# plexus

![](https://img.shields.io/badge/written%20in-Go-blue)

A table-driven parser/lexer with no precompilation phase.

Tags: PL

`plexus` is a minimal recursive descent parser and lexer that works entirely at runtime. Enter your grammar using a BNF-like syntax directly into a `map`. The parse function takes the grammar map and your source code as input, to return a tree of nodes. No DFA/NFA lifting is performed.

Rules can be used to customise the capture behaviour. Taking `FOO => BAR BAZ QUUX` as an example, a `FOO` node would only be created if all three child nodes could be parsed sequentially. By adding an RT_RULE into the list, this behaviour can be changed. For instance, `FOO => BAR RT_RULE_ONE_OF_THE_REMAINING BAZ QUUX` will match `BAR BAZ` and `BAR QUUX`. Various rules are supported to emulate plus, star, and optional groupings. Certain types of lookahead can be implemented via the `RT_RULE_NONE_OF_THE_REMAINING` rule. There is no support for backtracking.

By default, the parser skips `\s\t\r\n` characters in-between other tokens; but this can be overwritten at any time via the `WHITESPACE` rule, which specifies the character set to skip.

The project also includes `bnf2plexus`, a program that uses `plexus` to parse grammars in (E)BNF syntax, compiling the resulting AST into Go source code targeting the `plexus` library. At the time of writing, the BNF support was sufficiently complete to compile Python's upstream grammar file into a `plexus` parser.

This is an extended, standalone, Golang version of the C++ parser used by the [pexl-D](https://code.ivysaur.me/pexl-D/) compiler.

## Usage (library)

See `plexus_test.go` for a simple example of parsing arithmetic expressions.

See `cmd/bnf2plexus/bnfparse.go` for a more complex example of parsing BNF using custom rules.

## Usage (bnf2plexus)


```
Usage of bnf2plexus:
  --input filename

```


Output is written to `stdout`.

## Changelog

2017-03-12 0.32
- Initial public release
- [⬇️ plexus-0.32.zip](dist-archive/plexus-0.32.zip) *(7.48 KiB)*