Regex - Quantifier {x,y} following nothing

Question

I'm creating a basic text editor and I'm using regex to achieve a find and replace function. To do this I've gotten this code:

Private Function GetRegExpression() As Regex
    Dim result As Regex
    Dim regExString As [String]
    ' Get what the user entered
    If TabControl1.SelectedIndex = 0 Then
        regExString = txtbx_Find2.Text
    ElseIf TabControl1.SelectedIndex = 1 Then
        regExString = txtbx_Find.Text
    End If

    If chkMatchCase.Checked Then
        result = New Regex(regExString)
    Else
        result = New Regex(regExString, RegexOptions.IgnoreCase)
    End If

    Return result
End Function

And this is the Find method

 Private Sub FindText()
    ''
    Dim WpfTest1 As New Spellpad.Tb
    Dim ElementHost1 As System.Windows.Forms.Integration.ElementHost = frm_Menu.Controls("ElementHost1")
    Dim TheTextBox As System.Windows.Controls.TextBox = CType(ElementHost1.Child, Tb).ctrl_TextBox
    ''
    ' Is this the first time find is called?
    ' Then make instances of RegEx and Match
    If isFirstFind Then
        regex = GetRegExpression()
        match = regex.Match(TheTextBox.Text)
        isFirstFind = False
    Else
        ' match.NextMatch() is also ok, except in Replace
        ' In replace as text is changing, it is necessary to
        ' find again
        'match = match.NextMatch();
        match = regex.Match(TheTextBox.Text, match.Index + 1)

    End If

    ' found a match?
    If match.Success Then
        ' then select it
        Dim row As Integer = TheTextBox.GetLineIndexFromCharacterIndex(TheTextBox.CaretIndex)
        MoveCaretToLine(TheTextBox, row + 1)
        TheTextBox.SelectionStart = match.Index
        TheTextBox.SelectionLength = match.Length

    Else
        If TabControl1.SelectedIndex = 0 Then
            MessageBox.Show([String].Format("Cannot find ""{0}""   ", txtbx_Find2.Text), Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Information)
        ElseIf TabControl1.SelectedIndex = 1 Then
            MessageBox.Show([String].Format("Cannot find ""{0}""   ", txtbx_Find.Text), Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Information)
        End If
        isFirstFind = True
    End If
End Sub

When I run the program I get errors:

• For ?, parsing "?" - Quantifier {x,y} following nothing.; and
• For *, parsing "*" - Quantifier {x,y} following nothing.

It's as if I can't use these but I really need to. How can I solve this problem?

Answer 1

? and * are quantifiers in regular expressions:

• ? is used to specify that something is optional, for instance b?au can match both bau and au.
• * means the group with which it binds can be repeated zero, one or multiple times: for instance ba*u can bath bu, bau, baau, baaaaaaaau,...

Now most regular expressions use {l,u} as a third pattern with l the lower bound on the number of times something is repeated, and u the upper bound on the number of occurences. So ? is replaced by {0,1} and * by {0,}.

Now if you provide them without any character before them, evidently, the regex parser doesn't know what you mean. In other words if you do (used csharp, but the ideas are generally applicable):

$ csharp
Mono C# Shell, type "help;" for help

Enter statements below.
csharp> Regex r = new Regex("fo*bar");
csharp> r.Replace("Fooobar fooobar fbar fobar","<MATCH>");    
"Fooobar <MATCH> <MATCH> <MATCH>"
csharp> r.Replace("fooobar far qux fooobar quux fbar echo fobar","<MATCH>");
"<MATCH> far qux <MATCH> quux <MATCH> echo <MATCH>"

If you wish to do a "raw text find and replace", you should use string.Replace.

EDIT:

Another way to process them is by escaping special regex characters. Ironically enough, you can do this by replacing them by a regex ;).

Private Function GetRegExpression() As Regex
    Dim result As Regex
    Dim regExString As [String]
    ' Get what the user entered
    If TabControl1.SelectedIndex = 0 Then
        regExString = txtbx_Find2.Text
    ElseIf TabControl1.SelectedIndex = 1 Then
        regExString = txtbx_Find.Text
    End If

    'Added code
    Dim baseRegex As Regex = new Regex("[\\.$^{\[(|)*+?]")
    regExString = baseRegex.Replace(regExString,"\$0")
    'End added code

    If chkMatchCase.Checked Then
        result = New Regex(regExString)
    Else
        result = New Regex(regExString, RegexOptions.IgnoreCase)
    End If

    Return result
End Function