Expand description
nom, eating data byte by byte
nom is a parser combinator library with a focus on safe parsing, streaming patterns, and as much as possible zero copy.
Example
use nom::{
IResult,
bytes::complete::{tag, take_while_m_n},
combinator::map_res,
sequence::tuple};
#[derive(Debug,PartialEq)]
pub struct Color {
pub red: u8,
pub green: u8,
pub blue: u8,
}
fn from_hex(input: &str) -> Result<u8, std::num::ParseIntError> {
u8::from_str_radix(input, 16)
}
fn is_hex_digit(c: char) -> bool {
c.is_digit(16)
}
fn hex_primary(input: &str) -> IResult<&str, u8> {
map_res(
take_while_m_n(2, 2, is_hex_digit),
from_hex
)(input)
}
fn hex_color(input: &str) -> IResult<&str, Color> {
let (input, _) = tag("#")(input)?;
let (input, (red, green, blue)) = tuple((hex_primary, hex_primary, hex_primary))(input)?;
Ok((input, Color { red, green, blue }))
}
fn main() {
assert_eq!(hex_color("#2F14DF"), Ok(("", Color {
red: 47,
green: 20,
blue: 223,
})));
}The code is available on Github
There are a few guides with more details about how to write parsers, or the error management system. You can also check out the [recipes] module that contains examples of common patterns.
Looking for a specific combinator? Read the “choose a combinator” guide
If you are upgrading to nom 5.0, please read the migration document.
Parser combinators
Parser combinators are an approach to parsers that is very different from software like lex and yacc. Instead of writing the grammar in a separate syntax and generating the corresponding code, you use very small functions with very specific purposes, like “take 5 bytes”, or “recognize the word ‘HTTP’”, and assemble them in meaningful patterns like “recognize ‘HTTP’, then a space, then a version”. The resulting code is small, and looks like the grammar you would have written with other parser approaches.
This gives us a few advantages:
- The parsers are small and easy to write
- The parsers components are easy to reuse (if they’re general enough, please add them to nom!)
- The parsers components are easy to test separately (unit tests and property-based tests)
- The parser combination code looks close to the grammar you would have written
- You can build partial parsers, specific to the data you need at the moment, and ignore the rest
Here is an example of one such parser, to recognize text between parentheses:
use nom::{
IResult,
sequence::delimited,
// see the "streaming/complete" paragraph lower for an explanation of these submodules
character::complete::char,
bytes::complete::is_not
};
fn parens(input: &str) -> IResult<&str, &str> {
delimited(char('('), is_not(")"), char(')'))(input)
}It defines a function named parens which will recognize a sequence of the
character (, the longest byte array not containing ), then the character
), and will return the byte array in the middle.
Here is another parser, written without using nom’s combinators this time:
use nom::{IResult, Err, Needed};
fn take4(i: &[u8]) -> IResult<&[u8], &[u8]>{
if i.len() < 4 {
Err(Err::Incomplete(Needed::new(4)))
} else {
Ok((&i[4..], &i[0..4]))
}
}This function takes a byte array as input, and tries to consume 4 bytes. Writing all the parsers manually, like this, is dangerous, despite Rust’s safety features. There are still a lot of mistakes one can make. That’s why nom provides a list of functions to help in developing parsers.
With functions, you would write it like this:
use nom::{IResult, bytes::streaming::take};
fn take4(input: &str) -> IResult<&str, &str> {
take(4u8)(input)
}A parser in nom is a function which, for an input type I, an output type O
and an optional error type E, will have the following signature:
fn parser(input: I) -> IResult<I, O, E>;Or like this, if you don’t want to specify a custom error type (it will be (I, ErrorKind) by default):
fn parser(input: I) -> IResult<I, O>;IResult is an alias for the Result type:
use nom::{Needed, error::Error};
type IResult<I, O, E = Error<I>> = Result<(I, O), Err<E>>;
enum Err<E> {
Incomplete(Needed),
Error(E),
Failure(E),
}It can have the following values:
- A correct result
Ok((I,O))with the first element being the remaining of the input (not parsed yet), and the second the output value; - An error
Err(Err::Error(c))withcan error that can be built from the input position and a parser specific error - An error
Err(Err::Incomplete(Needed))indicating that more input is necessary.Neededcan indicate how much data is needed - An error
Err(Err::Failure(c)). It works like theErrorcase, except it indicates an unrecoverable error: We cannot backtrack and test another parser
Please refer to the “choose a combinator” guide for an exhaustive list of parsers. See also the rest of the documentation here.
Making new parsers with function combinators
nom is based on functions that generate parsers, with a signature like
this: (arguments) -> impl Fn(Input) -> IResult<Input, Output, Error>.
The arguments of a combinator can be direct values (like take which uses
a number of bytes or character as argument) or even other parsers (like
delimited which takes as argument 3 parsers, and returns the result of
the second one if all are successful).
Here are some examples:
use nom::IResult;
use nom::bytes::complete::{tag, take};
fn abcd_parser(i: &str) -> IResult<&str, &str> {
tag("abcd")(i) // will consume bytes if the input begins with "abcd"
}
fn take_10(i: &[u8]) -> IResult<&[u8], &[u8]> {
take(10u8)(i) // will consume and return 10 bytes of input
}Combining parsers
There are higher level patterns, like the alt combinator, which
provides a choice between multiple parsers. If one branch fails, it tries
the next, and returns the result of the first parser that succeeds:
use nom::IResult;
use nom::branch::alt;
use nom::bytes::complete::tag;
let mut alt_tags = alt((tag("abcd"), tag("efgh")));
assert_eq!(alt_tags(&b"abcdxxx"[..]), Ok((&b"xxx"[..], &b"abcd"[..])));
assert_eq!(alt_tags(&b"efghxxx"[..]), Ok((&b"xxx"[..], &b"efgh"[..])));
assert_eq!(alt_tags(&b"ijklxxx"[..]), Err(nom::Err::Error((&b"ijklxxx"[..], nom::error::ErrorKind::Tag))));The opt combinator makes a parser optional. If the child parser returns
an error, opt will still succeed and return None:
use nom::{IResult, combinator::opt, bytes::complete::tag};
fn abcd_opt(i: &[u8]) -> IResult<&[u8], Option<&[u8]>> {
opt(tag("abcd"))(i)
}
assert_eq!(abcd_opt(&b"abcdxxx"[..]), Ok((&b"xxx"[..], Some(&b"abcd"[..]))));
assert_eq!(abcd_opt(&b"efghxxx"[..]), Ok((&b"efghxxx"[..], None)));many0 applies a parser 0 or more times, and returns a vector of the aggregated results:
use nom::{IResult, multi::many0, bytes::complete::tag};
use std::str;
fn multi(i: &str) -> IResult<&str, Vec<&str>> {
many0(tag("abcd"))(i)
}
let a = "abcdef";
let b = "abcdabcdef";
let c = "azerty";
assert_eq!(multi(a), Ok(("ef", vec!["abcd"])));
assert_eq!(multi(b), Ok(("ef", vec!["abcd", "abcd"])));
assert_eq!(multi(c), Ok(("azerty", Vec::new())));Here are some basic combinators available:
opt: Will make the parser optional (if it returns theOtype, the new parser returnsOption<O>)many0: Will apply the parser 0 or more times (if it returns theOtype, the new parser returnsVec<O>)many1: Will apply the parser 1 or more times
There are more complex (and more useful) parsers like tuple, which is
used to apply a series of parsers then assemble their results.
Example with tuple:
use nom::{error::ErrorKind, Needed,
number::streaming::be_u16,
bytes::streaming::{tag, take},
sequence::tuple};
let mut tpl = tuple((be_u16, take(3u8), tag("fg")));
assert_eq!(
tpl(&b"abcdefgh"[..]),
Ok((
&b"h"[..],
(0x6162u16, &b"cde"[..], &b"fg"[..])
))
);
assert_eq!(tpl(&b"abcde"[..]), Err(nom::Err::Incomplete(Needed::new(2))));
let input = &b"abcdejk"[..];
assert_eq!(tpl(input), Err(nom::Err::Error((&input[5..], ErrorKind::Tag))));But you can also use a sequence of combinators written in imperative style,
thanks to the ? operator:
use nom::{IResult, bytes::complete::tag};
#[derive(Debug, PartialEq)]
struct A {
a: u8,
b: u8
}
fn ret_int1(i:&[u8]) -> IResult<&[u8], u8> { Ok((i,1)) }
fn ret_int2(i:&[u8]) -> IResult<&[u8], u8> { Ok((i,2)) }
fn f(i: &[u8]) -> IResult<&[u8], A> {
// if successful, the parser returns `Ok((remaining_input, output_value))` that we can destructure
let (i, _) = tag("abcd")(i)?;
let (i, a) = ret_int1(i)?;
let (i, _) = tag("efgh")(i)?;
let (i, b) = ret_int2(i)?;
Ok((i, A { a, b }))
}
let r = f(b"abcdefghX");
assert_eq!(r, Ok((&b"X"[..], A{a: 1, b: 2})));Streaming / Complete
Some of nom’s modules have streaming or complete submodules. They hold
different variants of the same combinators.
A streaming parser assumes that we might not have all of the input data. This can happen with some network protocol or large file parsers, where the input buffer can be full and need to be resized or refilled.
A complete parser assumes that we already have all of the input data. This will be the common case with small files that can be read entirely to memory.
Here is how it works in practice:
use nom::{IResult, Err, Needed, error::{Error, ErrorKind}, bytes, character};
fn take_streaming(i: &[u8]) -> IResult<&[u8], &[u8]> {
bytes::streaming::take(4u8)(i)
}
fn take_complete(i: &[u8]) -> IResult<&[u8], &[u8]> {
bytes::complete::take(4u8)(i)
}
// both parsers will take 4 bytes as expected
assert_eq!(take_streaming(&b"abcde"[..]), Ok((&b"e"[..], &b"abcd"[..])));
assert_eq!(take_complete(&b"abcde"[..]), Ok((&b"e"[..], &b"abcd"[..])));
// if the input is smaller than 4 bytes, the streaming parser
// will return `Incomplete` to indicate that we need more data
assert_eq!(take_streaming(&b"abc"[..]), Err(Err::Incomplete(Needed::new(1))));
// but the complete parser will return an error
assert_eq!(take_complete(&b"abc"[..]), Err(Err::Error(Error::new(&b"abc"[..], ErrorKind::Eof))));
// the alpha0 function recognizes 0 or more alphabetic characters
fn alpha0_streaming(i: &str) -> IResult<&str, &str> {
character::streaming::alpha0(i)
}
fn alpha0_complete(i: &str) -> IResult<&str, &str> {
character::complete::alpha0(i)
}
// if there's a clear limit to the recognized characters, both parsers work the same way
assert_eq!(alpha0_streaming("abcd;"), Ok((";", "abcd")));
assert_eq!(alpha0_complete("abcd;"), Ok((";", "abcd")));
// but when there's no limit, the streaming version returns `Incomplete`, because it cannot
// know if more input data should be recognized. The whole input could be "abcd;", or
// "abcde;"
assert_eq!(alpha0_streaming("abcd"), Err(Err::Incomplete(Needed::new(1))));
// while the complete version knows that all of the data is there
assert_eq!(alpha0_complete("abcd"), Ok(("", "abcd")));Going further: Read the guides, check out the [recipes]!
Re-exports
pub use self::bits::*;Modules
Bit level parsers
Choice combinators
Parsers recognizing bytes streams
Character specific parsers and combinators
General purpose combinators
Error management
Lib module to re-export everything needed from std or core/alloc. This is how serde does
it, albeit there it is not public.
Combinators applying their child parser multiple times
Parsers recognizing numbers
Combinators applying parsers in sequence
Macros
Creates a parse error from a nom::ErrorKind,
the position in the input and the next error in
the parsing tree
Creates a parse error from a nom::ErrorKind
and the position in the input
Structs
Implementation of Parser::and
Implementation of Parser::and_then
Implementation of Parser::flat_map
Implementation of Parser::into
Implementation of Parser::map
Implementation of Parser::or
Enums
Indicates whether a comparison was successful, an error, or if more data was needed
The Err enum indicates the parser was not successful
Contains information on needed data if a parser returned Incomplete
Traits
Helper trait for types that can be viewed as a byte slice
Transforms common types to a char for basic token parsing
Abstracts comparison operations
Equivalent From implementation to avoid orphan rules in bits parsers
Abstracts something which can extend an Extend.
Used to build modified input slices in escaped_transform
Look for a substring in self
Look for a token in self
Helper trait to convert a parser’s result to a more manageable type
Helper trait to show a byte slice as a hex dump
Abstracts common iteration operations on the input type
Abstract method to calculate the input length
Abstracts slicing operations
Methods to take as much input as possible until the provided function returns true for the current element.
Useful functions to calculate the offset between slices and show a hexdump of a slice
Used to integrate str’s parse() method
All nom parsers implement this trait
Slicing operations using ranges.
Helper trait to convert numbers to usize.
Dummy trait used for default implementations (currently only used for InputTakeAtPosition and Compare).
Type Definitions
Holds the result of parsing functions