Regex negated character disjunction

Question

Very quick and simple question.

Consider the vector of character strings ("AvAv", "AvAvAv")

Why does the pattern (Av)\1([^A]|$) match both strings?

The pattern says have an isntance of "Av", have another, then either have a character that is not an "A" or else come to an end. The first string clearly matches, the latter I do not see how it does. It has two copies of "Av" but then it fails to end (missing the second disjunct), and fails to be followed by a charavter other than "A" (missing the first disjunct), so how does the pattern successfully match it?

Thank you so much for your time and assistance. It is greatly appreciated.

because the regex not use `^`, so it can match start from the second `Av` — Kerwin, Jan 10 '18 at 03:59
I think the regex only needs to match a part of the string to be considered "a match". — iBug, Jan 10 '18 at 03:59
The answer is, as usual, [here](https://regex101.com/r/bLOF88/1), *EXPLANATION* pane is on the right. — Wiktor Stribiżew, Jan 10 '18 at 08:31

score 0 · Accepted Answer · answered Jan 10 '18 at 04:00

Here is an explanation:

AvAv    - matches (Av)\1$

In this case, we can match Av, followed by that captured quantity, followed by $ from the alternation. In the case of AvAvAv we also have a match:

AvAvAv  - again matches (Av)\1$
  ^^^^  last four letters match

It is the same logic here, except that in order to match, we have to skip the first Av.

If the pattern were ^(Av)\1([^A]|$) then only AvAv would be a match.

score 0 · Answer 2 · answered Jan 10 '18 at 04:00

A RegEx only needs to match a part of the string to be considered "a match".

In other words, your RegEx matches this part:

AvAvAv

for the second example.

If you don't want it to match the second one, use a caret ^

^(Av)\1([^A]|$)

In this way the second one won't be matched.

Regex negated character disjunction

2 Answers2