emphasis function js

em.js

var em = function(str, pattern, el) {
  if (!(str && pattern)) return "";
  pattern = pattern.toString().replace((/[^a-zA-Z1-9 ]/ig), "").split(" ").reduce(function(prev, curr){
    if(curr){
      prev.push(curr);
    } 
    return prev;
  },[]).join("|");

  el = el || "em";
  var ret = [],
      match,
      regex = new RegExp(pattern, 'ig');
  while(match = regex.exec(str)) {
    var index = match.index,
        before = str.substring(0, index),
        wrap = "<"+el+">"+match[0]+"</"+el+">";
    str = str.substr(regex.lastIndex);
    ret.push(before, wrap);
    regex.lastIndex = 0;
  }
  ret.push(str);
  return ret.join('');
};