Scan textfield for keyword and apply formatting

Question

I was interested in searching through text in a textfield, and applying text formatting to keywords. For example, every time the word 'the' appears, apply a text format that changes it to green and 14pt. Here is an example of a format and text applied to a textfield. How would I go about searching through the textfield and applying this format only to specific words?
my_txt.text = 'The cat jumped over the house.'
/// my format I want to apply
with (_lt_fmt) {
                align = 'left';
                blockIndent = 0;
                bold = false;
                bullet = false;
                color = _green;
                font = FontNames.ARIAL;
                indent = 0;
                italic = false;
                kerning = false;
                leading = 0;
                leftMargin = 0;
                letterSpacing = 0;
                rightMargin = 0;
                size = 14;
                tabStops = [];
                target = "";
                underline = false;
                url = "";

Answer 1

" I replaced some var names b/c they were reserved words"
There were no reserved words for the current or application scope.
"How can I keep all the words highlighted in the different formats?"
Comment out this line:
main_txt.setTextFormat(main_txt.defaultTextFormat);
Also, the code you showed is too verbose. You can combine declarations and and instantiation in one place and have 5 lines instead of 10:
var highLightFormat0:TextFormat = new TextFormat("Arial",14,0xff00ff,"bold");
var highLightFormat1:TextFormat = new TextFormat("Arial",7,0xff0000,"bold");
var highLightFormat2:TextFormat = new TextFormat("Arial",9,0xCCCCCC,"bold");
var highLightFormat3:TextFormat = new TextFormat("Arial", 8, 0xffEE00, "bold");
var main_txt:TextField = new TextField();
In addition, function  getTxtFmt and the way you deal with getting TExtFormats is an ovekill - conditionals are worse than direct references. So, I suggest your code is:
import flash.text.TextFormat;
// keywords to highlight
var wordsToSearch:Vector.<String> = new <String>['the','interested','text', 'applying'];
// TxtFormats
var highLightFormats:Vector.<TextFormat> = new <TextFormat>[new TextFormat("Arial", 14, 0xff00ff, "bold"), new TextFormat("Arial", 7, 0xff0000, "bold"), new TextFormat("Arial", 9, 0xCCCCCC, "bold"), new TextFormat("Arial", 8, 0xffEE00, "bold")];
// Create TextField and add to display list
var main_txt:TextField = new TextField();
with (main_txt) {
     multiline = main_txt.wordWrap = true;
     autoSize = "left";
     width = 400;
     defaultTextFormat = new TextFormat("Arial",12);
     x = main_txt.y = 20;
     text = "I was interested in searching through text in a textfield, and applying text formatting to keywords. For example, every time the word 'the' appears, apply a text format that changes it to green and 14pt. Here is an example of a format and text applied to a textfield. How would I go about searching through the textfield and applying this format only to specific words?";
addChild(main_txt);
// Iterate through Vector of keywords
for (var i:int; i < wordsToSearch.length; i++){
     search(wordsToSearch[i], i);
// find whole words
function search(keyword:String, fmtChoice:int):void {
     //main_txt.setTextFormat(main_txt.defaultTextFormat);
     var txt:String = main_txt.text;
     var pattern:RegExp = new RegExp("\(\?\<\=\\s)" + keyword + "\\s","ig");
     var theResult:Object = pattern.exec(txt);
     while (theResult) {
          main_txt.setTextFormat( highLightFormats[fmtChoice], theResult.index, theResult.index + keyword.length);
          theResult = pattern.exec(txt);