Match empty string at begin/end of symbol

[Joe Riel]

The regular expressions '\_<' and '\_>'
seem to be broken in Emacs 25.1.1.  Consider

(let ((str "3+ab"))
  (and (string-match "\\<[a-zA-Z][a-zA-Z0-9]*" str)
       (match-string 0 str)))
 
That returns "ab", as expected.  Change the "\\<" to "\\_<"
and it no longer matches.  Why not?

(let ((str "3+ab"))
  (and (string-match "\\_<[a-zA-Z][a-zA-Z0-9]*" str)
       (match-string 0 str)))

-- 
Joe Riel