使用perl中的regex匹配上次出现的事件

I have a text like this:

我有这样的文字：

hello world /* select a from table_b
*/ some other text with new line cha
racter and there are some blocks of 
/* any string */ select this part on
ly 
////RESULT rest string

The text is multilined and I need to extract from last occurrence of "*/" until "////RESULT". In this case, the result should be:

文本是多行的，我需要从最后一次出现的“* /”中提取，直到“//// RESULT”。在这种情况下，结果应该是：

 select this part on
ly

How to achieve this in perl?

如何在perl中实现这一目标？

I have attempted \\\*/(.|\n)*////RESULT but that will start from first "*/"

我试过\\\ * /（。| \ n）* ////结果但是从第一个“* /”开始

3 个解决方案

#1

A useful trick in cases like this is to prefix the regexp with the greedy pattern .*, which will try to match as many characters as possible before the rest of the pattern matches. So:

在这种情况下，一个有用的技巧是在regexp前加上贪婪模式。*，它会在模式的其余部分匹配之前尝试匹配尽可能多的字符。所以：

my ($match) = ($string =~ m!^.*\*/(.*?)////RESULT!s);

Let's break this pattern into its components:

让我们将这种模式分解为其组成部分：

^.* starts at the beginning of the string and matches as many characters as it can. (The s modifier allows . to match even newlines.) The beginning-of-string anchor ^ is not strictly necessary, but it ensures that the regexp engine won't waste too much time backtracking if the match fails.

^。*从字符串的开头开始，尽可能多地匹配字符。（s修饰符允许。甚至匹配换行符。）字符串开头的锚点^并不是绝对必要的，但它确保正则表达式引擎在匹配失败时不会浪费太多时间回溯。
\*/ just matches the literal string */.

\ * /只匹配文字字符串* /。
(.*?) matches and captures any number of characters; the ? makes it ungreedy, so it prefers to match as few characters as possible in case there's more than one position where the rest of the regexp can match.

（。*？）匹配并捕获任意数量的字符;的？使它不合适，所以它更喜欢匹配尽可能少的字符，以防正则表达式的其余部分可以匹配多个位置。
Finally, ////RESULT just matches itself.

最后，////结果只是匹配自己。

Since the pattern contains a lot of slashes, and since I wanted to avoid leaning toothpick syndrome, I decided to use alternative regexp delimiters. Exclamation points (!) are a popular choice, since they don't collide with any normal regexp syntax.

由于该模式包含很多斜线，并且因为我想避免倾斜牙签综合症，所以我决定使用替代的regexp分隔符。感叹号（！）是一种流行的选择，因为它们不会与任何正常的正则表达式语法冲突。

Edit: Per discussion with ikegami below, I guess I should note that, if you want to use this regexp as a sub-pattern in a longer regexp, and if you want to guarantee that the string matched by (.*?) will never contain ////RESULT, then you should wrap those parts of the regexp in an independent (?>) subexpression, like this:

编辑：每次与ikegami的讨论，我想我应该注意，如果你想在更长的正则表达式中使用这个正则表达式作为子模式，并且如果你想保证匹配的字符串（。*？）永远不会包含//// RESULT，那么你应该将regexp的那些部分包装在一个独立的（？>）子表达式中，如下所示：

my $regexp = qr!\*/(?>(.*?)////RESULT)!s;
...
my $match = ($string =~ /^.*$regexp$some_other_regexp/s);

The (?>) causes the pattern inside it to fail rather than accepting a suboptimal match (i.e. one that extends beyond the first substring matching ////RESULT) even if that means that the rest of the regexp will fail to match.

（？>）导致其中的模式失败而不是接受次优匹配（即超出匹配//// RESULT的第一个子串的匹配），即使这意味着正则表达式的其余部分将无法匹配。

#2

(?:(?!STRING).)*

matches any number of characters that don't contain STRING. It's like [^a], but for strings instead of characters.

匹配任意数量的不包含STRING的字符。它就像[^ a]，但是对于字符串而不是字符。

You can take shortcuts if you know certain inputs won't be encountered (like Kenosis and Ilmari Karonen did), but this is what what matches what you specified:

如果您知道不会遇到某些输入（如Kenosis和Ilmari Karonen所做的那样），您可以使用快捷方式，但这与您指定的匹配：

my ($segment) = $string =~ m{
    \*/
    ( (?: (?! \*/ ). )* )
    ////RESULT
    (?: (?! \*/ ). )*
    \z
}xs;

If you don't care if */ appears after ////RESULT, the following is the safest:

如果您不关心// // RESULT之后是否出现* /，则以下是最安全的：

my ($segment) = $string =~ m{
    \*/
    ( (?: (?! \*/ ). )* )
    ////RESULT
}xs;

You didn't specify what should happen if there are two ////RESULT that follow the last */. The above matches until the last one. If you wanted to match until the first one, you'd use

如果有两个//// RESULT跟随最后一个* /，则没有指定会发生什么。以上匹配到最后一个。如果你想匹配到第一个，你可以使用

my ($segment) = $string =~ m{
    \*/
    ( (?: (?! \*/ | ////RESULT ). )* )
    ////RESULT
}xs;

#3

Here's one option:

这是一个选项：

use strict;
use warnings;

my $string = <<'END';
hello world /* select a from table_b
*/ some other text with new line cha
racter and there are some blocks of 
/* any string */ select this part on
ly 
////RESULT
END

my ($segment) = $string =~ m!\*/([^/]+)////RESULT$!s;

print $segment;

Output:

输出：

 select this part on
ly

#1