You already got some ansers, but I can't resist to chime in :-)

You haven't exactly told us what you want to achieve. It looks to me like what you want is not necessarily RegExes, but a kind of template toolkit / templating engine.

Anyway, if you want to follow your initial approach (even if for learning purposes), you should at least read through

• How can I use a variable as a variable name? Update: link fixed
• Mark-Jason Dominus' "variable as variable name" text from 1998 (still relevant today) Part 1, Part 2, Part 3

These texts are Perl centered, but contain basics that are valid for any programming language.