Re: Match empty string at begin/end of symbol

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 4 Jul 2018 11:43:46 -0700
> From: Joe Riel <joer@san.rr.com>
> 
> 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)))

The result of the last form depends on the major mode of the buffer
where (or in whose minibuffer) you evaluate it.  If it's Lisp or its
derivatives, it indeed should not match because a Lisp symbol can
legitimately be named "3+ab", and so "ab" is not at a symbol
boundary.  But if you try the same in a buffer whose major mode is C
Mode, you surely get a match, because '+' is not a symbol-constituent
character in C.

IOW, I don't think there's a bug here.  It's behaving as intended.