[nene]

It's not really automatic interpolation. You have to very explicitly place the variable name inside ${var}. I'm puzzled in how you would think this will happen accidentally, or more accidentally than writing "+var+".

I've used Ruby quite a while and never experienced any problems with the very same construct in there.

[FuzzyDunlop]

You can also get away with doing just "$var", though, which is the problem. We encountered a strange bug the other day where having a string containing "$3" meant it never appeared on the page after (with it being converted to null).