Regular expressions are powerful tools for text processing and allow searching matches by a pattern. Regular expressions make text processing easier and more accurate, such as,
In VB.NET, Regex represents the regular expression engine. It can be used to quickly parse large amounts of text to find specific character patterns to extract, edit, replace, or delete text substrings.
Let's have a look into a simple example that creates a Regex from a pattern to match a word starting with char "S".
Public Sub Example1()
Dim customers As String = "John, samantha, Andy, Smith, Allen, Mark, Stella, Scarlett"
Dim pattern As String = "\b[S]\w+"
Dim expr As Regex = New Regex(pattern)
Dim matchedCustomers As MatchCollection = expr.Matches(customers)
Console.WriteLine("The following names of the customers start with ""S""" & vbLf)
For Each match In matchedCustomers
Console.WriteLine(match)
Next
End Sub
The \b[S]\w+ creates a pattern for a word that starts with the letter "S". Let's run the above code and you will see the following output.
The following names of the customers start with "S"
Smith
Stella
Scarlett
As you can see, the pattern only matches the names that start with "S" and ignore the name that starts with "s". You can use the RegexOptions.IgnoreCase parameter to make sure that Regex does not look for uppercase or lowercase.
Public Sub Example2()
Dim customers As String = "John, samantha, Andy, Smith, Allen, Mark, Stella, Scarlett"
Dim pattern As String = "\b[S]\w+"
Dim expr As Regex = New Regex(pattern, RegexOptions.IgnoreCase)
Dim matchedCustomers As MatchCollection = expr.Matches(customers)
Console.WriteLine("The following names of the customers start with ""S"" or or ""s""" & vbLf)
For Each match In matchedCustomers
Console.WriteLine(match)
Next
End Sub
Let's run the above code, and you will see the following output.
The following names of the customers start with "S" or or "s"
samantha
Smith
Stella
Scarlett
Let's consider another example where we want to validate an email Ids.
Private Function IsValidEmail(ByVal email As String) As Boolean
Dim strRegex As String = "\A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)\Z"
Dim re As Regex = New Regex(strRegex, RegexOptions.IgnoreCase)
If re.IsMatch(email) Then
Return (True)
Else
Return (False)
End If
End Function
You can call the IsValidEmail() bypassing the user email as a parameter, if it is a valid email it will return true, otherwise, false.
Public Sub EmailValidation()
Dim emails As String() =
{
"[email protected]",
"[email protected]",
"stella@gmail",
"[email protected]",
"mark.gmail.com",
"@gmail.com"
}
For Each email As String In emails
If IsValidEmail(email) Then
Console.WriteLine("{0, 20}: is a valid E-mail address.", email)
Else
Console.WriteLine("{0, 20}: is not a valid E-mail address.", email)
End If
Next
End Sub
Let's run the above code, and you will see the following output.
[email protected]: is a valid E-mail address.
[email protected]: is a valid E-mail address.
stella@gmail: is not a valid E-mail address.
[email protected]: is a valid E-mail address.
mark.gmail.com: is not a valid E-mail address.
@gmail.com: is not a valid E-mail address.