'oh {hello there' =~ /
   ([{]{0,1})  # Matches '' (after some backtracking)
   hello       # Matches 'hello'
   (?(1)\})    # Matches ''
/x;

##</code><code>##

'oh hello} there' =~ /
   ([{]{0,1})  # Matches ''
   hello       # Matches 'hello'
   (?(1)\})    # Matches ''
/x;

##</code><code>##

/
   {hello}   # '{hello}'
|
   (?<! { )  # Not preceded by '{'
   hello     # 'hello'
   (?! } )   # Not followed by '}'
/x