Crate sscanf[][src]

A sscanf (inverse of format!()) Macro based on Regex


sscanf is a C-function that takes a String, a format String with placeholders and several Variables (in the Rust version replaced with Types). It then parses the input String, writing the values behind the placeholders into the Variables (the Rust version returns a Tuple of the specified Types). This process can be thought of as reversing a call to format!():

let s = format!("Hello {} #{}", "World", 5);
assert_eq!(s, "Hello World #5");

let parsed = sscanf::scanf!(s, "Hello {} #{}", String, usize);
// parsed is Option<(String, usize)>
assert_eq!(parsed, Some((String::from("World"), 5)));

As can be seen in the example, scanf takes a format String like format!(), but instead of writing the values from the remaining parameters into the {} it instead extracts the contents of the input string. Those parts are then parsed according to the specified Types and returned as a Tuple, or None if the parsing failed or the strings don’t match.

let s = "Random Text";
let parsed = sscanf::scanf!(s, "Hello {} #{}", String, usize);
assert_eq!(parsed, None); // "Random Text" and "Hello..." do not match

Note that the original C-function (and this Crate) are called sscanf, which is the correct version in this context. scanf is itself a C-function with the same functionality, but reading the input from stdin instead of taking a String parameter. The macro itself is called scanf because that is shorter, can be pronounced without sounding too weird and nobody uses the stdin version anyway.

More examples of the capabilities of scanf:

use sscanf::scanf;

let input = "4-5 t: ftttttrvts";
let parsed = scanf!(input, "{}-{} {}: {}", usize, usize, char, String);
assert_eq!(parsed, Some((4, 5, 't', String::from("ftttttrvts"))));

let input = "<x=3, y=-6, z=6>";
let parsed = scanf!(input, "<x={}, y={}, z={}>", i32, i32, i32);
assert_eq!(parsed, Some((3, -6, 6)));

let input = "Goto N36E21";
let parsed = scanf!(input, "Goto {}{}{}{}", char, usize, char, usize);
assert_eq!(parsed, Some(('N', 36, 'E', 21)));

let input = "A Sentence. Another Sentence. Yet more Words with Spaces.";
let parsed = scanf!(input, "{}. {}. {}.", String, String, String);
let (a, b, c) = parsed.unwrap();
assert_eq!(a, "A Sentence");
assert_eq!(b, "Another Sentence");
assert_eq!(c, "Yet more Words with Spaces");

The parsing part of this macro has very few limitations, since it replaces the {} with a Regular Expression (regex) that corresponds to that type. For example:

  • char is just one Character (regex ".")
  • String is any sequence of Characters (regex ".+")
  • Numbers are any sequence of digits (regex "\d+")

And so on. The actual implementation for numbers tries to take the size of the Type into account and some other details, but that is the gist of the parsing.

This means that any sequence of replacements is possible as long as the Regex finds a combination that works. In the char, usize, char, usize example above it manages to assign the N and E to the chars because they cannot be matched by the usizes. If the input were slightly different then it might have matched the 6 of the 36 or the 2 of the 21 to the second char.

Custom Types

scanf works with the most primitive Types from std as well as String by default. The full list can be seen here: Implementations of RegexRepresentation.

More Types can easily be added, as long as they implement FromStr for the parsing and RegexRepresentation for scanf to obtain the Regex of the Type:

struct TimeStamp {
    year: usize, month: u8, day: u8,
    hour: u8, minute: u8,
impl sscanf::RegexRepresentation for TimeStamp {
    /// Matches "[year-month-day hour:minute]"
    const REGEX: &'static str = r"\[\d\d\d\d-\d\d-\d\d \d\d:\d\d\]";
impl std::str::FromStr for TimeStamp {
    // ...

let input = "[1518-10-08 23:51] Guard #751 begins shift";
let parsed = scanf!(input, "{} Guard #{} begins shift", TimeStamp, usize);
assert_eq!(parsed, Some((TimeStamp{
    year: 1518, month: 10, day: 8,
    hour: 23, minute: 51
}, 751)));

A Note on Error Messages

Errors in the format string would ideally point to the exact position in the string that caused the error. This is already the case if you compile/check with nightly, but not on stable, or at least until Rust Issue #54725 is far enough to allow for this method to be called from stable.

Error Messages on nightly currently look like this:

sscanf::scanf!("", "Some Text {}{}{} and stuff", usize);
error: Missing Type for given '{}'
4 | sscanf::scanf!("", "Some Text {}{}{} and stuff", usize);
  |                                 ^^

But on stable, you are limited to only pointing at the entire format string:

error: Missing Type for given '{}'.  At "Some Text {}{}" <--
4 | sscanf::scanf!("", "Some Text {}{}{} and stuff", usize);
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The current workaround is to point at the incorrect part of the string in the Error Message itself (the <--). The alternative is to use cargo +nightly check to see the better Errors whenever something goes wrong, or setting your Editor plugin to check with nightly.

This does not influence the functionality in any way. This Crate works entirely on stable with no drawbacks in functionality or performance. The only difference is the compiler errors that you get while writing format strings.


pub use const_format;
pub use regex;



A Macro to parse a String based on a format-String, similar to sscanf in C


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Same as scanf, but allows use of Regex in the format String.



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A Trait used by scanf to obtain the Regex of a Type