C# 5.0 Programmer's Reference

21
Regular Expressions
What’s in This Chapter
➤➤
➤➤
➤➤
Regular expression syntax
Using regular expressions to detect matches, find matches, and
make replacements
Using regular expressions to parse input
Wrox.com Downloads for This Chapter
Please note that all the code examples for this chapter are available as a part of this chapter’s
code download on the book’s website at www.wrox.com/go/csharp5programmersref on the
Download Code tab.
Many applications enable the user to type information but the information should match
some sort of pattern. For example, the string 784-36λ9 is not a valid phone number and
Rod@Stephens@C#Helper.com is not a valid e‐mail address.
One approach for validating this kind of input is to use string methods. You could use the string
class’s IndexOf, LastIndexOf, Substring, and other methods to break the input apart and see if
the pieces make sense. For all but the simplest situations, however, that would be a huge amount
of work.
Regular expressions provide another method for verifying that the user’s input matches a
pattern. A regular expression is a string that contains characters that define a pattern. For
example, the regular expression ^\d{3}-\d{4}$ represents a pattern that matches three
digits followed by a hyphen followed by four more digits as in 123-4567. (This isn’t a great
pattern for matching U.S. phone numbers because it enables many invalid combinations such
as 111-1111 and 000-0000.)
The .NET Framework includes classes that can use regular expressions to see if an input string
matches the pattern. They also provide methods for locating patterns within input text and for
making complex substitutions.
www.it-ebooks.info