grep 与正则表达式：除非添加断言，否则空格不匹配
GNU grep 2.5.4 on bash 4.1.5(1) on Ubuntu 10.04
这匹配
$ echo "this is a line" | grep 'a[[:space:]]\+line'
this is a line
但这不匹配
$ echo "this is a line" | grep 'a\s\+line'
但这也匹配
$ echo "this is a line" | grep 'a\s\+\bline'
this is a line
我不明白为什么#2不匹配（而#1匹配）并且#3也显示匹配。这里有什么区别？
GNU grep 2.5.4 on bash 4.1.5(1) on Ubuntu 10.04
This matches
$ echo "this is a line" | grep 'a[[:space:]]\+line'
this is a line
But this doesn't
$ echo "this is a line" | grep 'a\s\+line'
But this matches too
$ echo "this is a line" | grep 'a\s\+\bline'
this is a line
I don't understand why #2 does not match (whereas # 1 does) and #3 also shows a match. Whats the difference here?
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查看您的
grep联机帮助页。 Perl 添加了许多原始规范中没有的正则表达式扩展。然而，由于它们被证明非常有用，许多程序都采用了它们。不幸的是，
grep有时会停留在过去，因为您希望确保grep命令与旧版本的grep保持兼容。有些系统有
egrep和一些扩展。其他允许您使用 grep -E 来获取它们。还有一些有grep -P允许您使用 Perl 扩展。我相信Linux系统的grep命令可以使用大多数Unix系统中不可用的-P扩展，除非有人用GNU版本替换了grep。较新版本的 Mac OS X 也支持-P开关，但较旧版本不支持。Take a look at your
grepmanpage. Perl added a lot of regular expression extensions that weren't in the original specification. However, because they proved so useful, many programs adopted them.Unfortunately,
grepis sometimes stuck in the past because you want to make sure yourgrepcommand remains compatible with older versions ofgrep.Some systems have
egrepwith some extensions. Others allow you to usegrep -Eto get them. Still others have agrep -Pthat allows you to use Perl extensions. I believe Linux systems' grep command can use the-Pextension which is not available in most Unix systems unless someone has replaced the grep with the GNU version. Newer versions of Mac OS X also support the-Pswitch, but not older versions.grep不支持完整的正则表达式集，因此请尝试使用-P启用 Perl 正则表达式。您不需要转义+即grepdoesn't support the complete set of regular expressions, so try using-Pto enable perl regular expressions. You don't need to escape the+i.e.