TextMate snippets page says:

7.7 Transformations

There are situations where we want our placeholder text mirrored but with slight changes or where we want some text to appear depending on the value/presence of a placeholder.

We can accomplish this by doing a regular expression substitution on the placeholder text (when mirroring it). The syntax for this is: ${«tab stop»/«regexp»/«format»/«options»}.

It looks like transformations are not supported in VS Code. Would be a great addition.