Question

grep -i -o '<a[^>]\+href[ ]*=[ \t]*"\(ht\|f\)tps\?:[^"]\+"' | sed -e 's/^.*"\([^"]\+\)".*$/\1/g'

在拖网上找到我的作业问题的答案后，我终于得到了上述内容。但我并不完全理解sed和grep使用的两个正则表达式的含义。有人可以对我说清楚吗？提前谢谢。

Answer 1

grep命令查找包含与

匹配的任何行

'<a[^>]\+href[ ]*=[ \t]*"\(ht\|f\)tps\?:[^"]\+"'

是

<a     the characters <a
[^>]   not followed by a close '>'
\+     the last thing one or more times (this is really not necessary I think.
       with this, it would be "not followed by exactly one '>' which would be fine
href   followed by the string 'href'
[ ]*   followed by zero or more spaces (you don't really need the [], just ' *' would be enough)
=      followed by the equals sign
[ \t]* followed by zero or more space or tab ("white space")
"      followed by open quote (but only a double quote...)
\(     open bracket (grouping)
ht     characters 'ht'
\|     or
f      character f
\)     close group (of the either-or)
tp     characters 'tp'
s\?    optionally followed by s
       Note - the last few lines combined means 'http or https or ftp or ftps'
:      character :
[^"]\+ one or more characters that are not a double quote
       this is "everything until the next quote"

这会让你入手吗？你可以为下一位做同样的事情......

注意让你感到困惑 - 反斜杠用于改变某些特殊字符的含义，如()+;只是为了让每个人都保持警惕，无论这些是否具有特殊含义，无论是否使用反斜杠都不是由正则表达式语法定义的东西，而是由您使用它的命令（及其选项）定义的。例如，sed会根据您是否使用-E标记来更改事物的含义。

grep和sed正则表达式含义 - 从网页中提取网址

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