How to validate an expression in JavaScript?

You will have to use regular expression for that. See this.

To add a bit for details to Sarfraz answer your example would give this regular expression

var str = "2010-03".match(/\{\d{2,4}\}-\{\d{1,2}\}/g);

it would give you array with matching part if it match.

if ( /{\d{2,4}}-{\d{1,2}}/.test( str_date ) == true ) {
  // ok
}
else {
  // fail
}

You will match the input against a regular expression :

if(myInput.value.match(/\{\d+\}-\{\d+\}/)) {
  // input validated
} else {
  // validation failed
}

This regexp can be adjusted depending on what you need. Here is a quick tutorial of javascript regexp : http://www.w3schools.com/js/js_obj_regexp.asp .

Also, if you want to check if the input represents a valid date, you will have some extra work. It looks like you're accepting anything that looks like a year-month, so you can try this:

if(myInput.value.match(/\{(\d+)\}-\{(\d+)\}/)) {
  var year = parseInt(RegExp.$1);
  var month = parseInt(RegExp.$2);
  if(month<1 || month>12) return false;
  if(year < 100) year += 2000;
  if(year > 3000) return false;
  return true;
} else {
  // validation failed
  return false;
}