sscanf

A Rust crate with a sscanf (inverse of format!()) Macro based on Regex

sscanf is originally 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 (Rust: returns a Tuple). sscanf can be thought of as reversing a call to format!():

// format: takes format string and values, returns String
let s = format!("Hello {}_{}!", "World", 5);
assert_eq!(s, "Hello World_5!");

// scanf: takes String, format string and types, returns Tuple
let parsed = sscanf::scanf!(s, "Hello {}_{}!", String, usize);

// parsed is Option<(String, usize)>
assert_eq!(parsed, Some((String::from("World"), 5)));

scanf!() takes a format String like format!(), but doesn't write the values into the placeholders ({}), but extracts the values at those {} into the return Tuple.

If matching the format string failed, None is returned:

let s = "Text that doesn't match the format string";
let parsed = sscanf::scanf!(s, "Hello {}_{}!", String, usize);
assert_eq!(parsed, None); // No match possible

Note that the original C-function and this Crate are called sscanf, which is the technically correct version in this context. scanf (with one s) is a similar C-function that reads a console input 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 = "<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 = "Move to N36E21";
let parsed = scanf!(input, "Move to {}{}{}{}", char, usize, char, usize);
assert_eq!(parsed, Some(('N', 36, 'E', 21)));

let input = "Escape literal { } as {{ and }}";
let parsed = scanf!(input, "Escape literal {{ }} as {{{{ and {}", String);
assert_eq!(parsed, Some(String::from("}}")));

let input = "A Sentence with Spaces. Number formats: 0xab01 0o127 0b101010.";
let parsed = scanf!(input, "{}. Number formats: {x} {o} {b}.", String, usize, i32, u8);
let (a, b, c, d) = parsed.unwrap();
assert_eq!(a, "A Sentence with Spaces");
assert_eq!(b, 0xab01);
assert_eq!(c, 0o127);
assert_eq!(d, 0b101010);

The input in this case is a &'static stc, but in can be String, &str, &String, ... Basically anything with AsRef<str> and without taking Ownership.

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.

Format Options

All Options are inside '{' '}'. Literal '{' or '}' inside of a Format Option are escaped as '\{' instead of '{{' to avoid ambiguity.

Procedural macro don't have any reliable type information, so the Type must be the exact required Type without any path or alias (chrono imports happen automatically)

Radix Options:

Only work on primitive number types (u8, i8, u16, ...).

• x: hexadecimal Number (Digits 0-9 and A-F, optional Prefix 0x)
• o: octal Number (Digits 0-7, optional Prefix 0o)
• b: binary Number (Digits 0-1, optional Prefix 0b)
• r2 - r36: any radix Number

chrono integration (Requires chrono feature):

The types DateTime, NaiveDate, NaiveTime, NaiveDateTime, Utc and Local can be used and accept a Date/Time format string inside of the { }, that will then be used for both the Regex generation and parsing of the type.

Using DateTime returns a DateTime<FixedOffset> and requires the rules and limits that DateTime::parse_from_str has.

use chrono::prelude::*;

let input = "10:37:02";
let parsed = scanf!(input, "{%H:%M:%S}", NaiveTime);
assert_eq!(parsed, Some(NaiveTime::from_hms(10, 37, 2)));

let input = "Today is the 23. of May, 2020 at 09:05 pm and 7 seconds.";
let parsed = scanf!(input, "Today is the {%d. of %B, %Y at %I:%M %P and %-S} seconds.", Utc);
assert_eq!(parsed, Some(Utc.ymd(2020, 5, 23).and_hms(21, 5, 7)));

Note: The chrono feature needs to be active for this to work, because chrono is an optional dependency

Custom Types

scanf works with most of the 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)));