Please help me to understand why the following code works:
<script>
var re = RegExp('\\ba\\b') ;
alert(re.test('a')) ;
alert(re.test('ab')) ;
</script>
In the first line there is no new operator.
As far as I know, a contructor in JavaScript is a function that initialize objects created by the operator new and they are not meant to return anything.
In general, if something is documented as being a constructor, use new with it. But in this case, RegExp has a defined "factory" behavior for the situation where you've called it as a function instead. See Section 15.10.3 of the ECMAScript (JavaScript) specification (that links to the outgoing spec; the section number is the same in the new spec, which you can download from the ECMA front page [on the right-hand side]; I don't want to directly link to a ~4MB PDF file):
15.10.3 The RegExp Constructor Called as a Function
15.10.3.1 RegExp(pattern, flags)
If pattern is an object R whose [[Class]] property is "RegExp" and flags is undefined, then return R unchanged. Otherwise call the RegExp constructor (15.10.4.1), passing it the pattern and flags arguments and return the object constructed by that constructor.
You can actually define your own JavaScript constructor functions to allow omitting the new keyword (by detecting they've been called as a function instead, and turning around and calling themselves correctly), but I wouldn't suggest it as it leads to misleading code. (And you can't do it with class syntax, you have to use the older, clunkier function syntax.)