Question

Is there something in regex to match (conditionally) only if it exists ?

e.g.

the string maybe

question_1

or only

question

In case of the former, it should match the integer in the end, but as in the case of the latter, it should leave it.

Answer 1

The ? is the 0-1 quantifier in Regexes. \d? means 0 or 1 digit. The * is the 0-infinite quantifier. \d* means 0 or more digits. Is it what you want?
(additionally the + is the 1 or more quantifier, and not quantifier means exactly 1)

To elaborate on what you asked, I would say

question(_\d+)?

question followed by an optional (_ AND 1 or more digits)

Where the brackets are only to group the sub expression (they are “mathematical” brackets)