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0.4.4 | Jan 28, 2023 |
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0.4.3 | Jul 3, 2021 |
0.4.2 | Mar 30, 2021 |
0.4.0 | Sep 8, 2019 |
0.2.0 | Nov 11, 2018 |
#35 in Parser tooling
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pidgin
build non-recursive grammars
Pidgin is a Rust library for generating non-recursive grammars which allow one to convert strings to parse trees.
Pidgin's grammars are implemented underlyingly as regular expressions with named matching groups. Rust's regular expression engine is very fast but it has a limitation which makes parsing hierarchically structured patterns with repeating components difficult: you cannot use a particular match name more than once in a pattern. This makes a simple grammar such as
foo -> bar baz | baz bar
bar -> '1'
baz -> '2'
problematic. The foo
rule uses both bar
and baz
twice.
Pidgin allows you to work around this restriction by managing group renaming.
Another disadvantage of regular expressions is that the more expressive they are the more difficult they are to read. Pidgin allows you to construct expressive regular expressions without obscuring your intention.
Example
#![recursion_limit="256"]
#[macro_use]
extern crate pidgin;
fn experiment() -> Result<(), Box<Error>> {
let date = grammar!{
// comments are legal in grammars
(?ibB) // default flags for all rules -- case insensitive and enforce leading and trailing word boundaries
// the master rule; it has multiple alternates
date -> <weekday> (",") <month> <monthday> (",") <year>
date -> <month> <monthday> | <weekday> | <monthday> <month> <year>
date -> <month> <monthday> (",") <year>
date -> <numeric_date>
// sub-rules
numeric_date -> <year> ("/") <numeric_month> ("/") <numeric_day>
numeric_date -> <year> ("-") <numeric_month> ("-") <numeric_day>
numeric_date -> <numeric_month> ("/") <numeric_day> ("/") <year>
numeric_date -> <numeric_month> ("-") <numeric_day> ("-") <year>
numeric_date -> <numeric_day> ("/") <numeric_month> ("/") <year>
numeric_date -> <numeric_day> ("-") <numeric_month> ("-") <year>
year => r(r"\b[12][0-9]{3}|[0-9]{2}\b")
weekday => [
"Sunday Monday Tuesday Wednesday Thursday Friday Saturday"
.split(" ")
.into_iter()
.flat_map(|s| vec![s, &s[0..2], &s[0..3]])
.collect::<Vec<_>>()
]
weekday => (?-i) [["M", "T", "W", "R", "F", "S", "U"]]
monthday => [(1..=31).into_iter().collect::<Vec<_>>()]
numeric_day => [
(1..=31)
.into_iter()
.flat_map(|i| vec![i.to_string(), format!("{:02}", i)])
.collect::<Vec<_>>()
]
month => [
vec![
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December",
].into_iter().flat_map(|s| vec![s, &s[0..3]]).collect::<Vec<_>>()
]
numeric_month => [
(1..=31)
.into_iter()
.flat_map(|i| vec![i.to_string(), format!("{:02}", i)])
.collect::<Vec<_>>()
]
};
// compile this into a matcher
// (uncompiled grammar's can be used as elements of other grammars)
let matcher = date.matcher()?;
// we let whitespace vary
assert!(matcher.is_match(" June 6, 1969 "));
// we made it case-insensitive
assert!(matcher.is_match("june 6, 1969"));
// but we want to respect word boundaries
assert!(!matcher.is_match("jejune 6, 1969"));
// we can inspect the parse tree
let m = matcher.parse("2018/10/6").unwrap();
assert!(m.name("numeric_date").is_some());
assert_eq!(m.name("year").unwrap().as_str(), "2018");
let m = matcher.parse("Friday").unwrap();
assert!(!m.name("numeric_date").is_some());
assert!(m.name("weekday").is_some());
// still more crazy things we allow
assert!(matcher.is_match("F"));
assert!(matcher.is_match("friday"));
assert!(matcher.is_match("Fri"));
// but we said single-letter days had to be capitalized
assert!(!matcher.is_match("f"));
Ok(())
}
Limitations
Recursion
Because you must define sub-grammars before you use them, and because Rust's regular expression engine and formalism cannot represent it in any case, you cannot produce a recursive grammar in Pidgin. Something like this is impossible:
XP -> XP conj XP
Named Captures in Regex Elements
Pidgin allows you to provide regular expression elements to rules. Such an element
is a Rust expression which may be converted to a String
via to_string
bracketed
in the grammar!
macro body by r(
and )
, so, for example r(r"\d+")
. Regex
elements allow you to insert regex expressions into the grammar that the grammar formalism
doesn't provide itself, like anchors and character classes.
Rust's regular expression engine does not allow the reuse of names in named capturing
groups, however. (?<foo>bar) baz (?<foo>qux)
won't compile into a usable regex because
the name "foo" is reused. The following grammar produces just this situation:
grammar!{
TOP -> <foo> ("bar") <foo>
foo => r("(?P<baz>foo)")
};
The solution is to avoid named capturing groups in grammar regex elements. The grammar itself will handle the named capturing, so you should never need to do this.
Benchmarks
The principle motivation behind Pidgin is simply to produce something like an
abstract syntax tree one can use to better understand matched text, but the
regular expressions its [vec]
elements generate generally match as well as or better than
naively constructed regular expressions represented as alternations of
expressions provided the expressions are bounded. "Bounded" here means the
expression must match from the first character of the string to the last. An
unbounded expression simply must match somewhere in the string.
// good
let rx = regex::Regex::new("\A(?:cat|camel|canteloupe)\z").unwrap();
// better!
let rx = grammar!{ TOP => r("\A") [["cat", "camel", "canteloupe"]] r("\z") }.rx().unwrap();
In the unbounded case, the naive regular expression performs better.
There is a simple benchmark suite included in the benches/
directory demonstrating this. The suite compares matching
speed for "naive" regular expressions of the form foo|bar|baz
to those
generated by Pidgin, which involve no backtracking and, in this case, would
be something like foo|ba[rz]
. The benchmark suite compares non-matching to
matching and bounded to unbounded patterns. The mean match/non-match times for these 8
cases were
pidgin | match | bounded | time |
---|---|---|---|
✓ | ✓ | ✓ | 1.0029 ms |
✓ | ✓ | 7.3446 ms | |
✓ | ✓ | 151.75 µs | |
✓ | 1.3434 ms | ||
✓ | ✓ | 14.349 ms | |
✓ | 159.93 µs | ||
✓ | 17.577 ms | ||
53.637 µs |
The better time is bolded for each pair.
The full report can be generated by running
cargo bench
Full API
The complete Pidgin API is available at https://docs.rs/pidgin/.
Acknowledgments
In writing this I received considerable advice from TurkeyMcMac.
Dependencies
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