I'm guessing now ...

Here a "naive" approach by embedding Perl code.

When parsing a recursive syntax you are able to put a Perl code after each match pattern using (?{...}) and $^N will give you the content of this last match.

Compare this example from perlre

      $_ = "The brown fox jumps over the lazy dog";
      /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
      print "color = $color, animal = $animal\n";
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instead of storing the match you might concatenate it to a "result" string or print it to a channel

> and I want to change all identifiers to upper case in the text

depending on match pattern execute a uc before returning the result.