I have come across a regular expression that I don't fully understand - can somebody help me in deciphering it:
我遇到了一个我不完全理解的正则表达式 - 有人可以帮我解释它：
^home(?:\/|\/index\.asp)?(?:\?.+)?$
It is used in url matching and the above example matches the following urls:
它用于url匹配，上面的示例匹配以下url：
home
home/
home/?a
home/?a=1
home/index.asp
home/index.asp?a
home/index.asp?a=1
It seems to me that the question marks within the brackets (?: don't do anything. Can somebody enlighten me.
在我看来，括号内的问号（？：什么都不做。有人可以启发我。
The version of regex being used is the one supplied with Classic ASP and is being run on the server if that helps at all.
正在使用的正则表达式版本是Classic ASP提供的版本，如果有帮助的话，它正在服务器上运行。
2 个解决方案
#1
17
(?:) creates a non-capturing group. It groups things together without creating a backreference.
（？:)创建一个非捕获组。它将事物分组在一起而不会产生反向引用。
A backreference is a part you can refer to in the expression or a possible replacement (usually by saying \1 or $1 etc - depending on flavor). You can also usually extract them from a match afterwards when using regex in a programming language. The only reason for using (?:) is to avoid creating a new backreference, which avoids incrementing the group number, and saves (a usually negligible amount of) memory
反向引用是您可以在表达式中引用的部分或可能的替换（通常通过说\ 1或$ 1等 - 取决于风格）。在编程语言中使用正则表达式时，通常也可以从匹配中提取它们。使用（？:)的唯一原因是避免创建新的反向引用，这可以避免增加组号，并节省（通常可忽略不计的数量）内存
#2
4
It's a non-capture group, which essentially is the same as using (...), but the content isn't retained (not available as a back reference).
它是一个非捕获组，基本上与使用（...）相同，但内容不会保留（不可用作后引用）。
If you're doing something like this: (abc)(?:123)(def) You'll get abc in $1 and def in $2, but 123 will only be matched.
如果你正在做这样的事情：（abc）（？：123）（def）你将获得1美元的abc和2美元的def，但123只会匹配。
#1
17
(?:) creates a non-capturing group. It groups things together without creating a backreference.
（？:)创建一个非捕获组。它将事物分组在一起而不会产生反向引用。
A backreference is a part you can refer to in the expression or a possible replacement (usually by saying \1 or $1 etc - depending on flavor). You can also usually extract them from a match afterwards when using regex in a programming language. The only reason for using (?:) is to avoid creating a new backreference, which avoids incrementing the group number, and saves (a usually negligible amount of) memory
反向引用是您可以在表达式中引用的部分或可能的替换（通常通过说\ 1或$ 1等 - 取决于风格）。在编程语言中使用正则表达式时，通常也可以从匹配中提取它们。使用（？:)的唯一原因是避免创建新的反向引用，这可以避免增加组号，并节省（通常可忽略不计的数量）内存
#2
4
It's a non-capture group, which essentially is the same as using (...), but the content isn't retained (not available as a back reference).
它是一个非捕获组，基本上与使用（...）相同，但内容不会保留（不可用作后引用）。
If you're doing something like this: (abc)(?:123)(def) You'll get abc in $1 and def in $2, but 123 will only be matched.
如果你正在做这样的事情：（abc）（？：123）（def）你将获得1美元的abc和2美元的def，但123只会匹配。