示例来自： Linux 期刊文章
正如本文所预期的那样，前两个示例有效，但第三个示例无效。有人可以解释为什么吗？我在 Ubuntu 22.04.1 和 RHEL9 上都尝试过，结果相同。
我的屏幕：
[primus@rhel9 sandbox]$ cat filename.doc
The fast dog is fast.
The faster dogs are faster.
A sick dog should see a dogdoc.
This file is filename.doc.
[primus@rhel9 sandbox]$ grep "fast*" filename.doc
The fast dog is fast.
The faster dogs are faster.
[primus@rhel9 sandbox]$ grep "dogs*" filename.doc
The fast dog is fast.
The faster dogs are faster.
A sick dog should see a dogdoc.
[primus@rhel9 sandbox]$ grep "*.doc" filename.doc
[primus@rhel9 sandbox]$
[primus@rhel9 sandbox]$
但作为扩展的正则表达式，第三个示例有效：
[primus@rhel9 sandbox]$
[primus@rhel9 sandbox]$ egrep "*.doc" filename.doc
A sick dog should see a dogdoc.
This file is filename.doc.
[primus@rhel9 sandbox]$ grep -E "*.doc" filename.doc
A sick dog should see a dogdoc.
This file is filename.doc.
[primus@rhel9 sandbox]$