问题描述：

Suppose I have the following line:

1309270927C1642,61N654NONREF

Now I want to get the C or D after the first digits. Now there are a few rules here

1. The first 6 digits are always there
2. The 4 digits after that are optional
3. After that you have a D or a C.

Now I wanted to solve that with a look behind:

/(?<=\\d{6,10})D|C/ but that is not allowed in PHP.

So I tried a non capturing group /(?:\\d{6,10})D|C/. But that captures 1309270927C in stead of just C.

So my question is how can I just capture the D or a C?

图片转代码服务由CSDN问答提供

感谢您的意见，我们尽快改进~

功能建议

假设我有以下行：</ p>

  1309270927C1642,61N654NONREF  
</ code> </ pre> 
现在我想在第一个数字后面得到 C </ code>或 D </ code>。 现在这里有一些规则</ p> 
  
前6位数始终存在</ li> 
之后的4位数字是可选的</ li>  
之后你有 D </ code>或 C </ code>。</ li> 
</ ol> 
现在我想要 看看后面的解决方法：</ p> 
  /（？＆lt; = \\ d {6,10}）D | C / </ code>但PHP中不允许这样做 。</ p> 
所以我尝试了非捕获组 /（？：\\ d {6,10}）D | C / </ code>。 但是它捕获 1309270927C </ code>而不仅仅是 C </ code>。</ p> 
所以我的问题是如何才能捕获 D <  / code>或 C </ code>？</ p> 
</ div> 
网友观点：
You can use PCRE \\K operator:
\\d{6,10}\\K[DC]
It will omit everything in the match up to D or C. You may further tweak this regex allowing or disallowing more characters to the character class [DC].
Have a look at the example.
Sample code:
$re = \\\"/\\\\d{6,10}\\\\K[DC]/\\\"; 
$str = \\\"1309270927C1642,61N654NONREF\\\"; 
preg_match_all($re, $str, $matches);
Also, here is some more information on \\K operator:
  
The \\K \\\"keep out\\\" verb, which is available in Perl, PCRE (C, PHP, R…)  and Ruby 2+. \\K tells the engine to drop whatever it has matched so  far from the match to be returned.
    
Instead of (?<=\\b\\d+_)[A-Z]+, you can therefore use \\b\\d+_\\K[A-Z]+
The limitations of \\K:
Compared with lookbehinds, both the \\K and capture group workarounds have limitations:
  
✽ When you look for multiple matches in a string, at the starting
position of each match attempt, a lookbehind can inspect the
characters behind the current position in the string. Therefore,
against 123, the pattern (?<=\\d)\\d (match a digit preceded by a digit)
will match both 2 and 3. In contrast, \\d\\K\\d can only match 2, as the
starting position after the first match is immediately before the 3,
and there are not enough digits left for a second match. Likewise,
\\d(\\d) can only capture 2. 
    
✽ With lookbehinds, you can impose multiple conditions (similar to our
password validation technique) by using multiple lookbehinds. For
instance, to match a digit that is preceded by a lower-case Greek
letter, you can use (?<=\\p{Ll})(?<=\\p{Greek})\\d. The first lookbehind
(?<=\\p{Ll}) ensures that the character immediately to the left is a
lower-case letter, and the second lookbehind (?<=\\p{Greek}) ensures
that the character immediately to the left belongs to the Greek
script. With the workarounds, you could use \\p{Greek}\\K\\d to match a
digit preceded by a character in the Greek script (or \\p{Greek}(\\d) to
capture it), but you cannot impose a second condition. To get over
this limitation, you could capture the Greek character and use a
second regex to check that it is a lower-case letter.
Output: 
C
    
I would use a capturing subpattern, like this:
$string = \\\"1309270927C1642,61N654NONREF\\\";
$pattern = \'/\\d{6,10}(C|D)/\';
preg_match($pattern, $string, $matches);
// $matches[1] contains the contents of the first subpattern
echo $matches[1];
    
正则表达式指定字符后插入数据
需求背景
睡眠助理添加了古诗词显示效果，API接口返回的古诗词数据如下：