“未实现可变长度向后看".但这不是可变长度 [英] "Variable length lookbehind not implemented" but it isn't variable length

问题描述

我正在尝试诊断一个非常疯狂的正则表达式.它也很长，但是我将其简化为以下脚本.使用Strawberry Perl v5.26.2运行.

I have a very crazy regex that I'm trying to diagnose. It is also very long, but I have cut it down to just the following script. Run using Strawberry Perl v5.26.2.

use strict;
use warnings;

my $text = "M Y H A P P Y T E X T";
my $regex = '(?i)(?<!(Mon|Fri|Sun)day |August )abcd(?-i)';

if ($text =~ m/$regex/){
    print "true\n";
}
else {
    print "false\n";
}

这会产生错误正则表达式中未实现可变长度后视".

This gives the error "Variable length lookbehind not implemented in regex."

我希望您可以解决一些问题:

I am hoping you can help with several issues:

1. 我不知道为什么会发生此错误，因为所有可能的lookbehind值都是7个字符:星期一"，星期五"，星期日"，八月".
2. 我自己没有编写此正则表达式，并且不确定如何解释语法(?i)和(?-i).当我摆脱(?i)时，错误实际上消失了. Perl将如何解释正则表达式的这一部分?我认为前两个字符被评估为可选的文字括号"，除了不对括号进行转义，而且在这种情况下，我会遇到另一种语法错误，因为结束括号将不匹配.
3. 此行为至少在草莓Perl中开始于Perl 5.16.3_64和5.26.1_64之间.前一个版本可以使用代码，而后者则不行.为什么开始?

1. I don't see why this error would occur, because all of the possible lookbehind values are 7 characters: "Monday ", "Friday ", "Sunday ", "August ".
2. I did not write this regex myself, and I am not sure how to interpret the syntax (?i) and (?-i). When I get rid of the (?i) the error actually goes away. How will perl interpret this part of the regex? I would think the first two characters are evaluated to "optional literal parentheses" except that the parentheses isn't escaped and also in that case I would get a different syntax error because the closing parentheses would then not be matched.
3. This behavior starts somewhere between Perl 5.16.3_64 and 5.26.1_64, at least in Strawberry Perl. The former version is fine with the code, the latter is not. Why did it start?

推荐答案

我已将您的问题归结为:

I have reduced your problem to this:

my $text = 'M Y H A P P Y T E X T';
my $regex = '(?<!st)A';
print ($text =~ m/$regex/i ? "true\n" : "false\n");

由于存在/i(不区分大小写)修饰符和某些字符组合(例如"ss"或"st")，可以用

Due to presence of /i (case insensitive) modifier and presence of certain character combinations such as "ss" or "st" that can be replaced by a Typographic_ligature causing it to be a variable length (/August/i matches for instance on both AUGUST (6 characters) and august (5 characters, the last one being U+FB06)).

但是，如果我们删除/i(不区分大小写)修饰符，则它会起作用，因为印刷连字不匹配.

However if we remove /i (case insensitive) modifier then it works because typographic ligatures are not matched.

解决方案:使用aa修饰符，即:

/(?<!st)A/iaa

或者在您的正则表达式中:

Or in your regex:

my $text = 'M Y H A P P Y T E X T';
my $regex = '(?<!(Mon|Fri|Sun)day |August )abcd';
print ($text =~ m/$regex/iaa ? "true\n" : "false\n");

来自 perlre :

要禁止ASCII/非ASCII匹配(如带有"\ N {KELVIN SIGN}"的"k")，请指定两次"a"，例如/aai或/aia. (第一个出现的"a"会限制\d等，第二个出现会添加"/i"限制.)但是请注意，ASCII范围以外的代码点将使用Unicode规则进行/i匹配，因此修饰符并没有真正将事情限制为仅ASCII； 它只是禁止将ASCII和非ASCII混用.

To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"), specify the "a" twice, for example /aai or /aia. (The first occurrence of "a" restricts the \d, etc., and the second occurrence adds the "/i" restrictions.) But, note that code points outside the ASCII range will use Unicode rules for /i matching, so the modifier doesn't really restrict things to just ASCII; it just forbids the intermixing of ASCII and non-ASCII.

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