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 (xxb) of a word,thus btonb 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 (xxB) of a word,thus BtonB will find wantons but not tons,but tonB will find both wantons and tons.