Crate sscanf

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

sscanf

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 World5!");

// 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 = 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:

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 }}}}");
assert_eq!(parsed, Some(()));

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

let input = "Formats:  0xab01  0o127  101010  1Z";
let parsed = scanf!(input, "Formats:  {x}  {o}  {b}  {r36}", usize, i32, u8, u32);
let (a, b, c, d) = parsed.unwrap();
assert_eq!(a, 0xab01);     // Hex
assert_eq!(b, 0o127);      // Octal
assert_eq!(c, 0b101010);   // Binary

assert_eq!(d, 71);         // any radix (r36 = Radix 36)
assert_eq!(d, u32::from_str_radix("1Z", 36).unwrap());

The input in this case is a &'static str, 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.

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 info and can only compare types by name. This means that the number options below only work with a literal type like “i32”, NO Paths (std::i32) or Wrappers (struct Wrapper(i32);) or Aliases (type Alias = i32;). ONLY i32, usize, u16, …

config	description	possible types
{/ <regex> /}	custom regex	any
{x}	hexadecimal numbers	numbers
{o}	octal numbers	numbers
{b}	binary numbers	numbers
{r2} - {r36}	radix 2 - radix 32 numbers	numbers
{ <chrono format> }	chrono format	chrono types

Custom Regex:

• {/.../}: Match according to the Regex between the / /

For example:

let input = "random Text";
let parsed = scanf!(input, "{/[^m]+/}{}", String, String);

// regex  [^m]+  matches anything that isn't an 'm'
// => stops at the 'm' in 'random'
assert_eq!(parsed, Some((String::from("rando"), String::from("m Text"))));

As mentioned above, '{' '}' have to be escaped with a '\'. This means that:

• "{" or "}" would give a compiler error
• "\{" or "\}" lead to a "{" or "}" inside of the regex
  • curly brackets have a special meaning in regex as counted repetition
• "\\{" or "\\}" would give a compiler error
  • first '\' escapes the second one, leaving just the brackets
• "\\\{" or "\\\}" lead to a "\{" or "\}" inside of the regex
  • the first '\' escapes the second one, leading to a literal '\' in the regex. the third escapes the curly bracket as in the second case
  • needed in order to have the regex match an actual curly bracket

Works with non-String types too:

let input = "Match 4 digits of 123456";
let parsed = scanf!(input, r"Match 4 digits of {/\d\{4\}/}{}", usize, usize);
                           // raw string (r"") to write \d instead of \\d

// regex  \d{4}  matches 4 digits
assert_eq!(parsed, Some((1234, 56)));

Note that changing the regex of a non-String type might cause that type’s FromStr to fail

Number Options:

Only work on primitive number types (u8, …, u128, i8, …, i128, usize, isize).

• 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 (no prefix)

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 { }. This 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 expected = Utc.ymd(2020, 5, 23).and_hms(21, 5, 7);

// DateTime<*> directly implements FromStr and doesn't need a config
let input = "2020-05-23T21:05:07Z";
let parsed = scanf!(input, "{}", DateTime<Utc>);
assert_eq!(parsed, Some(expected));

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(expected));

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

Custom Types

scanf works with 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)));

Implementing RegexRepresentation isn’t strictly necessary if you always supply a custom Regex when using the type by using the {/.../} format option, but this tends to make your code less readable.

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:

scanf!("", "Some Text {}{}{} and stuff", usize);

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

The current workaround is to replicate that behavior in the Error Message itself. 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.

Macros

scanf

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

scanf_get_regex

Same as scanf, but returns the Regex without running it. Useful for Debugging or Efficiency.

scanf_unescaped

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

Structs

FullF32

A Wrapper around f32 whose RegexRepresentation also includes special floating point values like nan, inf, 2.0e5, …

FullF64

A Wrapper around f64 whose RegexRepresentation also includes special floating point values like nan, inf, 2.0e5, …

HexNumberDeprecated

Matches a Hexadecimal Number with optional 0x prefix. Deprecated in favor of format options

Traits

RegexRepresentation

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