GNU sed 中的基本正则与扩展正则在使用起来有些差异.有时为了简化语法会使用扩展正则.但是扩展正则后有些简单的语法亦有可能变得复杂.因此对两者之间的差异与细节有一个基本的了解很有必要;
字符加号 +
基本语法 [ BRE ]
$ echo 'a+b=c' > foo
$ sed -n '/a+b/p' foo
a+b=c
扩展语法 [ ERE ]
$ echo 'a+b=c' > foo
$ sed -E -n '/a\+b/p' foo
a+b=c
一个以上 a 跟随着字母 b [ 加号作为特殊元字符 ]
基本语法 [ BRE ]
$ echo aab > foo
$ sed -n '/a\+b/p' foo
aab
扩展语法 [ ERE ]
$ echo aab > foo
$ sed -E -n '/a+b/p' foo
aab
BRE 语法概览
| 语法 | 说明 | 备忘 |
|---|---|---|
char |
单个普通字符,匹配自身 | 无 |
* |
Matches a sequence of zero or more instances of matches for the preceding regular expression, which must be an ordinary character, a special character preceded by \, a .,a grouped regexp (see below), or a bracket expression. As a GNU extension, a postfixed regular expression can also be followed by *; for example, a** is equivalent to a*. POSIX 1003.1-2001 says that * stands for itself when it appears at the start of a regular expression or subexpression, but many nonGNU implementations do not support this and portable scripts should instead use \* in these contexts. |
|
. |
Matches any character, including newline. | |
^ |
||
$ |
It is the same as ^, but refers to end of pattern space.$ also acts as a special character only at the end of the regular expression orsubexpression (that is, before \) or |), and its use at the end ofa subexpression is not portable. |
|
\+ |
As *, but matches one or more. It is a GNU extension. |
|
\? |
As *, but only matches zero or one. It is a GNU extension. |
|
\{i\} |
As *, but matches exactlyisequences (iis a decimal integer; for portability,keep it between 0 and 255 inclusive). |
|
\{i,j\} |
||
\{i,\} |
||
\(regexp\) |
||
regexp1|regexp2 |
||
regexp1regexp2 |
||
\digit |
Matches thedigit-th \(…\) parenthesized subexpression in the regular expression.This is called a_back reference_. Subexpressions are implicitly numbered by counting occurrences of \( left-to-right. |
|
\n |
Matches the newline character. | |
\char |
Matcheschar, wherecharis one of $,*,.,[,\, or ^. Note that the only C-like backslash sequences that you can portably assume to be interpreted are \n and \\; in particular \t is not portable, and matches a ‘t’ under most implementations of sed, rather than a tab character. |
|
[list] or [^list] |
ERE 语法概览
The only difference between basic and extended regular expressions is in the behavior of a few characters: ‘?’, ‘+’, parentheses, braces (‘{}’), and ‘|’. While basic regular expressions require these to be escaped if you want them to behave as special characters, when using extended regular expressions you must escape them if you want them_to match a literal character_. ‘|’ is special here because ‘|’ is a GNU extension – standard basic regular expressions do not provide its functionality.
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