html-lexer
v0.5.0
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An HTML5 lexer
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An HTML5 lexer for safe template languages
A standard compliant, incremental/ streaming HTML5 lexer.
This is an HTML5 lexer designed to be used a basis for safe and HTML-context aware template languages, IDEs or syntax highlighters. It is different from the other available tokenizers in that it preserves all the information of the input string, e.g. formatting, quotation style and other idiosyncrasies. It does so by producing annotated chunks of the input string rather than the slightly more high level tokens that are described in the specification. However, it does do so in a manner that is compatible with the language defined in the HTML5 specification.
The main motivation for this project is a jarring absence of safe HTML template languages. By safe, I mean that the template placeholders are typed according to their context, and that the template engine ensures that the strings that come to fill the placeholders are automatically and correctly escaped to yield valid HTML.
Usage
The produced tokens are simply tuples (arrays) [type, chunk]
of a token type
and a chunk of the input string.
The lexer has a ‘push parser’ API.
The Lexer
constructor takes as its single argument a delegate object with
methods: write (token)
and end ()
.
Example:
const Lexer = require ('html-lexer')
const delegate = {
write: (token) => console.log (token),
end: () => null
}
const lexer = new Lexer (delegate)
lexer.write ('<h1>Hello, World</h1>')
lexer.end ()
Results in:
[ 'startTagStart', '<' ]
[ 'tagName', 'h1' ]
[ 'tagEnd', '>' ]
[ 'data', 'Hello,' ]
[ 'space', ' ' ]
[ 'data', 'World' ]
[ 'endTagStart', '</' ]
[ 'tagName', 'h1' ]
[ 'tagEnd', '>' ]
The lexer is incremental: delegate.write
will be called as soon as a token is
available and you can split the input across multiple writes:
const lexer = new Lexer (delegate)
lexer.write ('<h')
lexer.write ('1>Hello, W')
lexer.write ('orld</h1>')
lexer.end ()
Token types
The tokens emitted are simple tuples [type, chunk]
.
The type of a token is just a string, and it is one of:
attributeAssign
attributeName
attributeValueData
attributeValueEnd
attributeValueStart
bogusCharRef
charRefDecimal
charRefHex
charRefLegacy
charRefNamed
commentData
commentEndBogus
commentEnd
commentStartBogus
commentStart
data
endTagStart
lessThanSign
uncodedAmpersand
newline
nulls
plaintext
rawtext
rcdata
space
startTagStart
tagEndAutoclose
tagEnd
tagName
tagSpace
The uncodedAmpersand
is emitted for ampersand &
characters that do not start a character reference.
The tagSpace
is emitted for 'space' between attributes in
element tags.
Otherwise the names should be self explanatory.
Limitations
Doctype
The lexer still interprets doctypes as 'bogus comments'.CDATA
The lexer interprets CDATA sections as 'bogus comments'.
(CDATA is only allowed in foreign content - svg and mathml.)Script tags
The lexer interprets script tags as rawtext elements. This has no dire consequences, other than that html begin and end comment tags that may surround it, are not marked as such.
Changelog
0.5.0
The projet has been rewritten to use the fast, hand-written DFA-based lexer, from my related html-parser project. I have been inspired by the techniques described by Sean Barrett on their page about table-driven lexical analyis.
- NB Small changes have been made to the token types:
- The
endTagPrefix
token has been removed: anrcdata
orrawtext
token is emitted instead. - The
bogusCharRef
token has been removed: anuncodedAmpersand
token is emitted for an ampersand&
that does not start a character reference instead. - Stretches of NUL-characters, whitespace, and individual newlines are now emitted as separate tokens of type
nulls
,space
, andnewline
, respectively.
0.4.0
- NB The token types have changed to use a more consistent naming scheme.
- Added a Makefile for building a browser version.
- Added a browser based test page.
License
The source code for this project is licensed under the Mozilla Public License Version 2.0, copyright Alwin Blok 2016–2018, 2020–2021, 2023.