\(ab\)* 后向引用
\1，引用第一个括号内包涵的内容
\2，第二个
\3，第三个
$ grep '\(l..e\).*\1' 12.txt #前后一致
he loves me, lover
he like him , like her
he like her ,like me
前提回顾：
基本表达
.
[]
[^]
次数
*
\?
\{m,n\}
.*
铆钉
^
$
\b, \<
\b, \>
分组
\(\)
后向引用
\1,\2,\3
grep -i
grep -o
grep --color
grep -v
grep -E ：扩展正则表达式
grep -A：下面2行
grep -B：上面2行
grep -C: 上下各2行，context
$ grep -A 2 '\blove\b' 12.txt
he like me , love me
he like him , like her
he like her ,like me
grep -E ：扩展正则表达式 or egrep
字符不一样：
?: 不用加\
+：匹配其前面的字符至少1次
{m,n}
$ grep -E 'l{1,5}..e.*' 12.txt
he loves me, lover
he like me , love me
he like him , like her
he like her ,like me
或者
|: or
$ grep -E 'H|he' 12.txt
he loves me, lover
he like me , love me
he like him , like her
he like her ,like me
He likes me
$ grep -E '(H|h)e' 12.txt
he loves me, lover
he like me , love me
he like him , like her
he like her ,like me
He likes me
$ grep -E '^[[:space:]]+' /boot/grub/grub.cfg
set have_grubenv=true
load_env
set default="${next_entry}"
set next_entry=
$ egrep '\<([1-9]|[1-9][1-9])\>' /boot/grub/grub.cfg #找出1-99的整数
匹配元字符本身的意义
比如找. 需要加\.
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