unfmt

unfmt is a compile-time pattern matching library that reverses the interpolation process of format!.

You can think of it as an extremely lightweight regular expression engine without the runtime pattern-compilation cost.

Installation

cargo add -D unfmt

Usage

let value = "My name is Rho.";

// Unnamed captures are returned as tuples.
assert_eq!(
    unformat!("My {} is {}.", value),
    Some(("name", "Rho"))
);

// You can put indices as well; just make sure ALL captures use indices
// otherwise it's not well defined.
assert_eq!(
    unformat!("My {1} is {0}.", value),
    Some(("Rho", "name"))
);

// You can also name captures using variables, but make sure you check the
// return is not None.
let mut subject = None;
let mut object = None;
assert_eq!(
    unformat!("My {subject} is {object}.", value),
    Some(())
);
assert_eq!((subject, object), (Some("name"), Some("Rho")));

// If you want to ensure the entire string matches, you can add `true` to the end of the macro.
assert_eq!(unformat!("{1} is {0}", value, true), None);

// If a type implements `FromStr`, you can use it as a type argument. This
// is written as `{:Type}`.
assert_eq!(
    unformat!("Listening on {:url::Url}", "Listening on http://localhost:3000"),
    Some((url::Url::from_str("http://localhost:3000").unwrap(),))
);

In general, captures are written as {<index-or-variable>:<type>}. Multiple captures in a row (i.e. {}{}) are not supported as they aren't well-defined.

Limitations

• There is no backtracking.