Rust parser combinator - nom
This is the manual of nom
from docs.rs.
1 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
extern crate nom;
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 the design of nom macros, 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.
See also the FAQ.
2 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:
#[macro_use]
extern crate nom;
use nom::{IResult, Err, Needed};
fn main() {
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 function and macros 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)
}
With macros, you would write it like this:
#[macro_use]
extern crate nom;
fn main() {
named!(take4, take!(4));
}
nom has used macros for combinators from versions 1 to 4, and from version 5, it proposes new combinators as functions, but still allows the macros style (macros have been rewritten to use the functions under the hood). For new parsers, we recommend using the functions instead of macros, since rustc messages will be much easier to understand.
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::ErrorKind};
type IResult<I, O, E = (I,ErrorKind)> = 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))
withc
an 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.Needed
can indicate how much data is needed - An error
Err(Err::Failure(c))
. It works like theError
case, 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.
3 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
}
4 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:
#[macro_use] extern crate nom;
#[cfg(feature = "alloc")]
fn main() {
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())));
}
[cfg(not(feature = "alloc"))]
fn main() {}
Here are some basic combining macros available:
opt
: Will make the parser optional (if it returns theO
type, the new parser returnsOption<O>
)many0
: Will apply the parser 0 or more times (if it returns theO
type, 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
:
#[macro_use] extern crate nom;
fn main() {
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:
#[macro_use] extern crate nom;
fn main() {
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})));
}
5 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!
6 List of macros parsers and combinators
Note: this list is meant to provide a nicer way to find a nom macros than reading through the documentation on docs.rs, since rustdoc puts all the macros at the top level. Function combinators are organized in module so they are a bit easier to find.
6.1 Basic elements
Those are used to recognize the lowest level elements of your grammar, like, "here is a dot", or "here is an big endian integer".
combinator | usage | input | output | comment |
---|---|---|---|---|
char | char!('a') | "abc" | Ok(("bc", 'a')) | Matches one character (works with non ASCII chars too) |
is_a | is_a!("ab") | "ababc" | Ok(("c", "abab")) | Matches a sequence of any of the characters passed as arguments |
is_not | is_not!("cd") | "ababc" | Ok(("c", "abab")) | Matches a sequence of none of the characters passed as arguments |
one_of | one_of!("abc") | "abc" | Ok(("bc", 'a')) | Matches one of the provided characters (works with non ASCII characters too) |
none_of | none_of!("abc") | "xyab" | Ok(("yab", 'x')) | Matches anything but the provided characters |
tag | tag!("hello") | "hello world" | Ok((" world", "hello")) | Recognizes a specific suite of characters or bytes |
tag_no_case | tag_no_case!("hello") | "HeLLo World" | Ok((" World", "HeLLo")) | Case insensitive comparison. Note that case insensitive comparison is not well defined for unicode, and that you might have bad surprises |
take | take!(4) | "hello" | Ok(("o", "hell")) | Takes a specific number of bytes or characters |
take_while | take_while!(is_alphabetic) | "abc123" | Ok(("123", "abc")) | Returns the longest list of bytes for which the provided function returns true. take_while1 does the same, but must return at least one character |
take_till | take_till!(is_alphabetic) | "123abc" | Ok(("abc", "123")) | Returns the longest list of bytes or characters until the provided function returns true. take_till1 does the same, but must return at least one character. This is the reverse behaviour from take_while : take_till!(f) is equivalent to take_while!(\|c\| !f(c)) |
take_until | take_until!("world") | "Hello world" | Ok(("world", "Hello ")) | Returns the longest list of bytes or characters until the provided tag is found. take_until1 does the same, but must return at least one character |
6.2 Choice combinators
combinator | usage | input | output | comment |
---|---|---|---|---|
alt | alt!(tag!("ab") \| tag!("cd")) | "cdef" | Ok(("ef", "cd")) | Try a list of parsers and return the result of the first successful one |
switch | switch!(take!(2), "ab" => tag!("XYZ") \| "cd" => tag!("123")) | "cd1234" | Ok(("4", "123")) | Choose the next parser depending on the result of the first one, if successful, and returns the result of the second parser |
permutation | permutation!(tag!("ab"), tag!("cd"), tag!("12")) | "cd12abc" | Ok(("c", ("ab", "cd", "12")) | Succeeds when all its child parser have succeeded, whatever the order |
6.3 Sequence combinators
combinator | usage | input | output | comment |
---|---|---|---|---|
delimited | delimited!(char!('('), take!(2), char!(')')) | "(ab)cd" | Ok(("cd", "ab")) | |
preceded | preceded!(tag!("ab"), tag!("XY")) | "abXYZ" | Ok(("Z", "XY")) | |
terminated | terminated!(tag!("ab"), tag!("XY")) | "abXYZ" | Ok(("Z", "ab")) | |
pair | pair!(tag!("ab"), tag!("XY")) | "abXYZ" | Ok(("Z", ("ab", "XY"))) | |
separated_pair | separated_pair!(tag!("hello"), char!(','), tag!("world")) | "hello,world!" | Ok(("!", ("hello", "world"))) | |
tuple | tuple!(tag!("ab"), tag!("XY"), take!(1)) | "abXYZ!" | Ok(("!", ("ab", "XY", "Z"))) | Chains parsers and assemble the sub results in a tuple. You can use as many child parsers as you can put elements in a tuple |
do_parse | do_parse!(tag: take!(2) >> length: be_u8 >> data: take!(length) >> (Buffer { tag: tag, data: data}) ) | &[0, 0, 3, 1, 2, 3][..] | Buffer { tag: &[0, 0][..], data: &[1, 2, 3][..] } | do_parse applies sub parsers in a sequence. It can store intermediary results and make them available for later parsers |
6.4 Applying a parser multiple times
combinator | usage | input | output | comment |
---|---|---|---|---|
count | count!(take!(2), 3) | "abcdefgh" | Ok(("gh", vec!("ab", "cd", "ef"))) | Applies the child parser a specified number of times |
many0 | many0!(tag!("ab")) | "abababc" | Ok(("c", vec!("ab", "ab", "ab"))) | Applies the parser 0 or more times and returns the list of results in a Vec. many1 does the same operation but must return at least one element |
many_m_n | many_m_n!(1, 3, tag!("ab")) | "ababc" | Ok(("c", vec!("ab", "ab"))) | Applies the parser between m and n times (n included) and returns the list of results in a Vec |
many_till | many_till!(tag!( "ab" ), tag!( "ef" )) | "ababefg" | Ok(("g", (vec!("ab", "ab"), "ef"))) | Applies the first parser until the second applies. Returns a tuple containing the list of results from the first in a Vec and the result of the second |
separated_list | separated_list!(tag!(","), tag!("ab")) | "ab,ab,ab." | Ok((".", vec!("ab", "ab", "ab"))) | separated_nonempty_list works like separated_list but must returns at least one element |
fold_many0 | fold_many0!(be_u8, 0, \|acc, item\| acc + item) | [1, 2, 3] | Ok(([], 6)) | Applies the parser 0 or more times and folds the list of return values. The fold_many1 version must apply the child parser at least one time |
fold_many_m_n | fold_many_m_n!(1, 2, be_u8, 0, \|acc, item\| acc + item) | [1, 2, 3] | Ok(([3], 3)) | Applies the parser between m and n times (n included) and folds the list of return value |
length_count | length_count!(number, tag!("ab")) | "2ababab" | Ok(("ab", vec!("ab", "ab"))) | Gets a number from the first parser, then applies the second parser that many times |
6.5 Integers
Parsing integers from binary formats can be done in two ways: With parser functions, or combinators with configurable endianness:
- configurable endianness:
i16!
,i32!
,i64!
,u16!
,u32!
,u64!
are combinators that take as argument anom::Endianness
, like this:i16!(endianness)
. If the parameter isnom::Endianness::Big
, parse a big endiani16
integer, otherwise a little endiani16
integer. - fixed endianness: The functions are prefixed by
be_
for big endian numbers, and byle_
for little endian numbers, and the suffix is the type they parse to. As an example,be_u32
parses a big endian unsigned integer stored in 32 bits. be_f32
,be_f64
,le_f32
,le_f64
: Recognize floating point numbersbe_i8
,be_i16
,be_i24
,be_i32
,be_i64
: Big endian signed integersbe_u8
,be_u16
,be_u24
,be_u32
,be_u64
: Big endian unsigned integersle_i8
,le_i16
,le_i24
,le_i32
,le_i64
: Little endian signed integersle_u8
,le_u16
,le_u24
,le_u32
,le_u64
: Little endian unsigned integers
6.6 Streaming related
eof!
: Returns its input if it is at the end of input datacomplete!
: Replaces anIncomplete
returned by the child parser with anError
6.7 Modifiers
cond!
: Conditional combinator. Wraps another parser and calls it if the condition is metflat_map!
:map!
: Maps a function on the result of a parsermap_opt!
: Maps a function returning anOption
on the output of a parsermap_res!
: Maps a function returning aResult
on the output of a parsernot!
: Returns a result only if the embedded parser returnsError
orIncomplete
. Does not consume the inputopt!
: Make the underlying parser optionalopt_res!
: Make the underlying parser optionalparse_to!
: Uses the parse method fromstd::str::FromStr
to convert the current input to the specified typepeek!
: Returns a result without consuming the inputrecognize!
: If the child parser was successful, return the consumed input as the produced valueconsumed()
: If the child parser was successful, return a tuple of the consumed input and the produced output.return_error!
: Prevents backtracking if the child parser failstap!
: Allows access to the parser's result without affecting itverify!
: Returns the result of the child parser if it satisfies a verification function
6.8 Error management and debugging
add_return_error!
: Add an error if the child parser failsdbg!
: Prints a message if the parser failsdbg_dmp!
: Prints a message and the input if the parser failserror_node_position!
: Creates a parse error from anom::ErrorKind
, the position in the input and the next error in the parsing tree. If theverbose-errors
feature is not activated, it defaults to only the error codeerror_position!
: Creates a parse error from anom::ErrorKind
and the position in the input. If theverbose-errors
feature is not activated, it defaults to only the error codefix_error!
: Translate parser result fromIResult
toIResult
with a custom type
6.9 Text parsing
escaped!
: Matches a byte string with escaped charactersescaped_transform!
: Matches a byte string with escaped characters, and returns a new string with the escaped characters replaced
6.10 Binary format parsing
length_data!
: Gets a number from the first parser, then takes a subslice of the input of that size, and returns that subslicelength_bytes!
: Alias for length_datalength_value!
: Gets a number from the first parser, takes a subslice of the input of that size, then applies the second parser on that subslice. If the second parser returnsIncomplete
,length_value!
will return an error
6.11 Bit stream parsing
bits!
: Transforms the current input type (byte slice&[u8]
) to a bit stream on which bit specific parsers and more general combinators can be appliedbytes!
: Transforms its bits stream input back into a byte slice for the underlying parsertag_bits!
: Matches an integer pattern to a bitstream. The number of bits of the input to compare must be specifiedtake_bits!
: Generates a parser consuming the specified number of bits
6.12 Remaining combinators
apply!
: Emulate function currying:apply!(my_function, arg1, arg2, ...)
becomesmy_function(input, arg1, arg2, ...)
call!
: Used to wrap common expressions and function as macrosmethod!
: Makes a method from a parser combinationnamed!
: Makes a function from a parser combinationnamed_args!
: Makes a function from a parser combination with argumentsnamed_attr!
: Makes a function from a parser combination, with attributestry_parse!
: A bit likestd::try!
, this macro will return the remaining input and parsed value if the child parser returnedOk
, and will do an early return forError
andIncomplete
. This can provide more flexibility thando_parse!
if neededsuccess
: Returns a value without consuming any input, always succeeds
6.13 Character test functions
Use these functions with a combinator like take_while!
:
is_alphabetic
: Tests if byte is ASCII alphabetic:[A-Za-z]
is_alphanumeric
: Tests if byte is ASCII alphanumeric:[A-Za-z0-9]
is_digit
: Tests if byte is ASCII digit:[0-9]
is_hex_digit
: Tests if byte is ASCII hex digit:[0-9A-Fa-f]
is_oct_digit
: Tests if byte is ASCII octal digit:[0-7]
is_space
: Tests if byte is ASCII space or tab:[ \t]
alpha
: Recognizes one or more lowercase and uppercase alphabetic characters:[a-zA-Z]
alphanumeric
: Recognizes one or more numerical and alphabetic characters:[0-9a-zA-Z]
anychar
:begin
:crlf
:digit
: Recognizes one or more numerical characters:[0-9]
double
: Recognizes floating point number in a byte string and returns af64
eol
:float
: Recognizes floating point number in a byte string and returns af32
hex_digit
: Recognizes one or more hexadecimal numerical characters:[0-9A-Fa-f]
hex_u32
: Recognizes a hex-encoded integerline_ending
: Recognizes an end of line (both\n
and\r\n
)multispace
: Recognizes one or more spaces, tabs, carriage returns and line feedsnewline
: Matches a newline character\n
non_empty
: Recognizes non empty buffersnot_line_ending
:oct_digit
: Recognizes one or more octal characters:[0-7]
rest
: Return the remaining inputshift
:sized_buffer
:space
: Recognizes one or more spaces and tabstab
: Matches a tab character\t