my $g_nested_brackets;
$g_nested_brackets = qr{
    (?>                             # Atomic matching
       [^\[\]]+                     # Anything other than brackets
     | 
       \[
         (??{ $g_nested_brackets }) # Recursive set of nested brackets
       \]
    )*
}x;
my $anchors = qr{
    (                # wrap whole match in $1
      \[
        ($g_nested_brackets)    # link text = $2
      \]
      \(            # literal paren
        [ \t]*
        <?(.*?)>?   # href = $3
        [ \t]*
        (           # $4
          (['"])    # quote char = $5
          (.*?)     # Title = $6
          \5        # matching quote
        )?          # title is optional
      \)
    )
}xs;