function isCartesianIndexString( prop ) {
	return RE_CARTESIAN_INDEX.test( prop );
}
Tests if an indexing expression is a serialized Cartesian index.

@private
@param {string} prop - property name
@returns {boolean} result

@example
var out = isCartesianIndexString( '1,2,3' );
// returns true

@example
var out = isCartesianIndexString( '1' );
// returns false

@example
var out = isCartesianIndexString( ':' );
// returns false