Recursive parentheses in syntax

I'm writing a syntax file for a string template DSL (somewhat similar to Bash parameter expansion) which distinguishes between (a) `$(...)` (call it "placeholder") and (b) `(...)` (call it "parens") where placeholder is preferably an Operator and parens is preferably a String where the parentheses themselves are Special, and where placeholders can occur inside non-highlighted text and both can be nested inside (certain parts of) placeholders and inside parens which are inside a placeholder.

I know that Vim regex do not support recursive patterns but I'm pretty confident that this can be pulled off in a syntax definition using regions which contain other regions, but I'm not quite sure if/how. I'm wondering if someone can tell or point me to a relevant example. In particular I'm not sure how/if you can nest a region with some delimiters inside another region with the same delimiters without failing to match the outer closing delimiter or matching past it.

In Perl 5.10+ it corresponds to a regex like this (in lieu of a formal syntax), with the `\(` and `\)` which are part of the syntax replaced with `\{` and `\}` for clarity, and each `$foo` representing some more or less complicated subexpression whose exact shape is (I think) irrelevant to the recursion issue.  Thus this is not the actual regex used to match a placeholder, but it is like an MWE which illustrates how it is supposed to work.

``````perl

my $placeholder_re = qr{

  (?<placeholder>

    \$\{

      $varname_re

      (?:

        $operator_A_re

        (?<parens>

          \s*

          \{

            (?<content>

              (?:

                [^\{\}\$]++

              | \$ (?! \{ ) 

              | (?&placeholder)

              | (?&parens)

              )*?

            )

          \}

          \s*

        )

        $operator_B_re

        (?<parens_B> (?&parens) )

      |

        $some_operator_re

        (?<content_B> (?&parens) | (?&content) )

      )

    \}

  )

}msx;

``````

    

TIA,

/bpj

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