What matches the regular expression (? <N> a) (b) \ 2 (c)?

Question

What matches this regular expression?

(?<n>a)(b)\2(c) 

It does not match abc , abac , abbc , abcc or ab\x02c and does not throw an exception.

---

If you uncheck (c) , it will match aba . I understand that unnamed captures are first numbered, and then named captures are captured. So (c) should get 2, except that I am trying to follow the backlink before defining it, so I thought that maybe it will match a and a will be renumbered when it reaches c , but that’s not how seems also.

Answer 1

\2 in your regular expression refers to group (c) , as your experiments showed. Unfortunately, your regular expression never matches anything.

You can find the link in the documentation _{^{(although this is not too clear and it seems like an unrelated example follows)}} :

If a group has not written any substrings, the link to this group will be undefined and will never match.

Answer 2

It corresponds to "abbc" for me. I use the Perl regex flavor and I get "a" as group 1, "b" as group 2, and c - group 3. the \ 2 refers to the second group, which is "b"

(?<n>a) - Named capture group: name "n" regular expression "a"
(b)\2 - match capture group "b", then reference to the second group, which is "b"
(c) - comparison of the third capture group "c"

Answer 3

the back link is "n". b link is "1" and the third is "2"

 (?<n>a)(b)\1(c) matches abbc where n is a, 1 is b and 2 is c 

it displays unnamed backlinks with 1 and counts. Missing parentheses are not numbered. Thus, he cannot match anything.

Regular-Expressions.info in brackets