Meta-Character and regexp

Regular expressions use meta-characters. The shells also have meta-characters.Meta-characters are simply characters that have a special meaning.The problem occurs when you want to use a regular expression in a shell script. Will the shell do something special with the character?Or will it be passed unchanged to the program?The "$" character is a good example.It could be the beginning of a variable name, or it could be part of a regular expression.If you need a regular expression, you must know if any of the characters of the expression are meta-characters, and must know the right way to quote that character, so that it is passed to the program without being modified by the shell.

Here is a chart of the meta-characters the Bourne and C shell know about.I have also included several combinations of characters just to make this table more complete. There is a lot of detail on this chart.

Character	Where	Meaning
<RETURN>	csh, sh	Execute command
#	csh, sh, ASCII files	Start a comment
<SPACE>	csh, sh	Argument separator
`	csh, sh	Command substitution
"	csh, sh	Weak Quotes
'	csh, sh	Strong Quotes
\	csh, sh	Single Character Quote
variable	sh, csh	Variable
variable	csh, sh	Same as variable
|	csh, sh	Pipe character
^	sh	Pipe Character
&	csh, sh	Run program in background
?	csh, sh	Match one character
*	csh, sh	Match any number of characters
;	csh, sh	Command separator
;;	sh	End of Case statement
~	csh	Home Directory
~user	csh	User's Home Directory
!	csh	History of Commands
-	Programs	Start of optional argument
$#	csh, sh	Number of arguments to script
$*	csh, sh	Arguments to script
$@	sh	Original arguments to script
$-	sh	Flags passed to shell
$?	sh	Status of previous command
$$	sh	Process identification number
$!	sh	PID of last background job
&&	sh	Short-circuit AND
||	sh	Short-circuit OR
.	csh, sh	Typ. filename extension
.	sh	Source a file and execute as command
:	sh	Nothing command
:	sh	Separates Values in environment variables
:	csh	Variable modifier
Character	Where	Meaning
[ ]	csh, sh	Match range of characters
[ ]	sh	Test
%job	csh	Identifies job Number
(cmd;cmd)	csh. sh	Runs cmd;cmd as a sub-shell
{ }	csh	In-line expansions
{cmd;cmd }	sh	Like (cmd;cmd ) without a subshell
>file	csh, sh	Standard output
>>file	csh, sh	Append to standard output
<file	csh, sh	Standard Input
<<word	csh, sh	Read until word, substitute variables
<<\word	csh, sh	Read until word, no substitution
<<-word	sh	Read until word, ignoring TABS
>>!file	csh	Append to file, ignore error if not there
>!file	csh	Output to new file, ignore error if not there
>&file	csh	Send standard & error output to file
<&digit	sh	Switch Standard Input to file
<&-	sh	Close Standard Input
>&digit	sh	Switch Standard Output to file
>&-	sh	Close Standard Output
digit1<&digit2	sh	Connect digit2 to digit1
digit<&-	sh	Close file digit
digit2>&digit1	sh	Connect digit2 to digit1
digit>&-	sh	Close file digit

这两天我一直纠结关于正则表达式在emacs，grep,find中何时该加引号，何时不该加引号的问题。虽然我前面的两篇博客“浅谈lisp与shell中转义字符和正则表达”也基本上理清了，因为本身shell/emacs会有一些特殊字符我们称为Meta-Character，而正则表达式也是利用Meta-Character实现的；所以说这个时候就会有一些情况，两个字符在双方都是Meta-character，因为我们既然是正则表达的话，肯定是传给正则表达它所想要的字符形式，但是如果你在shell这边穿过去的时候，它的形式变了。比如$test,你传给正则的不是形式$test而是它的值。所以这就需要进行引号引用或转义了。

唉，我感觉我前两篇写的核心思想，被这个哥们上面的几句话，给结束了；太受挫了。悲了个剧！！！！最后看到它的简介以后心里平衡了，原来是出过书的大牛。。。

Meta-Character in a Unix command that the shell interprets rather than passing to the command也就是说shell碰见下面的这些符号的话，不会说把它传给Command，而是自己对它进行解析。

比如下面[] 和?经过echo值仍旧是[et]?，所以我们就可以安心的传给正则表达式函数了，而不需要加引号。具体是否变化可以通过echo来检验

UNIX SHELL Quote Tutorial

The Grymoire - home for UNIX wizards

浅谈lisp与shell中转义字符和正则表达 (一)

浅谈lisp与shell中转义字符和正则表达 (二)