What are Regular Expressions?

Although there is no standards body governing the regular expression language, Perl 5, by virtue of it's popularity, has set the standard for regular expression syntax. The .NET Framework Regular Expressions library is designed to be compatible with Perl 5 regular expressions, as well as including additional features not found elsewhere.

To give you a taste of how regular expressions work, let us look at an example. I once had a professor who proclaimed that history was summed up by the -ism's [a term referring to words ending in i-s-m. Example, existentialism.] Suppose you were given a document, and asked to extract the ism's mentioned inside of it. Our sample contains the following passage:

How would you extract the isms? You could manually proceed line by line, without the help of a computer, and record the isms you see. That would work, but it would take a while, and after all, you are a programmer. You could also write a string parsing utility to separate the document into words, and then see which words ended in ism. That too would work, but would require more effort. With regular expressions, you can extract the isms with this pattern:

This pattern says to look for a series of zero or more characters (\w*) ending in ism. Running this pattern against the above passage results in the following matches extracted for you:

Regular expressions can perform more complex searches, and in this guide to Regular Expressions, we'll cover the syntax of regular expressions, the classes in the System.Text.RegularExpressions, and how to use them. Using our online RegEx Tester, you can test regular expressions online. We'll finish off our discussion with some examples, such as how to extract information from web pages.