PyParsing -- A Python Parsing Module

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Introduction

The pyparsing module is an alternative approach to creating and executing simple grammars, vs. the traditional lex/yacc approach, or the use of regular expressions. The pyparsing module provides a library of classes that client code uses to construct the grammar directly in Python code.

[Since first writing this description of pyparsing in late 2003, this technique for developing parsers has become more widespread, under the name Parsing Expression Grammars - PEGs. See more information on PEGs at https://en.wikipedia.org/wiki/Parsing_expression_grammar .]

Here is a program to parse "Hello, World!" (or any greeting of the form "salutation, addressee!"):

.. code:: python

from pyparsing import Word, alphas
greet = Word(alphas) + "," + Word(alphas) + "!"
hello = "Hello, World!"
print(hello, "->", greet.parseString(hello))

The program outputs the following::

Hello, World! -> ['Hello', ',', 'World', '!']

The Python representation of the grammar is quite readable, owing to the self-explanatory class names, and the use of '+', '|' and '^' operator definitions.

The parsed results returned from parseString() can be accessed as a nested list, a dictionary, or an object with named attributes.

The pyparsing module handles some of the problems that are typically vexing when writing text parsers:

• extra or missing whitespace (the above program will also handle "Hello,World!", "Hello , World !", etc.)
• quoted strings
• embedded comments

The examples directory includes a simple SQL parser, simple CORBA IDL parser, a config file parser, a chemical formula parser, and a four- function algebraic notation parser, among many others.

Documentation

There are many examples in the online docstrings of the classes and methods in pyparsing. You can find them compiled into online docs at https://pyparsing-docs.readthedocs.io/en/latest/. Additional documentation resources and project info are listed in the online GitHub wiki, at https://github.com/pyparsing/pyparsing/wiki. An entire directory of examples is at https://github.com/pyparsing/pyparsing/tree/master/examples.

License

MIT License. See header of pyparsing.py

History

See CHANGES file.

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