在Bash正则表达式中，`^`和`$`是指行还是整个字符串? [英] In Bash regular expressions do `^` and `$` refer to lines, or to the entire string?
问题描述
在 Linux文档项目中(我没有在 Bash手册)中找不到有关正则表达式元字符的详细信息，元字符 ^ 和 $ 被定义为匹配行:
In The Linux Documentation Project (I didn't find details about the regex metacharacters in the Bash manual), the metachars ^ and $ are defined as matching lines:
^:匹配行[...]
开头的空字符串$:匹配行尾的空字符串
^: Matches the empty string at the beginning of a line [...]
$: Matches the empty string at the end of a line
但是，当我尝试时，这是不正确的:
however, when I try, this is not correct:
$ string="a
> b
> c"
$ [[ $string =~ ^a ]] && echo BOS match
BOS match
$ [[ $string =~ ^b ]] && echo BOL match
# nothing
这些手册真的不对吗，还是我遗漏了一些东西?
Are the manuals really wrong, or I am missing something?
推荐答案
^ 匹配整个输入字符串的开头，而 $ 匹配整个输入字符串的结尾POSIX正则表达式(Bash使用POSIX ERE).您链接到的文档中提到行，因为大多数文本处理工具(例如 sed ， grep 或 awk )默认情况下逐行读取输入，并且在大多数情况下，字符串与该行重合.
^ matches start of the whole input string and $ matches the end of the whole input string in a POSIX regex (Bash uses POSIX ERE). The document you link to mentions lines because most text processing tools, like sed, grep or awk read the input line by line by default, and string coincides with the line in the majority of cases.
请参见 POSIX正则表达式文档:
9.3.8 BRE表达锚定
可以将BRE限制为匹配以行开头或结尾的字符串.这就是所谓的锚定".在以下情况下，应将抑扬符和美元符号的特殊字符视为BRE锚点:
9.3.8 BRE Expression Anchoring
A BRE can be limited to matching strings that begin or end a line; this is called "anchoring". The circumflex and dollar sign special characters shall be considered BRE anchors in the following contexts:
-
回旋符(
'^')在用作整个BRE的第一个字符时应为锚.当用作子表达式的第一个字符时，该实现可以将抑扬符视为锚点.回旋符号应将表达式(或子表达式)锚定到字符串的开头；BRE只能匹配从字符串的第一个字符开始的序列.例如，BRE"^ ab"匹配字符串"abcdef"中的"ab"，但不匹配字符串"cdefab".BRE(^ ab)"可能与前一个字符串匹配.便携式BRE必须在子表达式中避开领先的抑扬符，以匹配文字抑扬符.
A circumflex (
'^') shall be an anchor when used as the first character of an entire BRE. The implementation may treat the circumflex as an anchor when used as the first character of a subexpression. The circumflex shall anchor the expression (or optionally subexpression) to the beginning of a string; only sequences starting at the first character of a string shall be matched by the BRE. For example, the BRE "^ab" matches "ab" in the string "abcdef", but fails to match in the string "cdefab". The BRE "(^ab)" may match the former string. A portable BRE shall escape a leading circumflex in a subexpression to match a literal circumflex.
美元符号('$')在用作整个BRE的最后一个字符时应作为锚点.当用作子表达式的最后一个字符时，该实现可以将美元符号视为锚点.美元符号应将表达式(或子表达式)锚定在要匹配的字符串的末尾；可以说美元符号与最后一个字符后的字符串结尾匹配.
A dollar sign ( '$' ) shall be an anchor when used as the last character of an entire BRE. The implementation may treat a dollar sign as an anchor when used as the last character of a subexpression. The dollar sign shall anchor the expression (or optionally subexpression) to the end of the string being matched; the dollar sign can be said to match the end-of-string following the last character.
由'^'和'$'锚定的BRE仅匹配整个字符串.例如，BRE "^ abcdef $" 匹配仅由"abcdef" 组成的字符串.
A BRE anchored by both '^' and '$' shall match only an entire string. For example, the BRE "^abcdef$" matches strings consisting only of "abcdef".
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