4.25. How do I join two lines if line #1 ends in a [certain string]?

This question appears in the section on one-line sed scripts, but it comes up so many times that it needs a place here also. Suppose a line ends with a particular string (often, a line ends with a backslash). How do you bring up the second line after it, even in cases where several consecutive lines all end in a backslash?

     sed -e :a -e '/\\$/N; s/\\\n//; ta' file   # all seds
     sed ':a; /\\$/N; s/\\\n//; ta' file        # GNU sed, ssed, HHsed

Note that this replaces the backslash-newline with nothing. You may want to replace the backslash-newline with a single space instead.