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The only difference between the two regexps is that the email/header.py
version looks for::
 (?=[ \t]|$) # whitespace or the end of the string
at the end (with re.MULTILINE, so $ also matches '\n').
To expand on "There is nothing about that thing in RFC 2047", it says::
 IMPORTANT: 'encoded-word's are designed to be recognized as 'atom's
 by an RFC 822 parser.
RFC 822 says::
 atom = 1*<any CHAR except specials, SPACE and CTLs>
 ...
 specials = "(" / ")" / "<" / ">" / "@" ; Must be in quoted-
 / "," / ";" / ":" / "\" / <"> ; string, to use
 / "." / "[" / "]" ; within a word.
So an example of mis-parsing is::
 >>> import email.header
 >>> h = '=?utf-8?q?=E2=98=BA?=(unicode white smiling face)'
 >>> email.header.decode_header(h)
 [('=?utf-8?q?=E2=98=BA?=(unicode white smiling face)', None)]
The correct result would be::
 >>> email.header.decode_header(h)
 [('\xe2\x98\xba', 'utf-8'), ('(unicode white smiling face)', None)]
which is what you get if you insert a space before the '(' in h.