Good idea. It reminds me of a similar idea that is at the heart of many parsers. You make your regex of the form (things to be left alone)|(things to mangle), for example:

s{
    ( [(] [^()]* [)] ) # Leave text in parens alone
  | <(\w+)>            # Replace <word> with $replace{word}
}{
    $1 ? $1 : $replace($2)
}gex;
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