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| Title | Use regular expressions in VB .NET |
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| Description | This example shows how to use regular expressions in VB .NET. It uses the Regex class to validate text entered by the user. |
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| Keywords | regular expression, string, parsing, parse |
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| Categories | Strings, Algorithms |
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When the user changes the text in the txtTestExp text box, the program makes a Regex object, passing the constructor the regular expression to match. It calls the object's IsMatch method, passing it the text that the user entered and sets the text box's background color to yellow if the text doesn't match the expression.
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Private Sub txtTestExp_TextChanged(ByVal sender As _
System.Object, ByVal e As System.EventArgs) Handles _
txtTestExp.TextChanged
Dim reg_exp As New Regex(txtRegExp.Text)
If reg_exp.IsMatch(txtTestExp.Text) Then
txtTestExp.BackColor = Color.White
Else
txtTestExp.BackColor = Color.Yellow
End If
End Sub
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The example program starts with the regular expression "^([2-9]{3}-)?[2-9]{3}-\d{4}$". The pieces of this expression have the following meanings:
| ^ | Match the beginning of the string |
| [2-9]{3}- | Match the characters 2 through 9 exactly 3 times, followed by a - |
| ([2-9]{3}-)? | Match the expression "[2-9]{3}-" zero or 1 times |
| [2-9]{3}- | Again, match the characters 2 through 9 exactly 3 times, followed by a - |
| \d{4} | Match any digit exactly 4 times |
| $ | Match the end of the string |
The result is that the string must have the form 222-2222 or 222-222-2222.
Also note that the Regex class has a shared IsMatch method that you can use without creating a Regex object. Creating an object as in this example allows the object to compile the regular expression so it can later evaluate the expression more quickly. This is useful if you need to use the expression many times.
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