MrNobo1024's answer is unfortunately incorrect. It will match and replace the $fo part of $foo[5], for example.

You need to stop that by adding \w to the negative lookahead. s/(?<!\\)\$\w+(?![\[{\w])/whatever()/ge

I've replaced [a-zA-Z0-9_] with the more compact and equivalent \w, and tested for $bar{baz} as well.

This is still not perfect; it will munge $foo [5], which is valid (if rare) syntax. The easiest way to fix that is to use another regex, e.g. s/(\w) +([\[{])/$1$2/g

hdp.