detect changes to the tables themselves (only changes to the list of active tables).