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The livelex lexer

Project description

This module parses text into tokens, and is able to reparse only modified parts of the text, using the earlier generated tokens. Tokenized text lives in a tree structure with powerful quering methods for finding tokens and contexts.

The livelex module is designed to be fast, and can tokenize in a background thread, so that even when using very large documents, GUI applications that need to be responsive do not grind to a halt.

Main use case: syntax highlighting in text editors, but also understanding the meaning of text to be able to provided context sensitive editing features.

The livelex module is written and maintained by Wilbert Berendsen.

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The module is designed to parse text using rules, which are regular-expression based. Rules are grouped into lexicons, and lexicons are grouped into a Language object. Every lexicon has its own set of rules that describe the text that is expected in that context.

A rule consists of three parts: a pattern, an action and a target.

  • The pattern is a either a regular expression string, or an object that inherits Pattern. In that case its build() method is called to get the pattern. If the pattern matches, a match object is created. If not, the next rule is tried.

  • The action can be any object, and is streamed together with the matched part of the text. It can be seen as a token. If the action is an instance of DynamicAction, its filter_actions() method is called, which can yield zero or more tokens. The special skip action skips the matching text.

  • The target is a list of objects, which can be integer numbers or references to a different lexicon. A positive number pushes the same lexicon on the stack, while a negative number pops the current lexicon(s) off the stack, so that lexing the text continues with a previous lexicon. It is also possible to pop a lexicon and push a different one.

    Instead of a list of objects, a DynamicTarget object can also be used, which can change the target based on the match object.

Using a special rule, a lexicon may specify a default action, which is streamed with text that is not recognized by any other rule in the lexicon. A lexicon may also specify a default target, which is chosen when no rule matches the current text.


Parsing (better: lexing) text always starts in a lexicon, which is called the root lexicon. The rules in that lexicon are tried one by one. As soon as there is a match, a Token is generated with the matching text, the position of the text and the action that was specified in the rule. And if a target was specified, parsing continues in a different lexicon.

The tokens are put in a tree structure. Every active lexicon creates a Context list that holds the tokens and child contexts. If a target pops back to a previous lexicon, the previous context becomes the current one again.

All tokens and contexts point to their parents, so it is possible to manipulate and query the tree structure in various ways.

The structure of the tree is built by the TreeBuilder, see the tree and the treebuilder module. At the root is the Context carrying the root lexicon. The root context contains Tokens and/or other Contexts.

The TreeBuilder is capable of tokenizing the text in a background thread and also to rebuild just a changed part of the text, smartly reusing earlier generated tokens if possible.

Iterating and Querying

Both Token and Context have many methods for iterating over the tree, for getting at the parent, child or sibling nodes. Context has various find() methods to quickly find a token or context at a certain position in the text.

Using the Context.query property you can build XPath-like chains of filtering queries to quickly find tokens or contexts based on text, action or lexicon. This is described in the query module.


Here are some examples of how to create a Language class and then use it:

import livelex

from livelex import (
    Language, lexicon,
    words, bygroup, bymatch, bytext,

class MyLang(Language):
    """A Language represents a set of Lexicons comprising a specific language.

    A Language is never instantiated. The class itself serves as a namespace
    and can be inherited from.


    def root(cls):
        yield r'"', "string", cls.string
        yield r'\(', "paren", cls.parenthesized
        yield r'\d+', "number"
        yield r'%', "comment", cls.comment
        yield r'[,.!?]', "punctuation"
        yield r'\w+', "word"

    def string(cls):
        yield r'\\[\\"]', 'string escape'
        yield r'"', "string", -1
        yield default_action, "string"

    def comment(cls):
        yield r'$', "comment", -1
        yield r'XXX|TODO', "todo"
        yield default_action, "comment"

    def parenthesized(cls):
        yield r'\)', "paren", -1
        yield from cls.root()

s = r"""
This is (an example) text with 12 numbers
and "a string with \" escaped characters",
and a % comment that TODO lasts until the end
of the line.

>>> import livelex
>>> tree = livelex.root(MyLang.root, s)
>>> tree.dump()
<Context MyLang.root at 1-144 (20 children)>
 ├╴<Token 'This' at 1 (word)>
 ├╴<Token 'is' at 6 (word)>
 ├╴<Token '(' at 9 (paren)>
 ├╴<Context MyLang.parenthesized at 10-21 (3 children)>
   ├╴<Token 'an' at 10 (word)>
   ├╴<Token 'example' at 13 (word)>
   ╰╴<Token ')' at 20 (paren)>
 ├╴<Token 'text' at 22 (word)>
 ├╴<Token 'with' at 27 (word)>
 ├╴<Token '12' at 32 (number)>
 ├╴<Token 'numbers' at 35 (word)>
 ├╴<Token 'and' at 43 (word)>
 ├╴<Token '"' at 47 (string)>
 ├╴<Context MyLang.string at 48-84 (4 children)>
   ├╴<Token 'a string with ' at 48 (string)>
   ├╴<Token '\\"' at 62 (string escape)>
   ├╴<Token ' escaped characters' at 64 (string)>
   ╰╴<Token '"' at 83 (string)>
 ├╴<Token ',' at 84 (punctuation)>
 ├╴<Token 'and' at 86 (word)>
 ├╴<Token 'a' at 90 (word)>
 ├╴<Token '%' at 92 (comment)>
 ├╴<Context MyLang.comment at 93-131 (3 children)>
   ├╴<Token ' comment that ' at 93 (comment)>
   ├╴<Token 'TODO' at 107 (todo)>
   ╰╴<Token ' lasts until the end' at 111 (comment)>
 ├╴<Token 'of' at 132 (word)>
 ├╴<Token 'the' at 135 (word)>
 ├╴<Token 'line' at 139 (word)>
 ╰╴<Token '.' at 143 (punctuation)>
>>> tree.find_token(50)
<Token 'a string with ' at 48 (string)>
>>> tree.find_token(50).parent
<Context MyLang.string at 48-84 (4 children)>

>>> d = livelex.Document(MyLang.root, s)
>>> d
<Document '\nThis is (an example) text w...'>

>>> print(d.text())

This is (an example) text with 12 numbers
and "a string with \" escaped characters",
and a % comment that TODO lasts until the end
of the line.

>>> d[50:56]
>>> with d:
...  d[9:12] = '(a "much longer'
...  d[20:20] = '"'
>>> print(d.text())

This is (a "much longer example") text with 12 numbers
and "a string with \" escaped characters",
and a % comment that TODO lasts until the end
of the line.

>>> d.get_root()[3].dump()
<Context MyLang.parenthesized at 10-34 (4 children)>
 ├╴<Token 'a' at 10 (word)>
 ├╴<Token '"' at 12 (string)>
 ├╴<Context MyLang.string at 13-33 (2 children)>
   ├╴<Token 'much longer example' at 13 (string)>
   ╰╴<Token '"' at 32 (string)>
 ╰╴<Token ')' at 33 (paren)>

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