Crate sscanf 

A Rust crate providing an sscanf-like macro (inverse of format!()), with near unlimited parsing capabilities

sscanf is originally a C function that takes a string, a format string with placeholders, and several variables. It parses the input and writes matched values into those variables. In Rust, this crate returns a tuple instead. You can think of it as reversing a call to format!():

// format: takes format string and values, returns String
let msg = format!("Hello {}{}!", "World", 5);
assert_eq!(msg, "Hello World5!");

// sscanf: takes string, format string and types, returns tuple
let parsed = sscanf::sscanf!(msg, "Hello {}{}!", &str, usize);

// parsed is Option<(&str, usize)>
assert_eq!(parsed.unwrap(), ("World", 5));

// alternative syntax:
let parsed2 = sscanf::sscanf!(msg, "Hello {&str}{usize}!");
assert_eq!(parsed2.unwrap(), ("World", 5));

sscanf!() takes a format string like format!(), but instead of writing values into {} placeholders, it extracts the values at those positions into the returned tuple.

If matching the format string fails, None is returned:

let msg = "Text that doesn't match the format string";
let parsed = sscanf::sscanf!(msg, "Hello {&str}{usize}!");
assert!(parsed.is_none());

Types in Placeholders:

The types can either be given as a separate parameter after the format string, or directly inside the {} placeholder.
The first allows for autocomplete while typing, syntax highlighting and better compiler errors generated by sscanf in case that the wrong types are given.
The second mirrors the captured identifiers in format strings. This option has less helpful compiler errors on stable Rust, but is otherwise identical to the first.

More examples of the capabilities of sscanf:

use sscanf::sscanf;
use std::num::NonZeroUsize;

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

let input = "Move to N36E21";
let parsed = sscanf!(input, "Move to {char}{usize}{char}{usize}");
assert_eq!(parsed.unwrap(), ('N', 36, 'E', 21));

let input = "Escape literal { } as {{ and }}";
let parsed = sscanf!(input, "Escape literal {{ }} as {{{{ and }}}}");
assert_eq!(parsed.unwrap(), ());

let input = "Indexing types: N36E21";
let parsed = sscanf!(input, "Indexing types: {1}{0}{1}{0}", NonZeroUsize, char);
// output is in the order of the placeholders
assert_eq!(parsed.unwrap(), ('N', NonZeroUsize::new(36).unwrap(),
                             'E', NonZeroUsize::new(21).unwrap()));

let input = "A Sentence with Spaces. Another Sentence.";
// &str and String do the same, but String clones from the input string
// to take ownership instead of borrowing.
let (a, b) = sscanf!(input, "{String}. {&str}.").unwrap();
assert_eq!(a, "A Sentence with Spaces");
assert_eq!(b, "Another Sentence");

// Number format options
let input = "ab01  127  101010  1Z";
let parsed = sscanf!(input, "{usize:x}  {i32:o}  {u8:b}  {u32:r36}");
let (a, b, c, d) = parsed.unwrap();
assert_eq!(a, 0xab01);     // Hexadecimal
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());

let input = "color: #D4AF37";
// Number types take their size into account, and hexadecimal u8 can
// have at most 2 digits => only possible match is 2 digits each.
let (r, g, b) = sscanf!(input, "color: #{u8:x}{u8:x}{u8:x}").unwrap();
assert_eq!((r, g, b), (0xD4, 0xAF, 0x37));

The input here is a &'static str, but it can be String, &str, &String, … Basically anything that auto-derefs to str without taking ownership. See input examples for a few examples of possible inputs.

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

• char is just one character (regex ".")
• str 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 '{' '}' and after a :, so either as {<type>:<option>} or as {:<option>}. Note: The type might still have a path that contains ::. Any double colons are ignored and only single colons are used to separate the options.

Custom Regex:

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

For example:

let input = "random Text";
let parsed = sscanf::sscanf!(input, "{&str:/[^m]+/}{&str}");

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

The regex uses the same escaping logic as JavaScripts /.../ syntax, meaning that the normal regex escaping with \d for digits etc. is in effect, with the addition that any / need to be escaped as \/ since they are used to end the regex.

NOTE: You should use raw strings for a format string containing a regex, since otherwise you need to escape any \ as \\:

use sscanf::sscanf;
let input = "1234";
let parsed = sscanf!(input, r"{u8:/\d{2}/}{u8}"); // regex  \d{2}  matches 2 digits
let _ =      sscanf!(input, "{u8:/\\d{2}/}{u8}"); // the same with a non-raw string
assert_eq!(parsed.unwrap(), (12, 34));

See trait AcceptsRegexOverride for types supporting this option and instructions for adding support to custom types.

Radix Options:

Generally only work on primitive integer types (u8, …, u128, i8, …, i128, usize, isize).

• x: hexadecimal Number (Digits 0-9 and a-f or A-F), optional prefix 0x or 0X
• o: octal Number (Digits 0-7), optional prefix 0o or 0O
• b: binary Number (Digits 0-1), optional prefix 0b or 0B
• r2 - r36: any radix Number (Digits 0-9 and a-z or A-Z for higher radices)

If used alongside a #: makes the number require a prefix (0x, 0o, 0b).

A note on prefixes: r2, r8 and r16 match the same numbers as b, o and x respectively, but without a prefix. Thus:

• {:x} may have a prefix, matching numbers like 0xab or ab
• {:r16} has no prefix and would only match ab
• {:#x} must have a prefix, matching only 0xab
• {:#r16} gives a compile error

Custom Types

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

To add more types there are two options:

• Derive FromScanf for your type (simple, readable, fool proof (mostly))
• Manually implement FromScanfSimple (more flexible, more code)
• Manually implement FromScanf for your type (flexible, but requires more code)

The simplest option is to use derive:

#[derive(sscanf::FromScanf)] // The derive macro
#[derive(Debug, PartialEq)] // additional traits for assert_eq below. Not required for sscanf
#[sscanf(format = "{numerator}/{denominator}")] // Format string for the type, using the field names.
struct Fraction {
    numerator: isize,
    denominator: usize,
}
let parsed = sscanf::sscanf!("-10/3", "{Fraction}").unwrap();
assert_eq!(parsed, Fraction { numerator: -10, denominator: 3 });

Also works for enums:

#[derive(sscanf::FromScanf)]
enum HasChanged {
    #[sscanf(format = "received {added} additions and {deleted} deletions")]
    Yes {
        added: usize,
        deleted: usize,
    },
    #[sscanf("has not changed")] // the `format =` part can be omitted
    No
}

let input = "Your file has not changed since your last visit!";
let parsed = sscanf::sscanf!(input, "Your file {HasChanged} since your last visit!").unwrap();
assert!(matches!(parsed, HasChanged::No));

let input = "Your file received 325 additions and 15 deletions since your last visit!";
let parsed = sscanf::sscanf!(input, "Your file {HasChanged} since your last visit!").unwrap();
assert!(matches!(parsed, HasChanged::Yes { added: 325, deleted: 15 }));

More details can be found in the trait FromScanf and derive FromScanf documentations.

Changelog

See Changelog.md

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.

Compiler errors

Ideally, errors in the format string point to the exact position in the string that caused the error. This already works on nightly, but not on stable - at least until Rust Issue #54725 enables calling Literal::subspan from stable.

Errors on nightly currently look like this:

sscanf!("", "Too many placeholders: {}{}{}", usize);

error: more placeholders than types provided
  |
4 | sscanf!("", "Too many placeholders: {}{}{}", usize);
  |                                       ^^

On stable, diagnostics can only point at the entire format string:

4 | sscanf!("", "Too many placeholders: {}{}{}", usize);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The current workaround is to replicate that behavior inside the error message:

error: more placeholders than types provided:
       At "Too many placeholders: {}{}{}"
                                    ^^
  |
4 | sscanf!("", "Too many placeholders: {}{}{}", usize);
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Alternatively, use cargo +nightly check to see these better errors, or configure your editor to check with nightly.

This does not affect functionality. This crate works entirely on stable with no performance or feature drawbacks. The only difference is the quality of compiler errors while writing format strings.

Modules

advanced

Advanced parsing types and utilities.

changelog

Changelog

Macros

sscanf

Parses a string using a format string, similar to C’s sscanf.

sscanf_parser

Converts a format string and types into a Parser that can be reused to parse multiple inputs.

sscanf_parser_with_regex

Same as sscanf_parser, but allows using regex in the format string.

sscanf_with_regex

Same as sscanf, but allows using regex in the format string.

Structs

Parser

A parser that can be reused to parse multiple inputs.

Traits

FromScanf

A trait for parsing a type with sscanf.

FromScanfSimple

A simpler version of FromScanf for manual implementations.

Functions

parse

Parses the given input string into a value of type T.

Derive Macros

FromScanf

Derive macro for FromScanf.