After reading my previous post, you should have a pretty good understanding of what a BNF definition is all about. Let’s put this theory into practice, and write some basic parsers in Python, using Pyparsing!
Pyparsing allows a pretty one-to-one mapping of BNF to Python code: you can define sets and combinations, then parse any text fragment against it. This is something very important to notice: one basic BNF definition can (and should) be reused: if you once wrote a BNF definition for an integer value, you can easily reuse this definition in, eg, a basic integer math expression.
The most basic element using Pyparsing is a Word. In it’s most basic form this is a set of characters which will match any arbitrary length string, as long as the characters in this string are part of the Word character set.
A little introduction example: let’s write a parser which accepts words consisting of small-cap characters, or sentences which consist of words separated by spaces. First we define a formal definition using BNF:
