[SalesForce] IsNull AMPscript function

Unfortunately there is no direct way to do this in AMPScript.

You have 2 possibilities that you can use. 1 in AMPScript and 1 in SSJS.

AMPSCRIPT: (if retrieved rows is fairly low quantity)

1. First, you would have to add a row to your DE that contains a constant value (flag) inside of it. I usually name this field 'Secret' and use a value of 1.
   
   So for instance your DE would have 2 fields - My_Text_field_1 | Secret
2. You would do your lookup based on this 'Secret' field - LookupRows('yourDE', 'Secret', 1)
   
   This will return ALL rows in this DE up to 2500 I believe.
3. You would then utilize IndexOf() function for your returned value from the Cloudpage to do a 'Contains' inside of a For Loop

Putting it all together you get:

%%[
SET @Text = AttributeValue('Text_Input_1') 
/* or RequestParameter('Text_Input_1') if the value is being passed as a parameter */
SET @Rows = LookupRows('yourDE', 'Secret', 1)
SET @Count = 0

FOR @i=1 TO Rowcount(@Rows) DO

  SET @Row = Row(@Rows, @i)
  SET @Text_Input_1 = Field(@Row, 'Text_Input_1')

  IF IndexOf(@Text_Input_1, @Text) > 0 THEN

    SET @Count = ADD(@Count, 1)

    /* Set any other actions you want to take if the value contains the string */

  ENDIF

Next @i
]%%

SSJS: (Processes slow and has a larger resource draw)

For the SSJS solution, you would use the Rows.Retrieve() Function.

<script runat="server">

Platform.Load("core", "1.1.1");

var text_Input_1 = Platform.Request.GetFormField('Text_Input_1'); /* Get form field */
var yourDE = DataExtension.Init("yourDE");
var filter = {Property:"My_Text_field_1 ",SimpleOperator:"Like",Value: text_Input_1};
var data = yourDE.Rows.Retrieve(filter);
</script>

This will return a rowset containing all of the rows that are 'like' your text input value from the cloud page.

Since the approach is similar to a previous post, I have reused the same code and made some slight modifications.

I have used the mixture of SSJS and AMPscript to cover this use case for the LookUp Rows.

The script below uses try and catch within SSJS to execute AMPscript and SET a variable which then can be used later in your email to run logic based on the value of that resulted variable.

If you copy and run this straight within an email the result will be false assuming this DE doesn't exist in your BU. However, if you change it to valid DE Name and a valid field in SSJS and render the email the result will be true.

Example:

<script runat="server" language="javascript">
    Platform.Load("core", "1");
    try {
        var CheckDE = DataExtension.Init("DE_Name_TO_Check");
        var CheckDEStatus = CheckDE.Rows.Lookup(["FieldName"], [Value]);
</script>

    %%[SET @DEExists = "true"]%%

<script runat="server" language="javascript">
Platform.Load("core", "1");
    }
    catch (e) {
</script>

    %%[SET @DEExists = "false"]%%

<script runat="server" language="javascript">
Platform.Load("core", "1");
    }
</script>

This is to confirm if DE Exists: %%=v(@DEExists)=%%