can you think of a solution that uses the conditional notation

The algorithm:

• Note the preceding character (if any).
• Match 'hello'.
• If there was a character to note,
  • If the noted character is {,
    • Match }. ( Oops, I had the brace reversed. )
  • Else,
    • Negatively match }.
• Else,
  • Negatively match }.

As it turns out, the "if any" portion of the first step is hard to implement because look-behinds must be fixed-width. So let's make that conditional too:

• If we are at the start of the string,
  • Match 'hello'.
  • Negatively match }.
• Else,
  • Note the preceding character.
  • Match 'hello'.
  • If the noted character is {,
    • Match }. ( Oops, I had the brace reversed. )
  • Else,
    • Negatively match }.

/
   (?(?<=(.))
      # 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
[download]

Yikes!

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