Regular Expression problem

To answer your original question, there are some gotchas with regular expressions.

Firstly, because you write the regex as a string and this is JavaScript/JScript, you need to double the backslashes in the string, e.g. \. -> \\. and \d -> \\d. Similar to how you double the backslashes in paths --"C:\\MyFolder".

Secondly, StrMatches uses a slightly different syntax than native JavaScript/JScript regexes. Specifically, "|" is non-greedy so "green|red" matches "greened" and "greered", not "green" and "red" as in native JavaScript/JScript regexes. To make it greedy you need to put the parentheses around the alternatives.

The working version would be:

function testRegEx()
{
  // Attempt to use a regular exppression object with test method.
  var dateRegEx = new RegExp("(0[1-9]|[12][0-9]|3[01])\\.(0[1-9]|1[012])\\.(19|20)\\d\\d", "g");
  var testBool1 = dateRegEx.test("23.09.2016");
  Log.Message(testBool1);

  // Attempt to use aqString.StrMatches
  var testBool2 = aqString.StrMatches("(0[1-9])|([12][0-9])|(3[01])\\.(0[1-9])|(1[012])\\.(19)|(20)\\d\\d", "23.09.2016");
  Log.Message(testBool2);
}

But as already suggested, parsing the date is better than using a regex, because a regex won't catch invalid dates like February 31.