正则习点 --- 03

1.4.10 Parentheses andBackreferences

小括号的用途：

1.限制多选项（alternation）的范围；

2.使用「|」将若干字符组合为一个单元，受问号或星号之类量词的作用；

3.反向引用(backreference)；表现形式为元字符序列「\1…」

例如，

% egrep -i '\<([a-z]+) +\1\>' files…

1.4.11 The Great Escape

转义符：在除了字符组内部之外使用反斜线(backslash)，使元字符失去特殊含义，成为普通字符。

比如：

「\.」：转义的点号

1.5 Expanding the Foundation

1.5.1 A Few More Examples

1.5.1.1 A string within double quotes

A simple solution to matching a stringwithin double quotes might be: 「”[^”]*”」

两端的引号用来匹配字符串开头和结尾的引号。在这两个引号之间的文本可以包括双引号之外的任何字符。所以我们使用「[^*]」来匹配除双引号之外的任何字符，用「*」来表示两个引号之间可以存在任意数目的非双引号字符。

1.5.2 Regular Expression Nomenclature

1.5.2.1 Regex

正则表达式，简称正则(Regex)

1.5.2.2 Matching

正则表达式「a」不能匹配cat，但是能匹配cat中的a。

1.5.2.3 Metacharacter

只有在字符组外部并且是在未转义的情况下，才有意义。

1.5.2.4 Flavor

我们主要讲Perl流派。

1.5.2.5 Subexpression

“子表达式”指的是整个正则表达式中的一部分，通常是小括号内的表达式，或者是由「|」分隔的多选(alternation)分支。

1.5.2.6 Character

ASCII编码的字节

1.5.3 Summary

Egrep工具的元字符总结。

Table1-3. Egrep Metacharacter Summary

Items to Match a Single Character
Metacharacter	Matches
.	dot	Matches any one character
[…]	character class	Matches any one character listed
[^…]	negated character class	Matches any one character not listed
\char	escaped character	When char is a metacharacter, or the escaped combination is not otherwise special, matches the literal char
Items Appended to Provide “Counting”: The Quantifiers
?	question	One allowed, but it is optional
*	star	Any number allowed, but all are optional
+	plus	At least one required; additional are optional
{min, max}	specified range†	Min required, max allowed
Items That Match a Position
^	caret	Matches the position at the start of the line
$	dollar	Matches the position at the end of the line
\<	word boundary†	Matches the position at the start of a word
\>	word boundary†	Matches the position at the end of a word
Other
|	alternation	Matches either expression it separates
(…)	parentheses	Limits scope of alternation, provide grouping for the quantifies, and “captures” for backreferences
\1, \2, ...	backreference†	Matches text previously matched within first, second, etc., set of parentheses.