An HTML parser in Rust that reads pages the way a browser does, finds what you're after with CSS selectors, and lets you change the tree and write it back out.

If you use Lexbor today, directly or through selectolax, PHP's Dom\HTMLDocument, nokolexbor or lazy_html, Parsec is being built for you. Lexbor is fast and follows the spec. What we want to add is everything people keep asking it for: positions on nodes, a worst case that stays linear, documents you can share across threads, XPath, selectors that match everything a static page can decide, and no use-after-free bugs, because the parser is memory-safe Rust.

Parsec isn't released yet. This page says what works today and what's on the way, and the line between the two is kept sharp.

What works today

use parsec::css::SelectorList;

let doc = parsec::parse("<ul><li class=hit>one<li>two</ul>", false);
let hits = SelectorList::parse("li.hit").unwrap().select(&doc, doc.root());
assert_eq!(doc.text(hits[0]), "one");

• The HTML Standard's tokenizer and tree builder. It passes all 7,045 html5lib tokenizer tests and every web-platform-tests tree-construction case that doesn't need JavaScript, run with scripting on and with it off. cargo test runs both suites; the WPT files are pinned in crates/parsec/tests/tree-construction.
• Fragments parsed in the context of any element, the way innerHTML is, SVG and MathML included.
• CSS selectors, Level 4: :has(), :is(), :where(), :not() with lists, :nth-child(… of S), :scope, attribute flags, and the form states (:checked, :disabled, :required and friends).
• A tree you can edit: create, insert, move and remove nodes, set and rename attributes, then serialize as innerHTML, outerHTML or text.
• Limits you choose: a time budget per parse and a cap on input size. The slow-input cases in examples/hostile.rs, like thousands of nested formatting elements (the pattern that runs Lexbor out of memory), take time in proportion to their length.
• One dependency (memchr) and one unsafe block, with its reasoning written beside it.

Speed: Parsec's parser is the one inside Netweir, whose published benchmark pulls every price, title and link from a generated shop page in 3.0 ms where selectolax (Lexbor) takes 5.7 ms, and 31 ms against 64 ms at 10 MB. That run includes Python and the queries, so it isn't a parser-only number. A head-to-head harness against Lexbor's own C, in one process, on the same corpus, is the first thing on the roadmap, and its numbers will replace this paragraph.

What's coming

The roadmap has the order. In short, Parsec is set to cover every module Lexbor has, and then go past it:

	Lexbor	Parsec
HTML parsing, fragments	yes	yes
Bytes in, encoding sniffed	yes	planned
Chunked input	yes	planned
Parse errors with line and column	pointers only	planned
Source positions on nodes and attributes	no	planned
Linear worst case	no (#340)	planned
Thread-safe documents	no	planned, checked at compile time
:scope, :lang(), :dir(), :valid and other static pseudo-classes	parsed, never match	:scope yes, the rest planned
XPath	no	planned
WHATWG Encoding, 40 encodings	yes	planned
WHATWG URL	yes	planned, with WPT pass counts published
CSS parser and CSSOM	yes	planned, with var(), media queries and computed values
C API	yes	planned, with prebuilt libraries for Linux, macOS and Windows
Python, WebAssembly	through bindings	planned, built here

Every "yes" in the Parsec column is something cargo test checks. A "planned" becomes a "yes" only when it ships.

Using it

Parsec isn't on crates.io yet. To try it from git:

[dependencies]
parsec-html = { git = "https://github.com/parsec-html/parsec" }

The package is parsec-html and the crate you use is parsec.

The wiki is where the documentation lives: how parsing works, every option, the selector list, and a guide for moving from Lexbor.

Working on it

git clone --recurse-submodules https://github.com/parsec-html/parsec
cd parsec
cargo test

The parser core is shared with Netweir, so changes to crates/parsec start there. CONTRIBUTING.md explains how that works and how to get a change in. Questions go to Discussions, bugs to Issues.

Licence

Apache-2.0. See LICENSE and NOTICE.