正则表达式:PHP后向固定长度断言限制的解决方法 [英] Regex: Workaround for PHP's look-behind fixed-length assert limitation

问题描述

我试图了解有关环视断言的更多信息，并且发现了此线程 ，因为PHP要求在后面的断言必须是固定长度的，所以应该在某些引擎中使用它们的解决方案，但在PHP的引擎中不能使用它们.

I'm trying to understand more about look-around asserts and I found this thread, where their solution is supposed to work in some engines but not PHP's, because of PHP's requiring the look-behind assert to be fixed-length.

我想要的是使相同的方案在PHP中工作，或者至少知道是否有可能.

What I want is to make the same scenario work in PHP or at least know if it's possible at all.

我试图减少正则表达式规则的解释，因此它与上面提到的线程不同，但是遵循相同的原理.

I tried to reduce the regex rules explanation, so it's not the same as in the thread I mention above, but it follows the same principle.

需要匹配由三个部分组成的字符串:

Need to match a string built in three parts:

• 以任意数量的字母数字字符开头
• 不包含"abc-"，后跟3至5个数字和/或连字符
• 以".htm"或".html"结尾

因此，这些将匹配:

• xxxyz-123.html
• xx123-abc.htm
• xxabc123.html
• xxabc-123-45.htm

但是这些不匹配:

• xxabc-4324.htm
• xxabc-1-2.html
• xxac-12-34.txt
• xxabc-12345.htm

我一直在尝试下面的正则表达式模式的一些变体，但它无法正常工作-由于固定长度的限制，这种特殊情况是这样的:

I've been trying with some variations of the regex pattern below but it's not working - this particular case because of the fixed-length limitation:

.*(?<!abc-[\d-]{3,5})\.htm[^l]?$

我还使用了不同的测试字符串，而忽略了3-5的范围部分，只关注了准确的数字(例如3个数字和/或连字符)，并使用了下面的正则表达式，但仍然无法使用，这就是为什么我决定寻求帮助:

I also used different test strings and forgot about the 3-5 range part, focusing only on exactly , say, 3 numbers and/or hyphens, and used the regex below, and it still doesn't work, which is why I decided to ask for help on this:

.*(?<!abc-[\d-]{3})\.htm[^l]?$

你们中的任何一位正则表达式专家都可以在这里帮助我吗?

Could anyone of you regex gurus help me out here?

修改

这是我测试的PHP代码:

This is my testing PHP code:

$regex = "/^(?!.*abc-[\d-]{3,5})[a-zA-Z0-9-]+\.html?$/";

foreach ( $matching2 as $k => $v ) {
    $matches = preg_match( $regex, $v );

    echo '"', $v, '"', ( $matches != 0 ) ? ' matches' : ' doesn\'t match', '<br />';
}

推荐答案

为什么需要反向查看?为什么不只使用超前行呢?

Why do you need to need to look at that in reverse? Why not just use a lookahead?

^(?!.*abc-[\d-]{3,5}[^\d-])[a-zA-Z0-9-]+\.html?$

这将简单地开始查看字符串的开头，并且超前尝试尝试在字符串中的任意位置(.*)查找不允许的字符串.如果是这样，则超前使模式失败.这还包括以下要求:字符串只能由字母数字和连字符组成.

This will simply start looking at the beginning of the string and the lookahead tries to find the disallowed string anywhere (.*) in the string. If it does, the lookahead makes the pattern fail. This also include the requirement, that the string consists only of alphanumerics and hyphens.

这与您链接的问题所使用的解决方案相同. Perl也无法应付可变长度的回溯. 仅 .NET可以.

This is by the way the same solution that is used for the question you linked. Perl cannot cope with variable-length lookbehinds either. Only .NET can.

另一注:如果您遇到一个示例，其中您实际上确实需要在后面进行可变长的查找(但不需要在前面进行可变长的查找)...反转字符串(也包括模式) ， 当然). ;)

Another note: if you ever encounter an example where you actually do need a variable-length lookbehind (but not a variable-length lookahead)... reverse the string (and the pattern, too, of course). ;)

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