Data-format validation often triggers error pages or requires the client to re-enter incorrectly formatted data. 数据格式验证经常触发错误页面或要求客户机重新输入格式错误的数据。
Often, when writing a parser that is able to detect errors in the format of the input stream, we want it to communicate to the user where the error happened within that input. 通常，在编写一个可以检测输入流的格式错误的分析器时，我们会希望它可以告知用户，错误发生于输入的何处。
