function reNativeFunction() {
	var str = '';

	// Use a native function as a template:
	str += function2string( Fcn );

	// Escape special RegExp characters:
	str = str.replace( /([.*+?^=!:$(){}|[\]\/\\])/g, '\\$1' ); // eslint-disable-line no-useless-escape

	// Replace any mentions of `Function` to make template generic and replace `for ...` and additional info provided in other environments, such as Rhino.
	str = str.replace( /Function|(function).*?(?=\\\()| for .+?(?=\\\])/g, '$1.*?' );

	// Bracket the regular expression:
	str = '^' + str + '$';

	// Create the regular expression:
	return new RegExp( str );
}
Returns a regular expression to capture everything that is not a space immediately after the `function` keyword and before the first left parenthesis.

@returns {RegExp} regular expression

@example
var RE_NATIVE_FUNCTION = reNativeFunction();
var bool = RE_NATIVE_FUNCTION.test( Date.toString() );
// returns true