function getIdentifier( str ) {
	return str.substring( 11, str.length-1 ); // ArrayIndex<XX> => XX
}
Extracts an array index identifier from an array index indexing expression.

@private
@param {string} str - input string
@returns {string} identifier

@example
var str = 'ArrayIndex<0>';

var id = getIdentifier( str );
// returns '0'