"Hello, World!" on Steroids!
Pyparsing comes with a number of examples, including a basic "Hello, World!" parser. This simple example is also covered in the O'Reilly ONLamp.com article "Building Recursive Descent Parsers with Python" (http://www.onlamp.com/-pub/a/python/2006/01/26/pyparsing.html). In this section, I use this same example to introduce many of the basic parsing tools in pyparsing.
The current "Hello, World!" parsers are limited to greetings of the form:
word, word !
This limits our options a bit, so let's expand the grammar to handle more complicated greetings. Let's say we want to parse any of the following:
Hello, World! Hi, Mom! Good morning, Miss Crabtree! Yo, Adrian! Whattup, G? How's it goin', Dude? Hey, Jude! Goodbye, Mr. Chips!
The first step in writing a parser for these strings is to identify the pattern that they all follow. Following our best practice, we write up this pattern as a BNF. Using ordinary words to describe a greeting, we would say, "a greeting is made up of one or more words (which is the salutation), followed by a comma, followed by one or more additional words (which is the subject of the greeting, or greetee), and ending with either an exclamation point or a question mark." As BNF, this description looks like:
greeting ::= salutation comma greetee endpunc salutation ::= word+ comma ::= , greetee ::= word+ word ::= a collection of one or more characters, which are any alpha or ' or . endpunc ::= ! | ?
This BNF translates almost directly into ...