Find and Replace for an Textarea

I updated my earlier answer and finished the search and replace functionality based on the direction my earlier post outlined. I tested this in Chrome 14, IE8 and Firefox 3.6.

Find: will select the next instance of the searchterm.

Find/Replace: will replace the next occurrence of the search string and select the replacement

Replace All: will replace all occurences and select the piece of text that has been replaced last

Hope this is now, what you were looking for. You should definitely be able to go from here and style this or make a proper class out of this...

<script src="jquery.js" type="text/javascript"></script>    

<textarea id="txtArea" style="width: 300px; height:100px">
    This is a test. A test, i say. The word TEST is mentioned three times.
</textarea>

<p>
    <label for="termSearch">Search</label>
    <input type="text" id="termSearch" name="termSearch" value="test" /><br/>
    <label for="termReplace">Replace</label>
    <input type="text" id="termReplace" name="termReplace" value="solution" /><br/>
    <label for="caseSensitive">Case Sensitive</label>
    <input type="checkbox" name="caseSensitive" id="caseSensitive" /><br/>
    <a href="#" onclick="SAR.find(); return false;" id="find">FIND</a><br/>
    <a href="#" onclick="SAR.findAndReplace(); return false;" id="findAndReplace">FIND/REPLACE</a><br/>
    <a href="#" onclick="SAR.replaceAll(); return false;" id="replaceAll">REPLACE ALL</a><br/>
</p>

<script type="text/javascript">
    var SAR = {};

    SAR.find = function(){
        // collect variables
        var txt = $("#txtArea").val();
        var strSearchTerm = $("#termSearch").val();
        var isCaseSensitive = ($("#caseSensitive").attr('checked') == 'checked') ? true : false;

        // make text lowercase if search is supposed to be case insensitive
        if(isCaseSensitive == false){
            txt = txt.toLowerCase();
            strSearchTerm = strSearchTerm.toLowerCase();
        }

        // find next index of searchterm, starting from current cursor position
        var cursorPos = ($("#txtArea").getCursorPosEnd());
        var termPos = txt.indexOf(strSearchTerm, cursorPos);

        // if found, select it
        if(termPos != -1){
            $("#txtArea").selectRange(termPos, termPos+strSearchTerm.length);
        }else{
            // not found from cursor pos, so start from beginning
            termPos = txt.indexOf(strSearchTerm);
            if(termPos != -1){
                $("#txtArea").selectRange(termPos, termPos+strSearchTerm.length);
            }else{
                alert("not found");
            }
        }
    };

    SAR.findAndReplace = function(){
        // collect variables
        var origTxt = $("#txtArea").val(); // needed for text replacement
        var txt = $("#txtArea").val(); // duplicate needed for case insensitive search
        var strSearchTerm = $("#termSearch").val();
        var strReplaceWith = $("#termReplace").val();
        var isCaseSensitive = ($("#caseSensitive").attr('checked') == 'checked') ? true : false;
        var termPos;

        // make text lowercase if search is supposed to be case insensitive
        if(isCaseSensitive == false){
            txt = txt.toLowerCase();
            strSearchTerm = strSearchTerm.toLowerCase();
        }

        // find next index of searchterm, starting from current cursor position
        var cursorPos = ($("#txtArea").getCursorPosEnd());
        var termPos = txt.indexOf(strSearchTerm, cursorPos);
        var newText = '';

        // if found, replace it, then select it
        if(termPos != -1){
            newText = origTxt.substring(0, termPos) + strReplaceWith + origTxt.substring(termPos+strSearchTerm.length, origTxt.length)
            $("#txtArea").val(newText);
            $("#txtArea").selectRange(termPos, termPos+strReplaceWith.length);
        }else{
            // not found from cursor pos, so start from beginning
            termPos = txt.indexOf(strSearchTerm);
            if(termPos != -1){
                newText = origTxt.substring(0, termPos) + strReplaceWith + origTxt.substring(termPos+strSearchTerm.length, origTxt.length)
                $("#txtArea").val(newText);
                $("#txtArea").selectRange(termPos, termPos+strReplaceWith.length);
            }else{
                alert("not found");
            }
        }
    };

    SAR.replaceAll = function(){
        // collect variables
        var origTxt = $("#txtArea").val(); // needed for text replacement
        var txt = $("#txtArea").val(); // duplicate needed for case insensitive search
        var strSearchTerm = $("#termSearch").val();
        var strReplaceWith = $("#termReplace").val();
        var isCaseSensitive = ($("#caseSensitive").attr('checked') == 'checked') ? true : false;

        // make text lowercase if search is supposed to be case insensitive
        if(isCaseSensitive == false){
            txt = txt.toLowerCase();
            strSearchTerm = strSearchTerm.toLowerCase();
        }

        // find all occurances of search string
        var matches = [];
        var pos = txt.indexOf(strSearchTerm);
        while(pos > -1) {
            matches.push(pos);
            pos = txt.indexOf(strSearchTerm, pos+1);
        }

        for (var match in matches){
            SAR.findAndReplace();
        }
    };


    /* TWO UTILITY FUNCTIONS YOU WILL NEED */
    $.fn.selectRange = function(start, end) {
        return this.each(function() {
            if(this.setSelectionRange) {
                this.focus();
                this.setSelectionRange(start, end);
            } else if(this.createTextRange) {
                var range = this.createTextRange();
                range.collapse(true);
                range.moveEnd('character', end);
                range.moveStart('character', start);
                range.select();
            }
        });
    };

    $.fn.getCursorPosEnd = function() {
        var pos = 0;
        var input = this.get(0);
        // IE Support
        if (document.selection) {
            input.focus();
            var sel = document.selection.createRange();
            pos = sel.text.length;
        }
        // Firefox support
        else if (input.selectionStart || input.selectionStart == '0')
            pos = input.selectionEnd;
        return pos;
    };  
</script>

This answer is those looking for a more complete and polished version of Jens Fischer's solution with added functionalities such as find previous, find next, find and replace using regular expression, undo and redo.

Here is the link to the GitHub project: https://github.com/AlienKevin/SmartTextarea