gist.el

(defun yaml-company-backend (command &optional arg &rest ignored)
  "A very simple backend for completion of anchors in YAML mode."
  (interactive (list 'interactive))
  (cl-case command
    (interactive (company-begin-backend 'drdv-yaml-company-backend))
    ;; Dispatch to handle two cases:
    ;;  - words that starts with "*" (anchor suggestions)
    ;;  - empty space (job suggestions)
    (prefix (and (eq major-mode 'yaml-mode)
		 (cond ((looking-back "\\*.*") (match-string 0))
		       ((looking-back "") ""))))
    (sorted t)
    (candidates
     (cond ((string= arg "")
	    (list "DEVELOPER" "MANAGER" "ADMIN" "COMMERCIAL"))
	   (t (cl-remove-if-not
	       (lambda (c) (string-prefix-p arg c))
	       ((lambda (regexp string)
		  ;; https://emacs.stackexchange.com/a/7150
		  (save-match-data
		    (let ((pos 0) matches)
		      (while (string-match regexp string pos)
			(push (concat "*" (match-string 1 string)) matches)
			(setq pos (match-end 0)))
		      matches))) "&\\(.*\\)" (buffer-string))))))))

init.el

(use-package yaml-mode
  :ensure t
  :bind (("<backtab>" . company-complete))
  :config
  (use-package company)
  (add-hook 'yaml-mode-hook 'company-mode)
  (add-to-list 'company-backends 'yaml-company-backend))