Fix regex for restoring quotes in plain text construction

The character following the token is found through a look-ahead, to
avoid consuming the opening of the next token.  (There should be
newlines separating tokens, but in some poorly understood circumstances,
this doesn't happen.)  We don't use a look-behind at the beginning, so
that the second consecutive token doesn't insert a second newline
between the two.  (The first of the pair already gave us one.)

Closes: bgo #723742