正则表达式（转载整理而成）

字符匹配

正则表达式的关键之处在于确定你要搜索匹配的东西，如果没有这一概念，Res将毫无用处。

每一个表达式都包含需要查找的指令，如表A所示。

Table A: Character-matching regular expressions
操作	解释	例子	结果
.	Match any one character	grep .ord sample.txt	Will match “ford”, “lord”, “2ord”, etc. in the file sample.txt.
[ ]	Match any one character listed between the brackets	grep [cng]ord sample.txt	Will match only “cord”, “nord”, and “gord”
[^ ]	Match any one character not listed between the brackets	grep [^cn]ord sample.txt	Will match “lord”, “2ord”, etc. but not “cord” or “nord”
		grep [a-zA-Z]ord sample.txt	Will match “aord”, “bord”, “Aord”, “Bord”, etc.
		grep [^0-9]ord sample.txt	Will match “Aord”, “aord”, etc. but not “2ord”, etc.

重复操作符

重复操作符，或数量词，都描述了查找一个特定字符的次数。它们常被用于字符匹配语法以查找多行的字符，可参见表B。

Table B: Regular expression repetition operators
操作	解释	例子	结果
?	Match any character one time, if it exists	egrep “?erd” sample.txt	Will match “berd”, “herd”, etc. and “erd”
*	Match declared element multiple times, if it exists	egrep “n.*rd” sample.txt	Will match “nerd”, “nrd”, “neard”, etc.
+	Match declared element one or more times	egrep “[n]+erd” sample.txt	Will match “nerd”, “nnerd”, etc., but not “erd”
{n}	Match declared element exactly n times	egrep “[a-z]{2}erd” sample.txt	Will match “cherd”, “blerd”, etc. but not “nerd”, “erd”, “buzzerd”, etc.
{n,}	Match declared element at least n times	egrep “.{2,}erd” sample.txt	Will match “cherd” and “buzzerd”, but not “nerd”
{n,N}	Match declared element at least n times, but not more than N times	egrep “n[e]{1,2}rd” sample.txt	Will match “nerd” and “neerd”

锚

锚是指它所要匹配的格式，如图C所示。使用它能方便你查找通用字符的合并。例如，我用vi行编辑器命令:s来代表substitute，这一命令的基本语法是：

s/pattern_to_match/pattern_to_substitute/
 

Table C: Regular expression anchors
操作	解释	例子	结果
^	Match at the beginning of a line	s/^/blah /	Inserts “blah “ at the beginning of the line
$	Match at the end of a line	s/$/ blah/	Inserts “ blah” at the end of the line
\<	Match at the beginning of a word	s/\</blah/	Inserts “blah” at the beginning of the word
		egrep “\<blah” sample.txt	Matches “blahfield”, etc.
\>	Match at the end of a word	s/\>/blah/	Inserts “blah” at the end of the word
		egrep “\>blah” sample.txt	Matches “soupblah”, etc.
\b	Match at the beginning or end of a word	egrep “\bblah” sample.txt	Matches “blahcake” and “countblah”
\B	Match in the middle of a word	egrep “\Bblah” sample.txt	Matches “sublahper”, etc.

间隔

 

Res中的另一可便之处是间隔(或插入)符号。实际上，这一符号相当于一个OR语句并代表|符号。下面的语句返回文件sample.txt中的“nerd” 和 “merd”的句柄：

egrep “(n|m)erd” sample.txt

间隔功能非常强大，特别是当你寻找文件不同拼写的时候，但你可以在下面的例子得到相同的结果：

egrep “[nm]erd” sample.txt

当你使用间隔功能与Res的高级特性连接在一起时，它的真正用处更能体现出来。

一些保留字符

Res的最后一个最重要特性是保留字符(也称特定字符)。例如，如果你想要查找“ne*rd”和“ni*rd”的字符，格式匹配语句“n[ei]*rd”与“neeeeerd” 和 “nieieierd”相符合，但并不是你要查找的字符。因为‘*’(星号)是个保留字符，你必须用一个反斜线符号来替代它，即：“n[ei]\*rd”。其它的保留字符包括：

• ^ (carat)
• . (period)
• [ (left bracket}
• $ (dollar sign)
• ( (left parenthesis)
• ) (right parenthesis)
• | (pipe)
• * (asterisk)
• + (plus symbol)
• ? (question mark)
• { (left curly bracket, or left brace)
• \ backslash

一旦你把以上这些字符包括在你的字符搜索中，毫无疑问Res变得非常的难读。比如说以下的PHP中的eregi搜索引擎代码就很难读了。

eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*$",$sendto)

你可以看到，程序的意图很难把握。但如果你抛开保留字符，你常常会错误地理解代码的意思。如果你想阅览Open Group组织的规则的完整描述，你可以参见：Regular Expressions，欢迎你在其中的讨论区发表你的问题或观点。