Sorry, ho beccato un paio di bug… usa quest'altra versione:
codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
function checkRows(oField, oKeyEvent) {
var nKey = (oKeyEvent || /* old IE */ window.event || /* check is not supported! */ { keyCode: 38 }).keyCode,
// qui il numero massimo di caratteri per riga:
nCols = 30,
// qui il numero massimo di righe:
nRows = 5,
nSelS = oField.selectionStart, nSelE = oField.selectionEnd,
sVal = oField.value, nLen = sVal.length,
nBackward = nSelS >= nCols ? nSelS - nCols : 0,
nDeltaForw = sVal.substring(nBackward, nSelS).search(new RegExp("\\n(?!.{0," + String(nCols - 2) + "}\\n)")) + 1,
nRowStart = nBackward + nDeltaForw,
aReturns = (sVal.substring(0, nSelS) + sVal.substring(nSelE, sVal.length)).match(/\n/g),
nRowEnd = nSelE + nRowStart + nCols - nSelS,
sRow = sVal.substring(nRowStart, nSelS) + sVal.substring(nSelE, nRowEnd > nLen ? nLen : nRowEnd),
bKeepCols = nKey === 13 || nLen + 1 < nCols || /\n/.test(sRow) || ((nRowStart === 0 || nDeltaForw > 0 || nKey > 0) && (sRow.length < nCols || (nKey > 0 && (nLen === nRowEnd || sVal.charAt(nRowEnd) === "\n"))));
return (nKey !== 13 || (aReturns ? aReturns.length + 1 : 1) < nRows) && ((nKey > 32 && nKey < 41) || bKeepCols);
}
</script>
</head>
<body>
Textarea con numero fisso di caratteri per riga:
<textarea cols="50" rows="10" name="myInput" onkeypress="return checkRows(this, event);" onpaste="return false;" /></textarea></p>
</form>
</body>
</html>