7 pat t e r n m at c h I n g w I t h r e g u L a r e X p r e s s I o n s

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7
PAT T E R N M AT C H I N G W I T H 
R E G U L A R E X P R E S S I O N S
You may be familiar with searching for text 
by pressing 
ctrl
-F and entering the words 
you’re looking for. Regular expressions go one 
step further: they allow you to specify a pattern of 
text to search for. You may not know a business’s exact 
phone number, but if you live in the United States or
Canada, you know it will be three digits, followed by a hyphen, and then 
four more digits (and optionally, a three-digit area code at the start). This 
is how you, as a human, know a phone number when you see it: 415-555-
1234 is a phone number, but 4,155,551,234 is not. 
We also recognize all sorts of other text patterns every day: email 
addresses have @ symbols in the middle, US social security numbers have 
nine digits and two hyphens, website URLs often have periods and forward 
slashes, news headlines use title case, social media hashtags begin with # 
and contain no spaces, and more.

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162
Chapter 7
Regular expressions are helpful, but few non-programmers know about 
them even though most modern text editors and word processors, such as 
Microsoft Word or OpenOffice, have find and find-and-replace features 
that can search based on regular expressions. Regular expressions are 
huge time-savers, not just for software users but also for programmers. In 
fact, tech writer Cory Doctorow argues that we should be teaching regular 
expressions even before programming: 
Knowing [regular expressions] can mean the difference between 
solving a problem in 3 steps and solving it in 3,000 steps. When 
you’re a nerd, you forget that the problems you solve with a cou-
ple keystrokes can take other people days of tedious, error-prone 
work to slog through.
1
In this chapter, you’ll start by writing a program to find text patterns with-
out using regular expressions and then see how to use regular expressions to 
make the code much less bloated. I’ll show you basic matching with regular 
expressions and then move on to some more powerful features, such as string 
substitution and creating your own character classes. Finally, at the end of the 
chapter, you’ll write a program that can automatically extract phone numbers 
and email addresses from a block of text.