<script type="text/javascript">
function vireAccents(txt) {
return txt.toLowerCase()
.replace(/[éèêë]/g, "e")
.replace(/[àâ]/g, "a")
.replace(/[îï]/g, "i")
.replace(/ô/g, "o")
.replace(/ç/g, "c")
.replace(/ù/g, "u");
}
</script>
in : <input type="text" id="in" value=""
onkeyup="document.getElementById('out').value=vireAccents(this.value)"><br>
out : <input type="text" id="out" value="">