1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
|
<p align="center">
<img src="https://raw.github.com/pest-parser/pest/master/pest-logo.svg?sanitize=true" width="80%"/>
</p>
# pest. The Elegant Parser
[](https://gitter.im/pest-parser/pest?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
[](https://pest.rs/book)
[](https://docs.rs/pest)
[](https://github.com/pest-parser/pest/actions/workflows/ci.yml)
[](https://codecov.io/gh/pest-parser/pest)
<a href="https://blog.rust-lang.org/2021/11/01/Rust-1.61.0.html"><img alt="Rustc Version 1.61.0+" src="https://img.shields.io/badge/rustc-1.61.0%2B-lightgrey.svg"/></a>
[](https://crates.io/crates/pest)
[](https://crates.io/crates/pest)
pest is a general purpose parser written in Rust with a focus on accessibility,
correctness, and performance. It uses parsing expression grammars
(or [PEG]) as input, which are similar in spirit to regular expressions, but
which offer the enhanced expressivity needed to parse complex languages.
[PEG]: https://en.wikipedia.org/wiki/Parsing_expression_grammar
## Getting started
The recommended way to start parsing with pest is to read the official [book].
Other helpful resources:
* API reference on [docs.rs]
* play with grammars and share them on our [fiddle]
* find previous common questions answered or ask questions on [GitHub Discussions]
* leave feedback, ask questions, or greet us on [Gitter] or [Discord]
[book]: https://pest.rs/book
[docs.rs]: https://docs.rs/pest
[fiddle]: https://pest.rs/#editor
[Gitter]: https://gitter.im/pest-parser/pest
[Discord]: https://discord.gg/XEGACtWpT2
[GitHub Discussions]: https://github.com/pest-parser/pest/discussions
## Example
The following is an example of a grammar for a list of alphanumeric identifiers
where all identifiers don't start with a digit:
```rust
alpha = { 'a'..'z' | 'A'..'Z' }
digit = { '0'..'9' }
ident = { !digit ~ (alpha | digit)+ }
ident_list = _{ ident ~ (" " ~ ident)* }
// ^
// ident_list rule is silent which means it produces no tokens
```
Grammars are saved in separate .pest files which are never mixed with procedural
code. This results in an always up-to-date formalization of a language that is
easy to read and maintain.
## Meaningful error reporting
Based on the grammar definition, the parser also includes automatic error
reporting. For the example above, the input `"123"` will result in:
```
thread 'main' panicked at ' --> 1:1
|
1 | 123
| ^---
|
= unexpected digit', src/main.rs:12
```
while `"ab *"` will result in:
```
thread 'main' panicked at ' --> 1:1
|
1 | ab *
| ^---
|
= expected ident', src/main.rs:12
```
These error messages can be obtained from their default `Display` implementation,
e.g. `panic!("{}", parser_result.unwrap_err())` or `println!("{}", e)`.
## Pairs API
The grammar can be used to derive a `Parser` implementation automatically.
Parsing returns an iterator of nested token pairs:
```rust
use pest_derive::Parser;
use pest::Parser;
#[derive(Parser)]
#[grammar = "ident.pest"]
struct IdentParser;
fn main() {
let pairs = IdentParser::parse(Rule::ident_list, "a1 b2").unwrap_or_else(|e| panic!("{}", e));
// Because ident_list is silent, the iterator will contain idents
for pair in pairs {
// A pair is a combination of the rule which matched and a span of input
println!("Rule: {:?}", pair.as_rule());
println!("Span: {:?}", pair.as_span());
println!("Text: {}", pair.as_str());
// A pair can be converted to an iterator of the tokens which make it up:
for inner_pair in pair.into_inner() {
match inner_pair.as_rule() {
Rule::alpha => println!("Letter: {}", inner_pair.as_str()),
Rule::digit => println!("Digit: {}", inner_pair.as_str()),
_ => unreachable!()
};
}
}
}
```
This produces the following output:
```
Rule: ident
Span: Span { start: 0, end: 2 }
Text: a1
Letter: a
Digit: 1
Rule: ident
Span: Span { start: 3, end: 5 }
Text: b2
Letter: b
Digit: 2
```
### Defining multiple parsers in a single file
The current automatic `Parser` derivation will produce the `Rule` enum
which would have name conflicts if one tried to define multiple such structs
that automatically derive `Parser`. One possible way around it is to put each
parser struct in a separate namespace:
```rust
mod a {
#[derive(Parser)]
#[grammar = "a.pest"]
pub struct ParserA;
}
mod b {
#[derive(Parser)]
#[grammar = "b.pest"]
pub struct ParserB;
}
```
## Other features
* Precedence climbing
* Input handling
* Custom errors
* Runs on stable Rust
## Projects using pest
You can find more projects and ecosystem tools in the [awesome-pest](https://github.com/pest-parser/awesome-pest) repo.
* [pest_meta](https://github.com/pest-parser/pest/blob/master/meta/src/grammar.pest) (bootstrapped)
* [AshPaper](https://github.com/shnewto/ashpaper)
* [brain](https://github.com/brain-lang/brain)
* [cicada](https://github.com/mitnk/cicada)
* [comrak](https://github.com/kivikakk/comrak)
* [elastic-rs](https://github.com/cch123/elastic-rs)
* [graphql-parser](https://github.com/Keats/graphql-parser)
* [handlebars-rust](https://github.com/sunng87/handlebars-rust)
* [hexdino](https://github.com/Luz/hexdino)
* [Huia](https://gitlab.com/jimsy/huia/)
* [insta](https://github.com/mitsuhiko/insta)
* [jql](https://github.com/yamafaktory/jql)
* [json5-rs](https://github.com/callum-oakley/json5-rs)
* [mt940](https://github.com/svenstaro/mt940-rs)
* [Myoxine](https://github.com/d3bate/myoxine)
* [py_literal](https://github.com/jturner314/py_literal)
* [rouler](https://github.com/jarcane/rouler)
* [RuSh](https://github.com/lwandrebeck/RuSh)
* [rs_pbrt](https://github.com/wahn/rs_pbrt)
* [stache](https://github.com/dgraham/stache)
* [tera](https://github.com/Keats/tera)
* [ui_gen](https://github.com/emoon/ui_gen)
* [ukhasnet-parser](https://github.com/adamgreig/ukhasnet-parser)
* [ZoKrates](https://github.com/ZoKrates/ZoKrates)
* [Vector](https://github.com/timberio/vector)
* [AutoCorrect](https://github.com/huacnlee/autocorrect)
* [yaml-peg](https://github.com/aofdev/yaml-peg)
* [qubit](https://github.com/abhimanyu003/qubit)
* [caith](https://github.com/Geobert/caith) (a dice roller crate)
* [Melody](https://github.com/yoav-lavi/melody)
* [json5-nodes](https://github.com/jlyonsmith/json5-nodes)
* [prisma](https://github.com/prisma/prisma)
## Minimum Supported Rust Version (MSRV)
This library should always compile with default features on **Rust 1.61.0**.
## no_std support
The `pest` and `pest_derive` crates can be built without the Rust standard
library and target embedded environments. To do so, you need to disable
their default features. In your `Cargo.toml`, you can specify it as follows:
```toml
[dependencies]
# ...
pest = { version = "2", default-features = false }
pest_derive = { version = "2", default-features = false }
```
If you want to build these crates in the pest repository's workspace, you can
pass the `--no-default-features` flag to `cargo` and specify these crates using
the `--package` (`-p`) flag. For example:
```bash
$ cargo build --target thumbv7em-none-eabihf --no-default-features -p pest
$ cargo bootstrap
$ cargo build --target thumbv7em-none-eabihf --no-default-features -p pest_derive
```
## Special thanks
A special round of applause goes to prof. Marius Minea for his guidance and all
pest contributors, some of which being none other than my friends.
|
