扩展正则表达式

+  重复前面字符一次或多次
   要匹配 god ， good，  goood 使用：
   grep -E 'go+d' test
？ 重复前面字符0次或一次
   匹配gd 或 god， 使用
   grep -E 'go?d' test
|  表示 或
   匹配good或bad，使用
   grep -E 'good|bad' test
() 分组
   要查找glad 或good， g和d相同，把la 和oo分组：
   grep -E 'g(la|oo)d' test
()+ 多个重复组的判断
   匹配 AabcabcabcC 中间abc为多组
   grep -E 'A(abc)+C' test 

3. 简写

Character Class Abbreviations
\d	Match any character in the range 0 - 9 (equivalent of POSIX [:digit:])
\D	Match any character NOT in the range 0 - 9 (equivalent of POSIX [^[:digit:]])
\s	Match any whitespace characters (space, tab etc.). (equivalent of POSIX [:space:] EXCEPT VT is not recognized)
\S	Match any character NOT whitespace (space, tab). (equivalent of POSIX [^[:space:]])
\w	Match any character in the range 0 - 9, A - Z and a - z (equivalent of POSIX [:alnum:])
\W	Match any character NOT the range 0 - 9, A - Z and a - z (equivalent of POSIX [^[:alnum:]])
Positional Abbreviations
\b	Word boundary. Match any character(s) at the beginning (\bxx) and/or end (xx\b) of a word, thus \bton\b will find ton but not tons, but \bton will find tons.
\B	Not word boundary. Match any character(s) NOT at the beginning(\Bxx) and/or end (xx\B) of a word, thus \Bton\B will find wantons but not tons, but ton\B will find both wantons and tons.