c# - How to replace first occurrence of a substring within a string

String replace first
string-replace-first.aspx

<%@ Page Language="C#" AutoEventWireup="true"%>

<!DOCTYPE html>  
<script runat="server"> 
    protected void Button1_Click(object sender, System.EventArgs e)  
    {
        //this section create a string variable.
        string stringValue = "White gary red green white DarkBlue white";

        Label1.Text = "string..................<br />";
        Label1.Text += stringValue.ToString();

        //ignore case string replace using Regex
        Regex rex = new Regex("white", RegexOptions.IgnoreCase);

        // this line only replace first occurence of 'white'
        string returnString = rex.Replace(stringValue, "pink",1);

        Label1.Text += "<br /><br />replaced string (first occurence of 'white')........";
        Label1.Text += "<br />" + returnString.ToString();
    }  
</script>  
  
<html xmlns="http://www.w3.org/1999/xhtml">  
<head id="Head1" runat="server">  
    <title>c# example - string replace first</title>  
</head>  
<body>  
    <form id="form1" runat="server">  
    <div>  
        <h2 style="color:MidnightBlue; font-style:italic;">  
            c# example - string replace first
        </h2>  
        <hr width="550" align="left" color="Gainsboro" />  
        <asp:Label   
            ID="Label1"   
            runat="server"  
            Font-Size="X-Large"
            >  
        </asp:Label>  
        <br /><br />
        <asp:Button   
            ID="Button1"   
            runat="server"   
            Text="string replace first"  
            OnClick="Button1_Click"
            Height="40"  
            Font-Bold="true"  
            />  
    </div>  
    </form>  
</body>  
</html>
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