Beginning Python - From Novice to Professional

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CHAPTER 10 ■ BATTERIES INCLUDED 235 This interaction isn’t terribly interesting. I could have done exactly the same thing with an ordinary dictionary instead of the shelf object. But now that I’ve quit the program, let’s see what happens when I restart it—perhaps the following day? Enter command (? for help): lookup Enter ID number: 001 What would you like to know? (name, age, phone) name Name: Mr. Gumby Enter command (? for help): quit As you can see, the program reads in the file I created the first time, and Mr. Gumby is still there! Feel free to experiment with this program, and see if you can extend its functionality and improve its user-friendliness. Perhaps you can think of a version that you have use for yourself? How about a database of your record collection? Or a database to help you keep track of which friends have borrowed which of your books? (I know I could use that last one.) re Some people, when confronted with a problem, think “I know, I’ll use regular expressions.” Now they have two problems. —Jamie Zawinski The re module contains support for regular expressions. If you’ve heard about regular expressions before, you probably know how powerful they are; if you haven’t, prepare to be amazed. You should note, however, that mastering regular expressions may be a bit tricky at first. (Okay, very tricky, actually.) The key is to learn about them a little bit at a time—just look up (in the documentation) the parts you need for a specific task. There is no point in memorizing it all up front. This section describes the main features of the re module and regular expressions, and enables you to get started. ■Tip In addition to the standard documentation, Andrew Kuchling’s “Regular Expression HOWTO” (http://amk.ca/python/howto/regex/) is a useful source of information on regular expressions in Python. What Is a Regular Expression? A regular expression (also called a regex or regexp) is a pattern that can match a piece of text. The simplest form of regular expression is just a plain string, which matches itself. In other words, the regular expression 'python' matches the string 'python'. You can use this matching behavior for such things as searching for patterns in a text, for replacing certain patterns with some computed values, or for splitting a text into pieces.
236 CHAPTER 10 ■ BATTERIES INCLUDED The Wildcard A regexp can match more than one string, and you create such a pattern by using some special characters. For example, the period character (dot) matches any character (except a newline), so the regular expression '.ython' would match both the string 'python' and the string 'jython'. It would also match strings such as 'qython', '+ython', or ' ython' (in which the first letter is a single space), but not strings such as 'cpython' or 'ython' because the period matches a single letter, and neither two nor zero. Because it matches “anything” (any single character except a newline), the period is called a wildcard. Escaping Special Characters When you use special characters such as this, it’s important to know that you may run into problems if you try to use them as normal characters. For example, imagine you want to match the string 'python.org'. Do you simply use the pattern 'python.org'? You could, but that would also match 'pythonzorg', for example, which you probably wouldn’t want. (The dot matches any character except newline, remember?) To make a special character behave like a normal one, you escape it, just as I demonstrated how to escape quotes in strings in Chapter 1. You place a backslash in front of it. Thus, in this example, you would use 'python\\.org', which would match 'python.org', and nothing else. ■Note To get a single backslash, which is required here by the re module, you need to write two backslashes in the string—to escape it from the interpreter. Thus you have two levels of escaping here: (1) from the interpreter, and (2) from the re module. (Actually, in some cases you can get away with using a single backslash and have the interpreter escape it for you automatically, but don’t rely on it.) If you are tired of doubling up backslashes, use a raw string, such as r'python\.org'. Character Sets Matching any character can be useful, but sometimes you want more control. You can create a so-called character set by enclosing a substring in brackets. Such a character set will match any of the characters it contains, so '[pj]ython' would match both 'python' and 'jython', but nothing else. You can also use ranges, such as '[a-z]' to match any character from a to z (alphabetically), and you can combine such ranges by putting one after another, such as '[a-zA-Z0-9]' to match uppercase and lowercase letters and digits. (Note that the character set will match only one such character, though.) To invert the character set, put the character ^ first, as in '[^abc]' to match any character except a, b, or c.
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Beginning Python From Novice to Pro
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For Ranveig
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■CHAPTER 23 Project 4: In the New
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viii ■CONTENTS Strings . . . . .
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xxii ■CONTENTS Preparations . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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82 CHAPTER 5 ■ CONDITIONALS, LOOP
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110 CHAPTER 6 ■ ABSTRACTION But w
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140 CHAPTER 7 ■ MORE ABSTRACTION
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CHAPTER 9 ■ ■ ■ Magic Methods
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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314 CHAPTER 15 ■ PYTHON AND THE W
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358 CHAPTER 17 ■ EXTENDING PYTHON
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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■INDEX 595 Python C API 365 hand-
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■INDEX 603 further exploration 47
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