html/doc.go

// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package html implements an HTML5-compliant tokenizer and parser.

Tokenization is done by creating a Tokenizer for an io.Reader r. It is the
caller's responsibility to ensure that r provides UTF-8 encoded HTML.

	z := html.NewTokenizer(r)

Given a Tokenizer z, the HTML is tokenized by repeatedly calling z.Next(),
which parses the next token and returns its type, or an error:

	for {
		tt := z.Next()
		if tt == html.ErrorToken {
			// ...
			return ...
		}
		// Process the current token.
	}

There are two APIs for retrieving the current token. The high-level API is to
call Token; the low-level API is to call Text or TagName / TagAttr. Both APIs
allow optionally calling Raw after Next but before Token, Text, TagName, or
TagAttr. In EBNF notation, the valid call sequence per token is:

	Next {Raw} [ Token | Text | TagName {TagAttr} ]

Token returns an independent data structure that completely describes a token.
Entities (such as "<") are unescaped, tag names and attribute keys are
lower-cased, and attributes are collected into a []Attribute. For example:

	for {
		if z.Next() == html.ErrorToken {
			// Returning io.EOF indicates success.
			return z.Err()
		}
		emitToken(z.Token())
	}

The low-level API performs fewer allocations and copies, but the contents of
the []byte values returned by Text, TagName and TagAttr may change on the next
call to Next. For example, to extract an HTML page's anchor text:

	depth := 0
	for {
		tt := z.Next()
		switch tt {
		case html.ErrorToken:
			return z.Err()
		case html.TextToken:
			if depth > 0 {
				// emitBytes should copy the []byte it receives,
				// if it doesn't process it immediately.
				emitBytes(z.Text())
			}
		case html.StartTagToken, html.EndTagToken:
			tn, _ := z.TagName()
			if len(tn) == 1 && tn[0] == 'a' {
				if tt == html.StartTagToken {
					depth++
				} else {
					depth--
				}
			}
		}
	}

Parsing is done by calling Parse with an io.Reader, which returns the root of
the parse tree (the document element) as a *Node. It is the caller's
responsibility to ensure that the Reader provides UTF-8 encoded HTML. For
example, to process each anchor node in depth-first order:

	doc, err := html.Parse(r)
	if err != nil {
		// ...
	}
	var f func(*html.Node)
	f = func(n *html.Node) {
		if n.Type == html.ElementNode && n.Data == "a" {
			// Do something with n...
		}
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			f(c)
		}
	}
	f(doc)

The relevant specifications include:
https://html.spec.whatwg.org/multipage/syntax.html and
https://html.spec.whatwg.org/multipage/syntax.html#tokenization

# Security Considerations

Care should be taken when parsing and interpreting HTML, whether full documents
or fragments, within the framework of the HTML specification, especially with
regard to untrusted inputs.

This package provides both a tokenizer and a parser, which implement the
tokenization, and tokenization and tree construction stages of the WHATWG HTML
parsing specification respectively. While the tokenizer parses and normalizes
individual HTML tokens, only the parser constructs the DOM tree from the
tokenized HTML, as described in the tree construction stage of the
specification, dynamically modifying or extending the docuemnt's DOM tree.

If your use case requires semantically well-formed HTML documents, as defined by
the WHATWG specification, the parser should be used rather than the tokenizer.

In security contexts, if trust decisions are being made using the tokenized or
parsed content, the input must be re-serialized (for instance by using Render or
Token.String) in order for those trust decisions to hold, as the process of
tokenization or parsing may alter the content.
*/
package html // import "golang.org/x/net/html"

// The tokenization algorithm implemented by this package is not a line-by-line
// transliteration of the relatively verbose state-machine in the WHATWG
// specification. A more direct approach is used instead, where the program
// counter implies the state, such as whether it is tokenizing a tag or a text
// node. Specification compliance is verified by checking expected and actual
// outputs over a test suite rather than aiming for algorithmic fidelity.

// TODO(nigeltao): Does a DOM API belong in this package or a separate one?
// TODO(nigeltao): How does parsing interact with a JavaScript engine?