bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Gregory Heytings]

> I don't think it's this regexp which causes the overflow.

... but with a larger buffer it does, so apparently the regexp is 
problematic after all:

(with-current-buffer (get-buffer-create "*bug*")
  (let ((regexp "\\(\\(?:\\(xmlns\\)\\|[_[:alpha:]][-._[:alnum:]]*\\)\\(:[_[:alpha:]][-._[:alnum:]]*\\)?\\)[ \r\t\n]*=\\(?:[ 
\r\t\n]*\\('[^<'&\r\n\t]*\\([&\r\n\t][^<']*\\)?'\\|\"[^<\"&\r\n\t]*\\([&\r\n\t][^<\"]*\\)?\"\\)\\(?:\\([
 \r\t\n]*>\\)\\|\\(?:\\([ \r\t\n]*/\\)\\(>\\)?\\)\\|\\([ \r\t\n]+\\)\\)\\)?"))
    (erase-buffer)
    (insert "<id name=\"")
    (insert (make-string  266659 ?n))
    (goto-char 5)
    (looking-at regexp)))

Here 266658 does not overflow, and 266659 does.  If '=' is removed from 
the regexp it doesn't overflow anymore, even with a much larger string.

Trying to simplify the regexp gradually, I finally obtained the following 
minimal test case:

(with-current-buffer (get-buffer-create "*bug*")
  (let ((regexp "[[:alpha:]]*=\".*&.*\""))
    (erase-buffer)
    (insert "<id name=\"")
    (insert (make-string  266666 ?n))
    (goto-char 5)
    (looking-at regexp)))

Here it fails with 266666, and doesn't with 266665.