Paste a URL into a textarea as a [url=]<selected text>[/url]

<textarea id="txt1" cols="80" rows="5">The quick brown fox jumps over the lazy dog</textarea>

<script>
function getTextAreaSelection(textarea) {
    var start = textarea.selectionStart, end = textarea.selectionEnd;
    return {
        start: start,
        end: end,
        length: end - start,
        text: textarea.value.slice(start, end)
    };
}

function detectPaste(textarea, callback) {
    textarea.onpaste = function() {
        var sel = getTextAreaSelection(textarea);
        var initialLength = textarea.value.length;

        var textBefore = textarea.value.slice(0, sel.start);
        var textAfter = textarea.value.slice(sel.end, textarea.value.length);

        window.setTimeout(function() {
            var val = textarea.value;
            var pastedTextLength = val.length - (initialLength - sel.length);
            var end = sel.start + pastedTextLength;
            callback({
                start: sel.start,
                end: end,
                length: pastedTextLength,
                text: val.slice(sel.start, end),
                selection: sel,
                before: textBefore,
                after: textAfter
            });
        }, 1);
    };
}

function validateURL(textval) {
  var urlregex = new RegExp(
      "^(http|https|ftp)\://([a-zA-Z0-9\.\-]+(\:[a-zA-Z0-9\.&amp;%\$\-]+)*@)*((25[0-5]|2[0-4][0-9]|[0-1]{1}[0-9]{2}|[1-9]{1}[0-9]{1}|[1-9])\.(25[0-5]|2[0-4][0-9]|[0-1]{1}[0-9]{2}|[1-9]{1}[0-9]{1}|[1-9]|0)\.(25[0-5]|2[0-4][0-9]|[0-1]{1}[0-9]{2}|[1-9]{1}[0-9]{1}|[1-9]|0)\.(25[0-5]|2[0-4][0-9]|[0-1]{1}[0-9]{2}|[1-9]{1}[0-9]{1}|[0-9])|localhost|([a-zA-Z0-9\-]+\.)*[a-zA-Z0-9\-]+\.(com|edu|gov|int|mil|net|org|biz|arpa|info|name|pro|aero|coop|museum|[a-zA-Z]{2}))(\:[0-9]+)*(/($|[a-zA-Z0-9\.\,\?\'\\\+&amp;%\$#\=~_\-]+))*$");
  return urlregex.test(textval);
}

var textarea = document.getElementById("txt1");

detectPaste(textarea, function(pasteInfo) {
    // pasteInfo also has properties for the start and end character
    // index and length of the pasted text
    if (pasteInfo.text != null && pasteInfo.text.length > 0 && validateURL(pasteInfo.text)) {
      // Handle trailing spaces in selection (happens when the word was selected with double-click)
      while (pasteInfo.selection.text.length > 0 && pasteInfo.selection.text.charAt(pasteInfo.selection.text.length-1) == ' ') {
         pasteInfo.selection.text = pasteInfo.selection.text.substring(0, pasteInfo.selection.text.length-1);
         pasteInfo.after = " " + pasteInfo.after;
      }
      // Replace URL with [link] tag
      textarea.value = pasteInfo.before + "[url=" + pasteInfo.text + "]" + pasteInfo.selection.text + "[/url]" + pasteInfo.after;
      var newPosition = textarea.value.length - pasteInfo.after.length
      // if no text was selected for title, position cursor inside the [url] tag
      if (pasteInfo.selection.text.length == 0) {
         newPosition -= 6;
      }
      textarea.setSelectionRange(newPosition, newPosition);
    }
});
</script>

<br>
To test this feature, select any part of the text, and paste (Ctrl-V, or right-click + Paste) a valid URL.