Shopify/liquid ? reverse-engineered prompt

I want to build a secure, customer-facing template language in Ruby. The main goal is to let non-technical users customize the look of their web pages or emails within my app without being able to run dangerous, insecure code on my server. It needs to be totally sandboxed.

The syntax should be simple and elegant, using double curly braces like {{ product.name }} for outputting variables and percent-braces like {% for item in collection %} for basic logic like loops. Think of it like the kind of templating system Shopify uses for its themes. It needs to be stateless, so I can parse a template one time and then efficiently render it over and over with different data. It should also handle cases where variables are missing without crashing, and maybe have different strictness modes for parsing to help with debugging.