[−][src]Crate nom
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.
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.
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 a very specific purpose, like "take 5 bytes", or "recognize the word 'HTTP'", and assemble then 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 take4(i: &[u8]) -> IResult<&[u8], &[u8]>{ if i.len() < 4 { Err(Err::Incomplete(Needed::Size(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; 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 u32
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. .
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 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 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
:
use nom::{error::ErrorKind, Needed, number::streaming::be_u16, bytes::streaming::{tag, take}, sequence::tuple}; let 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::Size(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 intirely to memory.
Here is how it works in practice:
use nom::{IResult, Err, Needed, 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::Size(4)))); // but the complete parser will return an error assert_eq!(take_complete(&b"abc"[..]), Err(Err::Error((&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::Size(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!
Re-exports
pub extern crate regex; |
pub use self::methods::*; |
pub use self::bits::*; |
pub use self::whitespace::*; |
Modules
bits | Bit level parsers and combinators |
branch | choice combinators |
bytes | parsers recognizing bytes streams |
character | character specific parsers and combinators |
combinator | general purpose combinators |
error | Error management |
lib | Lib module to re-export everything needed from |
methods | method combinators |
multi | combinators applying their child parser multiple times |
number | parsers recognizing numbers |
sequence | combinators applying parsers in sequence |
whitespace | Support for whitespace delimited formats |
Macros
add_return_error | Add an error if the child parser fails |
alt | Try a list of parsers and return the result of the first successful one |
apply_m | do not use: method combinators moved to the nom-methods crate |
bits | Transforms its byte slice input into a bit stream for the underlying parser. This allows the given bit stream parser to work on a byte slice input. |
bytes | Counterpart to bits, bytes! transforms its bit stream input into a byte slice for the underlying parser, allowing byte-slice parsers to work on bit streams. |
call | Used to wrap common expressions and function as macros |
call_m | do not use: method combinators moved to the nom-methods crate |
char | matches one character: `char!(char) => &u8 -> IResult<&u8, char> |
complete | replaces a |
cond |
|
count |
|
dbg | Prints a message if the parser fails |
dbg_dmp | Prints a message and the input if the parser fails |
delimited |
|
do_parse |
|
eat_separator | helper macros to build a separator parser |
eof |
|
error_node_position | creates a parse error from a |
error_position | creates a parse error from a |
escaped |
|
escaped_transform |
|
exact |
|
fix_error | translate parser result from IResult<I,O,u32> to IResult<I,O,E> with a custom type |
flat_map |
|
fold_many0 |
|
fold_many1 |
|
fold_many_m_n |
|
i16 | if the parameter is nom::Endianness::Big, parse a big endian i16 integer, otherwise a little endian i16 integer |
i32 | if the parameter is nom::Endianness::Big, parse a big endian i32 integer, otherwise a little endian i32 integer |
i64 | if the parameter is nom::Endianness::Big, parse a big endian i64 integer, otherwise a little endian i64 integer |
i128 | if the parameter is nom::Endianness::Big, parse a big endian i64 integer, otherwise a little endian i64 integer |
is_a |
|
is_not |
|
length_count |
|
length_data |
|
length_value |
|
many0 |
|
many0_count |
|
many1 |
|
many1_count |
|
many_m_n |
|
many_till |
|
map |
|
map_opt |
|
map_res |
|
method | do not use: method combinators moved to the nom-methods crate |
named | Makes a function from a parser combination |
named_args | Makes a function from a parser combination with arguments. |
named_attr | Makes a function from a parser combination, with attributes |
none_of | matches anything but the provided characters |
not |
|
one_of | Character level parsers matches one of the provided characters |
opt |
|
opt_res |
|
pair |
|
parse_to |
|
peek |
|
permutation |
|
preceded |
|
re_bytes_capture |
|
re_bytes_capture_static |
|
re_bytes_captures |
|
re_bytes_captures_static |
|
re_bytes_find |
|
re_bytes_find_static |
|
re_bytes_match |
|
re_bytes_match_static |
|
re_bytes_matches |
|
re_bytes_matches_static |
|
re_capture |
|
re_capture_static |
|
re_captures |
|
re_captures_static |
|
re_find |
|
re_find_static |
|
re_match |
|
re_match_static |
|
re_matches |
|
re_matches_static |
|
recognize |
|
return_error | Prevents backtracking if the child parser fails |
sep | sep is the parser rewriting macro for whitespace separated formats |
separated_list |
|
separated_nonempty_list |
|
separated_pair |
|
switch |
|
tag |
|
tag_bits | Matches the given bit pattern. |
tag_no_case |
|
take |
|
take_bits | Consumes the specified number of bits and returns them as the specified type. |
take_str |
|
take_till |
|
take_till1 |
|
take_until |
|
take_until1 |
|
take_while |
|
take_while1 |
|
take_while_m_n |
|
tap |
|
terminated |
|
try_parse | A bit like |
tuple |
|
u16 | if the parameter is nom::Endianness::Big, parse a big endian u16 integer, otherwise a little endian u16 integer |
u32 | if the parameter is nom::Endianness::Big, parse a big endian u32 integer, otherwise a little endian u32 integer |
u64 | if the parameter is nom::Endianness::Big, parse a big endian u64 integer, otherwise a little endian u64 integer |
u128 | if the parameter is nom::Endianness::Big, parse a big endian u128 integer, otherwise a little endian u128 integer |
value |
|
verify |
|
wrap_sep | applies the separator parser before the other parser |
ws |
|
Enums
CompareResult | indicates wether a comparison was successful, an error, or if more data was needed |
Err | The |
Needed | Contains information on needed data if a parser returned |
Traits
AsBytes | Helper trait for types that can be viewed as a byte slice |
AsChar | transforms common types to a char for basic token parsing |
Compare | abstracts comparison operations |
ExtendInto | abtracts something which can extend an |
FindSubstring | look for a substring in self |
FindToken | look for a token in self |
HexDisplay | Helper trait to show a byte slice as a hex dump |
InputIter | abstracts common iteration operations on the input type |
InputLength | abstract method to calculate the input length |
InputTake | abstracts slicing operations |
InputTakeAtPosition | methods to take as much input as possible until the provided function returns true for the current element |
Offset | useful functions to calculate the offset between slices and show a hexdump of a slice |
ParseTo | used to integrate str's parse() method |
Slice | slicing operations using ranges |
ToUsize | Helper trait to convert numbers to usize |
UnspecializedInput | Dummy trait used for default implementations (currently only used for |
Functions
dbg_dmp | Prints a message and the input if the parser fails |
Type Definitions
IResult | Holds the result of parsing functions |