How to understand this regular expression?

Question

Regular Expressions

I met this here:

(?<=(?:.pdf))

What else does it do, besides extracting the string 'pdf' from the text?

Answer 1

(?<=) - to the right of this template, that is, the condition is present but it will not be included in the match. what will appear after this statement will get into the match.
.pdf - the bb template itself -
what was found
and that
(?:) - the usual group, but ?: says that it does not need to be allocated to a group, that is, it does not need to be turned into $ 1, then this question with a colon does not apply to the regular expression itself it's just a group, but because it doesn't need data, it doesn't matter. and in order to clog the conclusion, it must simply be forgotten.

Answer 2

Positive Lookbehind
more specific https://learn.javascript.ru/regexp-lookahead-lookbehind
A small life hack on regular expressions. Sometimes it helps to use the tools to compose them. For example, https://regex101.com/r/GYjBIY/1
In the upper right corner there will be an explanation of how it works. And in the field below, you can use your own examples to see how the regular season works in different situations.

Answer 3

Perhaps not all regular expressions are shown or cut off.
For example, it is also supplemented with the symbols "bb":
(?<=(?:.pdf))bb
see what was found on it
https://regex101.com/r/tAqWiW/1
+ Read the link to the Positive Lookbehind from the previous one (on Russian)
it also became clearer to me what it is)