C# 4 and .NET 4

218 ❘ ChaPTer 9 strinGs And reGulAr expressiOns
Microsoft ported it onto Windows, where up until now it has been used mostly with scripting
languages. Today, regular expressions are supported by a number of .NET classes in the namespace
System.Text.RegularExpressions . You can also fi nd the use of regular expressions in various parts
of the .NET Framework. For instance, you will fi nd that they are used within the ASP.NET Validation
server controls.
If you are not familiar with the regular expressions language, this section introduces both regular
expressions and their related .NET classes. If you are already familiar with regular expressions, you will
probably want to just skim through this section to pick out the references to the .NET base classes. You
might like to know that the .NET regular expression engine is designed to be mostly compatible with Perl 5
regular expressions, although it has a few extra features.
introduction to regular expressions
The regular expressions language is designed specifi cally for string processing. It contains two features:
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A set of escape codes for identifying specifi c types of characters. You will be familiar with the use of
the * character to represent any substring in DOS expressions. (For example, the DOS command
Dir Re* lists the fi les with names beginning with Re .) Regular expressions use many sequences like
this to represent items such as any one character , a word break , one optional character , and so on.
A system for grouping parts of substrings and intermediate results during a search operation.
With regular expressions, you can perform quite sophisticated and high - level operations on strings. For
example, you can:
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Identify (and perhaps either fl ag or remove) all repeated words in a string (for example, “ The
computer books books ” to “ The computer books ”)
Convert all words to title case (for example, “ this is a Title ” to “ This Is A Title ” )
Convert all words longer than three characters to title case (for example, “ this is a Title ” to “ This is
a Title ”)
Ensure that sentences are properly capitalized
Separate the various elements of a URI (for example, given http://www.wrox.com , extract the
protocol, computer name, fi le name, and so on)
Of course, all these tasks can be performed in C# using the various methods on System.String and
System.Text.StringBuilder . However, in some cases, this would require writing a fair amount of
C# code. If you use regular expressions, this code can normally be compressed to just a couple of lines.
Essentially, you instantiate a System.Text.RegularExpressions.RegEx object (or, even simpler, invoke
a static RegEx() method), pass it the string to be processed, and pass in a regular expression (a string
containing the instructions in the regular expressions language), and you ’ re done.
A regular expression string looks at fi rst sight rather like a regular string, but interspersed with escape
sequences and other characters that have a special meaning. For example, the sequence \b indicates the
beginning or end of a word (a word boundary), so if you wanted to indicate you were looking for the
characters th at the beginning of a word, you would search for the regular expression, \bth (that is, the
sequence word boundary - t - h ). If you wanted to search for all occurrences of th at the end of a word,
you would write th\b (the sequence t - h - word boundary). However, regular expressions are much more
sophisticated than that and include, for example, facilities to store portions of text that are found in a
search operation. This section merely scratches the surface of the power of regular expressions.
For more on regular expressions, please review the book Beginning Regular
Expressions, Wiley Publishing, 2005 (ISBN 978 - 0 - 7645 - 7489 - 4).
www.it-ebooks.info