function pick(str, re) {
  var result = str.match(re);
  return (result && result.length === 2) ? result[1] : result;
}

var str = "tel: 13512345678";
var tel_a = pick(str, /tel:\s?(\d+)/);
var tel_b = pick(str, /tel:(\d+)/);

// return 13512345678
alert(tel_a);

// return null
alert(tel_b);

http://oragg.com/2009/10/snipplr-for-wordpress.html

pick a certain substring from a string by regular expression

JavaScript