Questions about regular expression reverse reference

For new users, please refer to the following questions about reverse reference of regular expressions: Hello, everyone! I'm a Cainiao. I have something to ask.

I recently read "Let's talk about PHP" and read it in a book. each subexpression in a regular expression (included in ), when no non-capturing metacharacters are added, they are stored in the buffer according to the matching results of each sub-expression from left to right and referenced by numbers \ 1 to \ 99, I don't know if this is the correct description. There are two problems:
1. The book says that using a serial number to reference a subexpression requires a backslash before the serial number to escape the subexpression. This article is very tangled and I don't need it. is it related to any version?
2. is a subexpression added with a metacharacter? :. In this way, the matching results will not be stored, so the number of the buffer will not be added, and no number can be referenced. can this subexpression be called repeatedly?

Thank you!

Reply to discussion (solution)

Let's give an example.No.

Thanks to the moderator for replying. In PHP, I use preg_match for matching. the pattern is '^ ([a-zA-Z] *) \ d + \ 1 $ ', to match the 'demo123demo' string with letters, numbers, and letters, return yes. If a backslash is added before the number in the mode, false is returned.
I tried it myself. Then I have Baidu's several "reference manuals for regular expressions". I do not need to add them in some examples.
Attachment link (do not know whether the Post link will violate the posting code): https://translate.google.com.hk /? Hl = zh-CN & tab = wT, which does not exist in the example.
Okay. I just want to extend this question to see if there is a "version difference". what I really want to answer urgently is the second question, which is bothering the moderator!

What does repeated call and reference mean? If so, do you know the answer.

/^ ([A-zA-Z] *) \ d + \ 1 $/
This is notAdd backslash escape before the number?

Is that what you mean by the second question?
/^ (? = [A-zA-Z] +) \ d + (? = [A-zA-Z] +) $/

Haha, then you will understand. We are honored that both moderators have come out.
The first one is used to escape the backslash. the numbers written in the book I read are \ 1 to \ 99. I need to add a backslash when referencing it, that is, '\ 1'. the instance in the book is like this. I felt wrong last night and found that adding a backslash matches the string' \ 1 '. It seems that books cannot be fully written.
Thank you very much!