Since groups are numbered from left to right and a complex expression may use many groups, it can become difficult to keep track of the correct numbering, and modifying such a complex RE is annoying.

• 因为组是从左到右编号的，而且一个复杂的表达式也许会使用许多组，它可以使跟踪当前组号变得困难，而修改如此复杂的RE是十分麻烦的。