\<和 \>用于匹配word，但grep中的word不考虑标点，只考虑26个字母：
grep --color "\<ab\>" data.txt
hello (ab)
hello ab
(ab)和ab都match了

charactergrep egrep find wildcard
* >=0个前一个字符 >=0个前一个字符 >=0个前一个字符 >=0个任意字符.
ls m*l will get:
ml mal mbl m.l
指定m~n个 匹配g[2~3个o]d
grep "go\{2,3\}d" regex.txt
grep --color "\<c\{3\}99" data.txt # just c 3 times:
aa ccc99 bb
grep --color "\<c\{3,\}99" data.txt # c [3, infinity]:
aa ccc99 bb
aa cccc99 bb
 egrep "go{2,3}d" regex.txt find -regextype egrep -regex ".*go{2,3}d.*"
+ 仅egrep ≥1个前一个RE字符:
egrep --color 'go+d' data.txt
good
god
gooood
? 仅egrep 0或1个前一个RE字符:
egrep --color 'go?d' data.txt
god
gd
()+ 仅egrep ≥1个前一个组
egrep --color 'A(xyz)+C' data.txt
AxyzxyzC
AxyzxyzxyzC
() group 仅egrep egrep --color 'g(la|oo)d' data.txt
glad
good
[] []中的任意
egrep --color "4[89]3" data.txt
483 Sept 5AP1996 USP 65.00 LVX2C 189
如果需要匹配[]本身，需要\[和\]:
grep --color "\[file\]" data.txt
[file] 		范围(数字，字母)
grep --color "5..199[6,8]" data.txt
483 Sept 5AP1996 USP 65.00 LVX2C 189
483 may 5PA1998 USP 37.00 KVM9D 644
 1:单个列出. 2:指定范围
ls a[b,c,d].doc
ab.doc ac.doc ad.doc
ls a[b-z].doc
ab.doc ae.doc az.doc
? 一个任意字符
ls a?c
abc a.c axc
{} terms are separated by commas and each term must be
the name of something or a wildcard.
ls {*.doc,*.pdf}
a.doc b.doc c.pdf e.pdf
1:{}中不要有一个空格
2:{}项目个数至少两个，只有一个term又何必用{}?
php
item php perl compitable
* >=0个前个re字符
+ >=1个前个re字符
? 0或1个前个re字符
对反斜线'\'的转义 $content="a\\\\b";
echo "content length=".strlen($content)."\n";
echo $content."\n";
$pattern="/a\\\\{2}b/";
/*$pattern="/a\\\\\\\\b/";*/如果不用范围，匹配2个\需要8个
echo "pattern length=".strlen($pattern)."\n";
echo $pattern."\n";
if(preg_match($pattern, $content,$matches)){echo __LINE__; print_r($matches);}
输出：
content length=4
a\\b
pattern length=9
/a\\{2}b/
9Array
(
[0] => a\\b
)
mysql
character MySQL
% >=0个任意字符
_:下划线 1个任意字符 
https://www.gnu.org/software/sed/manual/html_node/Regular-Expressions.html
sed 去掉开头的空格
sed -i 's/^[ ]\{1,\}//g' xy.txt


sed需要转义的:^$.*\[


Letters, digits and (){}+?| must not be quoted (you can get away with quoting some of these in some implementations).
The sequences \(, \), \n, and in some implementations 
\{, \}, \+, \?, \| and other backslash+alphanumerics have special meanings.:

\( \)分组, \{ \}用于前一RE字符出现次数：
D.Gen[ice@ name]echo "|123787877878788787|ABCDEF" | sed 's/|\([0-9]\{1,\}\)|\([A-Z]\{1,\}\)/_\2_\1/g'
_ABCDEF_123787877878788787
\+
D.Gen[ice@ name]echo "|123787877878788787|ABCDEF|" | sed 's/|\([0-9]\+\)|\([A-Z]\+\)|/_\2_\1/g'
_ABCDEF_123787877878788787

关于list:
/* 其实记这些非特殊字符没有意义，因为非特殊太多了，应该记特殊字符
The characters $, ., *,\ and [ are normally not special within list. 
$ :
D.Gen[ice@ name]echo "|\$|" | sed 's/|[$]|/MM/g'
MM

.:
D.Gen[ice@ name]echo "_._"  | sed 's/_[.]_/MM/g'
MM

*:
D.Gen[ice@ name]echo "_*_"  | sed 's/_[*]_/MM/g'
MM

\
echo "_\\_"   | sed 's/_[\]_/MM/g'
MM

[
D.Gen[root@ stock]#echo "_[_"   | sed 's/_[[]_/MM/g'
MM
*/
特殊情况
1:
To include ] in the list, make it the first character (after the ^ if needed),
D.Gen[root@ stock]#echo "]]ab]]ab" | sed 's/[]]/_/g'
__ab__ab

2:
to include ^ put it after the first character.
D.Gen[root@ stock]#echo "^^ab^^ab"  | sed 's/[|^]/./g'
..ab..ab

3:
, to include - in the list, make it the first or last, but ] has occupy first, so - should occupy last
D.Gen[root@ stock]#echo "--ab--ab" | sed 's/[a-]/_/g'
___b___b


]:
D.Gen[root@ stock]#echo "]_]_"    | sed 's/[]]/MM/g'
MM_MM_
D.Gen[root@ stock]#echo "]_]_"    | sed 's/[^]]/MM/g'

re in python

python re
character Python RE
(pattern)? 0,1个pattern
(pattern)* >=0个前一个pattern
(pattern)+ >=1个前一个pattern
(pattern){m,n} >=m个, <=n个pattern