With anything less than a full-featured lexer and parser of the latest edition of the language in question, I'd say. Including understanding of whatever metaprogramming features or preprocessors that are in play.
It's even more than full-featured for a code editor, really, you really want a language parser that can more gracefully handle partial statements than even the core language parser itself can handle. von Neumann help you if your language was complicated to parse even before you added that requirement.
Visual Studio doesn't even do a full-blown AST - even though it has a full-blown internal parser. It frequently messes up auto-indenting when using macros involving blocks.
Lexing and producing tokens with a loop is probably enough given that auto-indentation works on a subset of states of the parser, if any. As my comment alluded to, using regex is harder to both write and read than a simple loop; it's also most likely slower.
