Regular Expressions
Posted: Monday, May 17, 2004 10:51 PM
by
dave
Filed under: .NET
Regular expressions are a very powerful tool for validating or parsing a string. I don't claim to be an expert in the use of Regular expressions by any means. But I have used them with great success on a number of occasions. They are very useful. I have used them to validate a special URL that is used to create hyperlinks to forms within a smart client application, or to parse the syntax of a Powerbuilder DataWindow to name just two.
In ASP.NET there is a Regular Expression validator control. This control lets you validate input based on Regular Expressions. The nice thing about this control is the hard work is done for you for a variety of different strings. For example there is a sample expression for internet email addresses. \w+([-+.]\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)* and one for an Internet URL http://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?
If you want to use Regex in .NET (C# or VB.NET) getting the expression right is the tough part, as you can see from the expressions above. The rest is pretty easy. To help you out there is a tool written by Eric Gunnerson called Regular Expression Workbench that I have found very useful on many occasions. You can down load it from gotdotnet. This wonderful tool will help you format your expression, interpret or execute your expression on a sample string, even tell you how efficient the operation is.
For those of you who don't really know how to use Regular Expressions here is a simple example to get you started. Using this example you can test a string for swear words.
First: to use Regular Expressions you must use the System.Text.RegularExpressions namespace.
Then create a regular expression and call the match method to search your string.
using System.Text.RegularExpressions;
Regex r; //Declare Regular expression
r = new Regex("hell"); //Create expression
Match m = r.Match(this.textBox1.Text); //Call match method
if (m.Success) //Check to see if a match was made
{
MessageBox.Show("You can't say that.");
}
else
{
MessageBox.Show("nothing found.");
}
The above example works to find the word hell but it will also find it inside the word hello. You can add the escape character /b to denote word boundaries. For example if your expression is /bhell/b it will only match on the word hell. /b has two uses in a regular expression it's a word boundary. When used within a [ ] character class it refers to the backspace character.
Fine so you can now validate a string to keep users from entering a swear word. "But Dave there are many words I need to check for.", you say. Not to worry this can be done in the same expression using a pipe. If you separate each word you want to find with a pipe Match( ) will look for all the words listed. See the example below.
using System.Text.RegularExpressions;
Regex r; //Declare Regular expression
r = new Regex("\\bhell\\b|\\bfrig\\b"); //Create expression
Match m = r.Match(this.textBox1.Text); //Call match
if (m.Success) //Check to see if a match was made
{
MessageBox.Show("You can't say that.");
}
else
{
MessageBox.Show("nothing found.");
}
If you would like to receive an email when updates are made to this post, please register here