Python解析器Pyparsing入门教程：快速构建解析器

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"《Getting Started with Pyparsing》(2007) 是一本由 Paul McGuire 著的文档，版权属于 O'Reilly Media，出版日期为2007年10月4日。本书旨在介绍如何利用 Pyparsing，这是一套纯Python的解析库，帮助开发者快速构建递归下降解析器，解决从文本文件、网页抓取数据，或创建具有用户自定义命令和搜索功能的灵活应用程序的问题。Pyparsing的独特之处在于其无需手工设计复杂的解析状态机，使得构建HTML解析器、日志数据提取工具以及复杂数据结构处理或命令处理器变得简单。书中内容涵盖了以下几个关键部分： 1. **什么是Pyparsing**：首先介绍了Pyparsing的基本概念，它为何不同于其他解析库，以及它在简化解析任务中的优势。 2. **基础Pyparsing程序示例**：通过一个“Hello, World!”的加强版示例，读者可以了解如何用Pyparsing实现基础的解析任务。 3. **特殊之处**：深入探讨了Pyparsing的高效性和灵活性，特别是如何利用内置的解析动作（Parse Actions）和解析结果（Parse Results）来解析数据。 4. **从表格中提取数据**：讲解如何运用Pyparsing处理表格数据，提取相关信息。 5. **网页数据提取**：演示如何通过Pyparsing从网页中抓取数据，展示了其在实际应用中的实用性。 6. **S-表达式解析**：从简单的S-表达式解析开始，逐步到完整版本的解析器实现，展示了如何处理更复杂的语法结构。 7. **解析搜索字符串**：介绍如何利用Pyparsing解析和处理用户输入的搜索查询。 8. **快速构建搜索引擎**：给出了一个100行代码的简短示例，展示了如何使用Pyparsing来构建一个小型的搜索引擎。 9. **结论**：总结全文，强调Pyparsing对简化编程任务的重要作用，并鼓励读者继续探索其潜力。 10. **索引**：提供了查找特定主题的参考指南。这本书不仅适合经验丰富的Python开发者，也适合那些想要学习如何在Python中高效解析文本和数据的新手，是提高编程效率和扩展应用程序功能的良好资源。"

Yuck! We might as well just have written a character-by-character scanner in the

first

place! Fortunately, we can avoid this drudgery by making our parser a bit

smarter.

Since we know that the salutation and greetee parts of the greeting are logical

groups, we can use pyparsing's Group class to give more structure to the returned

results. By changing the definitions of salutation and greetee to:

salutation = Group( OneOrMore(word) )

greetee = Group( OneOrMore(word) )

our results start to look a bit more organized:

[['Hello'], ',', ['World'], '!']

[['Hi'], ',', ['Mom'], '!']

[['Good', 'morning'], ',', ['Miss', 'Crabtree'], '!']

[['Yo'], ',', ['Adrian'], '!']

[['Whattup'], ',', ['G'], '?']

[["How's", 'it', "goin'"], ',', ['Dude'], '?']

[['Hey'], ',', ['Jude'], '!']

[['Goodbye'], ',', ['Mr.', 'Chips'], '!']

and we can use basic list-to-variable assignment to access the different parts:

for t in tests:

salutation, dummy, greetee, endpunc = greeting.parseString(t)

print salutation, greetee, endpunc

prints:

['Hello'] ['World'] !

['Hi'] ['Mom'] !

['Good', 'morning'] ['Miss', 'Crabtree'] !

['Yo'] ['Adrian'] !

['Whattup'] ['G'] ?

["How's", 'it', "goin'"] ['Dude'] ?

['Hey'] ['Jude'] !

['Goodbye'] ['Mr.', 'Chips'] !

Note that we had to put in the scratch variable dummy to handle the parsed comma

character. The comma is a very important element during parsing, since it shows

where the parser stops reading the salutation and starts the greetee. But in the

returned results, the comma is not really very interesting at all, and it would be

nice to suppress it from the returned results. You can do this by wrapping the

definition of comma in a pyparsing Suppress instance:

comma = Suppress( Literal(",") )

There are actually a number of shortcuts built into pyparsing, and since this func-

tion is so common, any of the following forms accomplish the same thing:

Getting Started with Pyparsing 11

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