Parsec document says the following about <|>
This combinator implements choice. The parser p <|> q first applies p. If it succeeds, the value of p is returned. If p fails without consuming any input, parser q is tried. This combinator is defined equal to the mplus member of the MonadPlus class and the (Control.Applicative.<|>) member of Control.Applicative.Alternative.
How to implement back-tracking version of <|>? It backtracks when p consumes any-input such the q can be applied to full input again.
To enable backtracking, you simply need to use try, e.g. try p <|> q.
However, note that excessive use of try can hurt the performance of your parser. Consider left factoring your grammar instead to eliminate common prefixes where possible.