Lazy quantifier {,}? not working as I would expect

Question

I have an issue with lazy quantifiers. Or most likely I misunderstand how I am supposed to use them.

Testing on Regex101 My test string is let\'s say: 12345678

Answer 1

Your regex is

.{1,5}?D

matches

123456789D123456789
    ------

But you said you expected 9D because using of "non-greedy quantifier".

Anyway, how about this?

D.{1,5}?

What is a result of matching?

Yes! as you expected it matches

123456789D123456789
         --

So, WHY?

OK, The first, I think you need to understand that normally regex engine will read characters from left to right-hand side of an input string. Considering your example which using non-greedy quantifier, once engine is matched

123456789D123456789
    ------

It will not go further to

123456789D123456789
     -----
123456789D123456789
      ----
...
123456789D123456789
        --

Because regex engine will evaluate text as less as possible, this is why it also called "Lazy quantifiers".

And it also work in the same way on my regex D.{1,5}? which should not go further to

123456789D123456789
         ---
123456789D123456789
         ----
...
123456789D123456789
         ------

But stop at the first match

123456789D123456789
         --