/
   (?(?<=(.))
      # We are not at the start of the string.
      # The preceding character is in $1.
      hello
      (?(?{ $1 eq "\{" })
         # The char before 'hello' is '{'.
         }
      |
         # The char before 'hello' is not '{'.
         (?! } )
      )
   |
      # We are at the start of the string.
      hello
      (?! } )
   )
/x