<script>
alert("123abc".replace(/(\d+).*/,"$1"));
alert("0abc".replace(/(\d+).*/,"$1"));
</script>
alert("123abc".replace(/(\d+).*/,"$1"));
alert("0abc".replace(/(\d+).*/,"$1"));
</script>
var str1=123abc
var str2=substring(0,2) //123
var str3=0abc
var str4=indexof(0) //0
<script>
alert('aa123'.match(/\d+/));
</script>