1// Copyright 2010 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5/*
6Package html implements an HTML5-compliant tokenizer and parser.
7
8Tokenization is done by creating a Tokenizer for an io.Reader r. It is the
9caller's responsibility to ensure that r provides UTF-8 encoded HTML.
10
11 z := html.NewTokenizer(r)
12
13Given a Tokenizer z, the HTML is tokenized by repeatedly calling z.Next(),
14which parses the next token and returns its type, or an error:
15
16 for {
17 tt := z.Next()
18 if tt == html.ErrorToken {
19 // ...
20 return ...
21 }
22 // Process the current token.
23 }
24
25There are two APIs for retrieving the current token. The high-level API is to
26call Token; the low-level API is to call Text or TagName / TagAttr. Both APIs
27allow optionally calling Raw after Next but before Token, Text, TagName, or
28TagAttr. In EBNF notation, the valid call sequence per token is:
29
30 Next {Raw} [ Token | Text | TagName {TagAttr} ]
31
32Token returns an independent data structure that completely describes a token.
33Entities (such as "<") are unescaped, tag names and attribute keys are
34lower-cased, and attributes are collected into a []Attribute. For example:
35
36 for {
37 if z.Next() == html.ErrorToken {
38 // Returning io.EOF indicates success.
39 return z.Err()
40 }
41 emitToken(z.Token())
42 }
43
44The low-level API performs fewer allocations and copies, but the contents of
45the []byte values returned by Text, TagName and TagAttr may change on the next
46call to Next. For example, to extract an HTML page's anchor text:
47
48 depth := 0
49 for {
50 tt := z.Next()
51 switch tt {
52 case html.ErrorToken:
53 return z.Err()
54 case html.TextToken:
55 if depth > 0 {
56 // emitBytes should copy the []byte it receives,
57 // if it doesn't process it immediately.
58 emitBytes(z.Text())
59 }
60 case html.StartTagToken, html.EndTagToken:
61 tn, _ := z.TagName()
62 if len(tn) == 1 && tn[0] == 'a' {
63 if tt == html.StartTagToken {
64 depth++
65 } else {
66 depth--
67 }
68 }
69 }
70 }
71
72Parsing is done by calling Parse with an io.Reader, which returns the root of
73the parse tree (the document element) as a *Node. It is the caller's
74responsibility to ensure that the Reader provides UTF-8 encoded HTML. For
75example, to process each anchor node in depth-first order:
76
77 doc, err := html.Parse(r)
78 if err != nil {
79 // ...
80 }
81 var f func(*html.Node)
82 f = func(n *html.Node) {
83 if n.Type == html.ElementNode && n.Data == "a" {
84 // Do something with n...
85 }
86 for c := n.FirstChild; c != nil; c = c.NextSibling {
87 f(c)
88 }
89 }
90 f(doc)
91
92The relevant specifications include:
93https://html.spec.whatwg.org/multipage/syntax.html and
94https://html.spec.whatwg.org/multipage/syntax.html#tokenization
95*/
96package html // import "golang.org/x/net/html"
97
98// The tokenization algorithm implemented by this package is not a line-by-line
99// transliteration of the relatively verbose state-machine in the WHATWG
100// specification. A more direct approach is used instead, where the program
101// counter implies the state, such as whether it is tokenizing a tag or a text
102// node. Specification compliance is verified by checking expected and actual
103// outputs over a test suite rather than aiming for algorithmic fidelity.
104
105// TODO(nigeltao): Does a DOM API belong in this package or a separate one?
106// TODO(nigeltao): How does parsing interact with a JavaScript engine?