Nested back-ref does not work

[User4martin]

A back-ref \1 matches the same literal text as was matched by the most recent successful capture of the group.

That is, if the group is matched in a loop, then it will be the text matched in the most recent iteration.

But

  IsMatching('nested ref', '(?i)^(.(?:(\1)|(.)))*',  'aAaaAa',  [1,5,  3,3, 4,2, 2,1]);

The \1 is within the group 1.

• In the first loop it has nothing to match.
• In the 2nd loop, it can and should match the result from the first iteration.

https://regex101.com/r/sstaLI/1

In TRegExpr it does not.

[Alexey-T]

Do all / most engines support \1 inside group1 ? it sounds scary. maybe it's rare feature.

[User4martin]

If they support \1 at all, then they supported it nested too.

ECMA handles it different (independent of nested or not) Given (?: (?:\1|\d) (\w) )*

In the first loop the capture 1 (\w) has not yet been maatched.

• All engines will therefore fail to match \1 in the first iteration. But may match it in the 2nd iteration.
• ECMA will match the empty string for that \1 in the first iteration.

Same holds true in the nested case. Which really is no different, from a users view. It only is different for the implementation, because the new match must be written in one go only when the ) is reached, so the last match is valid until the next is fully avail.