Find in Selection with ExtendScript

Thank you for answer.
My goal was to select the found text and then apply a conditional tag to it. The Find() function automatically selects found text. But I think I can use the solution you provided as all I need to do is to select found text. 
Here is final solution I have used.

#target framemaker

var doc, regex, textRange, text;

// Get the document object and the selected text range.
doc = app.ActiveDoc;
textRange = doc.TextSelection;

// A regular expression of the text I want to find.
regex = /what I want to find/;

// Get the text of the text range.
text = getText (textRange, doc);

// See if the text contains the regular expression.
var foundArray = text.match(regex); // this part was changed from inital solution
if (foundArray == null) {
    alert ("The regular expression is not found");
}
else {
  var foundTextRange = new TextRange();
  foundTextRange.beg.obj = textRange.beg.obj;
  foundTextRange.beg.offset = textRange.beg.offset + foundArray.index;
  foundTextRange.end.obj = textRange.end.obj;
  foundTextRange.end.offset = textRange.beg.offset + foundArray.index + foundArray[0].length;
  
  doc.TextSelection = foundTextRange;
}

function getText (textObj, doc) {
    
    // Gets the text from the text object or text range.

    var text = "", textItems, i;
    
    // Get a list of the strings in the text object or text range.
    if (textObj.constructor.name !== "TextRange") {
        textItems = textObj.GetText (Constants.FTI_String);
    } 
    else {
         textItems = doc.GetTextForRange(textObj, Constants.FTI_String);
    }
    // Concatenate the strings.
    for (i = 0; i < textItems.len; i += 1) {
        text += (textItems[i].sdata);
    }

    return text; // Return the text
}

 
Thanks.