Easy expression engine in JavaScript

expression.js

function replaceOutOfStrings(str) {
  var index = 0, length = str.length, ch;
  var new_str = "", inString = null, start = 0;
  while(index < length) {
    ch = str.charAt(index);
    if(ch === '\\') {
      index = index + 2;
      continue;
    }
    if(ch === '"' || ch === "'") {
      // closing a string
      if(inString === ch) {
        inString = null;
        new_str = new_str + str.slice(start, index);
        start = index;
      } else {
        // opening a string
        new_str = new_str + replaceNames(str.slice(start, index));
        start = index;
        inString = ch;
      }
    }
    index = index + 1;
  }
  new_str += replaceNames(str.slice(start, index));
  return new_str;
}

var nameReg = /[a-zA-Z_$][0-9a-zA-Z_$]*/gm;

function replaceNames(str) {
  return str.replace(nameReg, function(_name) {
    if(!_name.match(/^_ctx./)) {
      return '_ctx.get("'+_name+'")';
    }
    return _name;
  });
}

function jsExpression(source) {
  var newSource = replaceOutOfStrings(source);
  return new Function('_ctx', 'return ' + newSource);
}