VB.NET Regex.Match Examples: Regular Expressions

Match example. This program uses Regex. Please notice the System.Text.RegularExpressions namespace—this is important to get started with regular expressions.

Step 1 We create a Regex object. The Regex pattern "\w+" matches one or more word characters together.

Step 2 We invoke the Match Function on the Regex instance. This returns a Match (which we test next).

Step 3 We test if the match is successful. If it is, we print (with Console.WriteLine) its value—the string "do."

VB.NET program that uses Regex

Imports System.Text.RegularExpressions Module Module1 Sub Main() ' Step 1: create Regex. Dim regex As Regex = New Regex("\w+") ' Step 2: call Match on Regex. Dim match As Match = regex.Match(" do?") ' Step 3: test the Success bool. ' ... If we have Success, write the Value. If match.Success Then Console.WriteLine("RESULT: [{0}]", match.Value) End If End Sub End Module

RESULT: [do]

Pattern details:

\w Match word char. + Match 1 or more chars.

IgnoreCase. Next, we use different syntax, and an option, for Match. We call the Regex.Match shared Function—no Regex object is needed. We then specify an option, RegexOptions.IgnoreCase.

IgnoreCase This enum value, a constant, specifies that lower and uppercase letters are equal.

VB.NET program that uses RegexOptions.IgnoreCase

Imports System.Text.RegularExpressions Module Module1 Sub Main() ' Match ignoring case of letters. Dim match As Match = Regex.Match("I like that cat", "C.T", RegexOptions.IgnoreCase) If match.Success Then ' Write value. Console.WriteLine(match.Value) End If End Sub End Module

cat

Groups. This example uses Match and Groups. We specify the case of letters is unimportant with RegexOptions.IgnoreCase. And finally we test for Success on the Match object received.

Info When we execute this program, we see the target text was successfully extracted from the input.

Groups index We use the value 1 to get the first group from the Match. With Regex, indexing starts at 1 not 0 (don't ask why).

VB.NET program that uses Regex.Match

Imports System.Text.RegularExpressions Module Module1 Sub Main() ' The input string. Dim value As String = "/content/alternate-1.aspx" ' Invoke the Match method. Dim m As Match = Regex.Match(value, _ "content/([A-Za-z0-9\-]+)\.aspx$", _ RegexOptions.IgnoreCase) ' If successful, write the group. If (m.Success) Then Dim key As String = m.Groups(1).Value Console.WriteLine(key) End If End Sub End Module

alternate-1

Shared. A Regex object requires time to be created. We can instead share Regex objects, with the shared keyword. A shared Regex object is faster than shared Regex Functions.

Shared

Therefore Storing a Regex as a field in a module or class often results in a speed boost, when Match is called more than once.

Function The Match function is an instance function on a Regex object. This program has the same result as the previous program.

VB.NET program that uses Match on Regex field

Imports System.Text.RegularExpressions Module Module1 ''' <summary> ''' Member field regular expression. ''' </summary> Private _reg As Regex = New Regex("content/([A-Za-z0-9\-]+)\.aspx$", _ RegexOptions.IgnoreCase) Sub Main() ' The input string. Dim value As String = "/content/alternate-1.aspx" ' Invoke the Match method. ' ... Use the regex field. Dim m As Match = _reg.Match(value) ' If successful, write the group. If (m.Success) Then Dim key As String = m.Groups(1).Value Console.WriteLine(key) End If End Sub End Module

alternate-1

Match, NextMatch. The Match() Function returns the first match only. But we can call NextMatch() on that returned Match object. This is a match that is found in the text, further on.

Tip NextMatch can be called in a loop. This results in behavior similar to the Matches method (which may be easier to use).

For Each, For

VB.NET program that uses Match, NextMatch

Imports System.Text.RegularExpressions Module Module1 Sub Main() ' Get first match. Dim match As Match = Regex.Match("4 and 5", "\d") If match.Success Then Console.WriteLine(match.Value) End If ' Get next match. match = match.NextMatch() If match.Success Then Console.WriteLine(match.Value) End If End Sub End Module

4 5

IsMatch. This returns true if a String matches the regular expression. We get a Boolean that tells us whether a pattern matches. If no other results are needed, IsMatch is useful.

Here This program introduces the IsValid Boolean function, which computes the result of the Regex.IsMatch function on its parameter.

Note The regular expression pattern indicates any string of lowercase ASCII letters, uppercase ASCII letters, or digits.

VB.NET program that uses Regex.IsMatch function

Imports System.Text.RegularExpressions Module Module1 Function IsValid(ByRef value As String) As Boolean Return Regex.IsMatch(value, "^[a-zA-Z0-9]*$") End Function Sub Main() Console.WriteLine(IsValid("dotnetperls0123")) Console.WriteLine(IsValid("DotNetPerls")) Console.WriteLine(IsValid(":-)")) End Sub End Module

True True False

Start and ends. Matching the start and end of a String is commonly-needed. We use the metacharacters "^" and "$" to match the starts and ends of a string.

Note IsMatch() evaluates these metacharacters in the same way that Match (or Matches) can—the result is different for each function.

VB.NET program that uses IsMatch, tests start and end

Imports System.Text.RegularExpressions Module Module1 Sub Main() Dim value As String = "xxyy" ' Match the start with a "^" char. If Regex.IsMatch(value, "^xx") Then Console.WriteLine("ISMATCH START") End If ' Match the end with a "$" char. If Regex.IsMatch(value, "yy$") Then Console.WriteLine("ISMATCH END") End If End Sub End Module

ISMATCH START ISMATCH END

Pattern details:

^ Start of string. xx 2 x chars. yy 2 y chars. $ End of string.

Benchmark, Compiled Regex. To optimize Regex performance in VB.NET, we can use the RegexOptions.Compiled enum and store the Regex in a field. Here we test Compiled Regexes.

Version 1 This code uses a Regex field in the Module, and calls IsMatch on the field instance.

Version 2 Uses Regex.IsMatch directly with no stored Regex instance. This code does the same thing as version 1.

Result The optimizations applied (a field, and RegexOptions.Compiled) makes regular expression testing much faster.

VB.NET program that times RegexOptions.Compiled

Imports System.Text.RegularExpressions Module Module1 Dim _regex As Regex = New Regex("X.+0", RegexOptions.Compiled) Sub Version1() ' Use compiled regular expression stored as field. If _regex.IsMatch("X12340") = False Then Throw New Exception End If End Sub Sub Version2() ' Do not use compiled Regex. If Regex.IsMatch("X12340", "X.+0") = False Then Throw New Exception End If End Sub Sub Main() Dim m As Integer = 100000 Dim s1 As Stopwatch = Stopwatch.StartNew ' Version 1: use RegexOptions.Compiled. For i As Integer = 0 To m - 1 Version1() Next s1.Stop() Dim s2 As Stopwatch = Stopwatch.StartNew ' Version 2: do not compile the Regex. For i As Integer = 0 To m - 1 Version2() Next s2.Stop() Dim u As Integer = 1000000 Console.WriteLine(((s1.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns")) Console.WriteLine(((s2.Elapsed.TotalMilliseconds * u) / m).ToString("0.00 ns")) End Sub End Module

131.78 ns IsMatch, RegexOptions.Compiled 484.66 ns IsMatch

Matches. This function is used to locate and return parts of the source String in separate variables. To capture groups, we use parentheses in a Regex pattern.

Regex.Matches

Regex.Matches Quote

Some examples. In these programs, I remove HTML tags. Be careful—this does not work on all HTML. I also count words in English text (this is also not perfect).

HTML Tags

Word Counts

Files. See how you can read in the lines of a file, and parse each line, accessing only a part of the text from each line. The StreamReader type and Regex.Match Function are used.

Regex File