Match against 'root: email'
               01234567890

1) ok!                                                 pos = 0. (zw)
2) ok!   Found a 'root:' at pos 0 through 4.           pos = 5.
3) ok!   Found 1 '\s' at pos 5.                        pos = 6.
4) fail! Found 'email' at pos 6 through 10.            Backtrack!
3) ok!   Found 0 '\s' at pos 5.                        pos = 5. (zw)
4) ok!   Found something other than 'email' at pos 5.  pos = 5. (zwla)
         (found ' email')
5) fail! Did not find a '\S' at pos 5.                 Backtrack!
Nothing more to try.
No match!