"(??{ code })"

  WARNING: This extended regular expression
  feature is considered highly experimental, and
  may be changed or deleted without notice.  A
  simplified version of the syntax may be
  introduced for commonly used idioms.
  
  This is a "postponed" regular subexpression.
  The "code" is evaluated at run time, at the
  moment this subexpression may match.  The result
  of evaluation is considered as a regular
  expression and matched as if it were inserted
  instead of this construct.
  
  The "code" is not interpolated.  As before, the
  rules to determine where the "code" ends are
  currently somewhat convoluted.
  
  The following pattern matches a parenthesized
  group:
 
 
 
   $re = qr{
              \(
              (?:
                 (?> [^()]+ )    # Non-parens without backtracking
               |
                 (??{ $re })     # Group with matching parens
              )*
              \)
           }x;
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