html

Documentation

Overview

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 ErrorToken:
		return z.Err()
	case TextToken:
		if depth > 0 {
			// emitBytes should copy the []byte it receives,
			// if it doesn't process it immediately.
			emitBytes(z.Text())
		}
	case StartTagToken, EndTagToken:
		tn, _ := z.TagName()
		if len(tn) == 1 && tn[0] == 'a' {
			if tt == 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: http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html and http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html

Index

• func EscapeString(s string) string
• func Render(w io.Writer, n *Node) error
• func UnescapeString(s string) string
• type Attribute
• type Node
  • func Parse(r io.Reader) (*Node, error)
  • func ParseFragment(r io.Reader, context *Node) ([]*Node, error)
  • func (n *Node) AppendChild(c *Node)
  • func (n *Node) InsertBefore(newChild, oldChild *Node)
  • func (n *Node) RemoveChild(c *Node)
• type NodeType
• type Token
  • func (t Token) String() string
• type TokenType
  • func (t TokenType) String() string
• type Tokenizer
  • func NewTokenizer(r io.Reader) *Tokenizer
  • func NewTokenizerFragment(r io.Reader, contextTag string) *Tokenizer
  • func (z *Tokenizer) AllowCDATA(allowCDATA bool)
  • func (z *Tokenizer) Err() error
  • func (z *Tokenizer) Next() TokenType
  • func (z *Tokenizer) NextIsNotRawText()
  • func (z *Tokenizer) Raw() []byte
  • func (z *Tokenizer) TagAttr() (key, val []byte, moreAttr bool)
  • func (z *Tokenizer) TagName() (name []byte, hasAttr bool)
  • func (z *Tokenizer) Text() []byte
  • func (z *Tokenizer) Token() Token

Examples

• Parse

Functions

func EscapeString

func EscapeString(s string) string

EscapeString escapes special characters like "<" to become "&lt;". It escapes only five such characters: <, >, &, ' and ". UnescapeString(EscapeString(s)) == s always holds, but the converse isn't always true.

func Render

func Render(w io.Writer, n *Node) error

Render renders the parse tree n to the given writer.

Rendering is done on a 'best effort' basis: calling Parse on the output of Render will always result in something similar to the original tree, but it is not necessarily an exact clone unless the original tree was 'well-formed'. 'Well-formed' is not easily specified; the HTML5 specification is complicated.

Calling Parse on arbitrary input typically results in a 'well-formed' parse tree. However, it is possible for Parse to yield a 'badly-formed' parse tree. For example, in a 'well-formed' parse tree, no <a> element is a child of another <a> element: parsing "<a><a>" results in two sibling elements. Similarly, in a 'well-formed' parse tree, no <a> element is a child of a <table> element: parsing "<p><table><a>" results in a <p> with two sibling children; the <a> is reparented to the <table>'s parent. However, calling Parse on "<a><table><a>" does not return an error, but the result has an <a> element with an <a> child, and is therefore not 'well-formed'.

Programmatically constructed trees are typically also 'well-formed', but it is possible to construct a tree that looks innocuous but, when rendered and re-parsed, results in a different tree. A simple example is that a solitary text node would become a tree containing <html>, <head> and <body> elements. Another example is that the programmatic equivalent of "a<head>b</head>c" becomes "<html><head><head/><body>abc</body></html>".

func UnescapeString

func UnescapeString(s string) string

UnescapeString unescapes entities like "&lt;" to become "<". It unescapes a larger range of entities than EscapeString escapes. For example, "&aacute;" unescapes to "á", as does "&#225;" and "&xE1;". UnescapeString(EscapeString(s)) == s always holds, but the converse isn't always true.

Types

type Attribute

type Attribute struct {
	Namespace, Key, Val string
}

An Attribute is an attribute namespace-key-value triple. Namespace is non-empty for foreign attributes like xlink, Key is alphabetic (and hence does not contain escapable characters like '&', '<' or '>'), and Val is unescaped (it looks like "a<b" rather than "a&lt;b").

Namespace is only used by the parser, not the tokenizer.

type Node

type Node struct {
	Parent, FirstChild, LastChild, PrevSibling, NextSibling *Node

	Type      NodeType
	DataAtom  atom.Atom
	Data      string
	Namespace string
	Attr      []Attribute
}

A Node consists of a NodeType and some Data (tag name for element nodes, content for text) and are part of a tree of Nodes. Element nodes may also have a Namespace and contain a slice of Attributes. Data is unescaped, so that it looks like "a<b" rather than "a&lt;b". For element nodes, DataAtom is the atom for Data, or zero if Data is not a known tag name.

An empty Namespace implies a "http://www.w3.org/1999/xhtml" namespace. Similarly, "math" is short for "http://www.w3.org/1998/Math/MathML", and "svg" is short for "http://www.w3.org/2000/svg".

func Parse

func Parse(r io.Reader) (*Node, error)

Parse returns the parse tree for the HTML from the given Reader. The input is assumed to be UTF-8 encoded.

Example

package main

import (
	"fmt"
	"log"
	"strings"

	"code.google.com/p/go.net/html"
)

func main() {
	s := `<p>Links:</p><ul><li><a href="foo">Foo</a><li><a href="/bar/baz">BarBaz</a></ul>`
	doc, err := html.Parse(strings.NewReader(s))
	if err != nil {
		log.Fatal(err)
	}
	var f func(*html.Node)
	f = func(n *html.Node) {
		if n.Type == html.ElementNode && n.Data == "a" {
			for _, a := range n.Attr {
				if a.Key == "href" {
					fmt.Println(a.Val)
					break
				}
			}
		}
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			f(c)
		}
	}
	f(doc)
}

Output:

foo
/bar/baz

func ParseFragment

func ParseFragment(r io.Reader, context *Node) ([]*Node, error)

ParseFragment parses a fragment of HTML and returns the nodes that were found. If the fragment is the InnerHTML for an existing element, pass that element in context.

func (*Node) AppendChild

func (n *Node) AppendChild(c *Node)

AppendChild adds a node c as a child of n.

It will panic if c already has a parent or siblings.

func (*Node) InsertBefore

func (n *Node) InsertBefore(newChild, oldChild *Node)

InsertBefore inserts newChild as a child of n, immediately before oldChild in the sequence of n's children. oldChild may be nil, in which case newChild is appended to the end of n's children.

It will panic if newChild already has a parent or siblings.

func (*Node) RemoveChild

func (n *Node) RemoveChild(c *Node)

RemoveChild removes a node c that is a child of n. Afterwards, c will have no parent and no siblings.

It will panic if c's parent is not n.

type NodeType

type NodeType uint32

A NodeType is the type of a Node.

const (
	ErrorNode NodeType = iota
	TextNode
	DocumentNode
	ElementNode
	CommentNode
	DoctypeNode
)

type Token

type Token struct {
	Type     TokenType
	DataAtom atom.Atom
	Data     string
	Attr     []Attribute
}

A Token consists of a TokenType and some Data (tag name for start and end tags, content for text, comments and doctypes). A tag Token may also contain a slice of Attributes. Data is unescaped for all Tokens (it looks like "a<b" rather than "a&lt;b"). For tag Tokens, DataAtom is the atom for Data, or zero if Data is not a known tag name.

func (Token) String

func (t Token) String() string

String returns a string representation of the Token.

type TokenType

type TokenType uint32

A TokenType is the type of a Token.

const (
	// ErrorToken means that an error occurred during tokenization.
	ErrorToken TokenType = iota
	// TextToken means a text node.
	TextToken
	// A StartTagToken looks like <a>.
	StartTagToken
	// An EndTagToken looks like </a>.
	EndTagToken
	// A SelfClosingTagToken tag looks like <br/>.
	SelfClosingTagToken
	// A CommentToken looks like <!--x-->.
	CommentToken
	// A DoctypeToken looks like <!DOCTYPE x>
	DoctypeToken
)

func (TokenType) String

func (t TokenType) String() string

String returns a string representation of the TokenType.

type Tokenizer

type Tokenizer struct {
	// contains filtered or unexported fields
}

A Tokenizer returns a stream of HTML Tokens.

func NewTokenizer

func NewTokenizer(r io.Reader) *Tokenizer

NewTokenizer returns a new HTML Tokenizer for the given Reader. The input is assumed to be UTF-8 encoded.

func NewTokenizerFragment

func NewTokenizerFragment(r io.Reader, contextTag string) *Tokenizer

NewTokenizerFragment returns a new HTML Tokenizer for the given Reader, for tokenizing an exisitng element's InnerHTML fragment. contextTag is that element's tag, such as "div" or "iframe".

For example, how the InnerHTML "a<b" is tokenized depends on whether it is for a <p> tag or a <script> tag.

The input is assumed to be UTF-8 encoded.

func (*Tokenizer) AllowCDATA

func (z *Tokenizer) AllowCDATA(allowCDATA bool)

AllowCDATA sets whether or not the tokenizer recognizes <![CDATA[foo]]> as the text "foo". The default value is false, which means to recognize it as a bogus comment "<!-- [CDATA[foo]] -->" instead.

Strictly speaking, an HTML5 compliant tokenizer should allow CDATA if and only if tokenizing foreign content, such as MathML and SVG. However, tracking foreign-contentness is difficult to do purely in the tokenizer, as opposed to the parser, due to HTML integration points: an <svg> element can contain a <foreignObject> that is foreign-to-SVG but not foreign-to- HTML. For strict compliance with the HTML5 tokenization algorithm, it is the responsibility of the user of a tokenizer to call AllowCDATA as appropriate. In practice, if using the tokenizer without caring whether MathML or SVG CDATA is text or comments, such as tokenizing HTML to find all the anchor text, it is acceptable to ignore this responsibility.

func (*Tokenizer) Err

func (z *Tokenizer) Err() error

Err returns the error associated with the most recent ErrorToken token. This is typically io.EOF, meaning the end of tokenization.

func (*Tokenizer) Next

func (z *Tokenizer) Next() TokenType

Next scans the next token and returns its type.

func (*Tokenizer) NextIsNotRawText

func (z *Tokenizer) NextIsNotRawText()

NextIsNotRawText instructs the tokenizer that the next token should not be considered as 'raw text'. Some elements, such as script and title elements, normally require the next token after the opening tag to be 'raw text' that has no child elements. For example, tokenizing "<title>a<b>c</b>d</title>" yields a start tag token for "<title>", a text token for "a<b>c</b>d", and an end tag token for "</title>". There are no distinct start tag or end tag tokens for the "<b>" and "</b>".

This tokenizer implementation will generally look for raw text at the right times. Strictly speaking, an HTML5 compliant tokenizer should not look for raw text if in foreign content: <title> generally needs raw text, but a <title> inside an <svg> does not. Another example is that a <textarea> generally needs raw text, but a <textarea> is not allowed as an immediate child of a <select>; in normal parsing, a <textarea> implies </select>, but one cannot close the implicit element when parsing a <select>'s InnerHTML. Similarly to AllowCDATA, tracking the correct moment to override raw-text- ness is difficult to do purely in the tokenizer, as opposed to the parser. For strict compliance with the HTML5 tokenization algorithm, it is the responsibility of the user of a tokenizer to call NextIsNotRawText as appropriate. In practice, like AllowCDATA, it is acceptable to ignore this responsibility for basic usage.

Note that this 'raw text' concept is different from the one offered by the Tokenizer.Raw method.

func (*Tokenizer) Raw

func (z *Tokenizer) Raw() []byte

Raw returns the unmodified text of the current token. Calling Next, Token, Text, TagName or TagAttr may change the contents of the returned slice.

func (*Tokenizer) TagAttr

func (z *Tokenizer) TagAttr() (key, val []byte, moreAttr bool)

TagAttr returns the lower-cased key and unescaped value of the next unparsed attribute for the current tag token and whether there are more attributes. The contents of the returned slices may change on the next call to Next.

func (*Tokenizer) TagName

func (z *Tokenizer) TagName() (name []byte, hasAttr bool)

TagName returns the lower-cased name of a tag token (the `img` out of `<IMG SRC="foo">`) and whether the tag has attributes. The contents of the returned slice may change on the next call to Next.

func (*Tokenizer) Text

func (z *Tokenizer) Text() []byte

Text returns the unescaped text of a text, comment or doctype token. The contents of the returned slice may change on the next call to Next.

func (*Tokenizer) Token

func (z *Tokenizer) Token() Token

Token returns the next Token. The result's Data and Attr values remain valid after subsequent Next calls.