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Introduction to Combinatorial Parsing in Python

Elliot Cameron | http://3noch.com

What is parsing?

Let's start with a basic example of parsing.

# Get the user's name and parse his first and last name.
your_name = input('Enter your full name: ')

parts_of_name = your_name.split(' ')
first_name = parts_of_name[0]
last_name = parts_of_name[-1]

print('Parsed name:', parts_of_name)
print('Hi Mr.', last_name)

What did we do?

We took raw input and turned it into something that our program can understand and use intelligently.

Other examples:

  • The Python interpreter has a parser to turn my textual code into something it can execute.
  • The Markdown parser understands how to turn my text into something pretty.

Grammar

Since we're transforming one thing (text) into another (meaningful data), we can describe this transformation as a set of rules, called a grammar.

Here's a grammar for a person's name, written in Backus-Nuar Form (BNF), a common format for grammars:

<name>   ::= <word> " " <word>
<word>   ::= <letter><word> | ""
<letter> ::= "a" | "b" | "c" ...  # all the permissible symbols in a word
  • A name is defined by two words, separated by a space.
  • A word is defined recursively. I'll explain.
  • A letter is any one of many possible symbols.