I am writing a python regex that matches only string that consists of [A-Za-z0-9] and ? where question mark can exist in any position in the string. Here ? should be in the string.

For example, regex1: ^[A-Za-z0-9?]+$ returns strings with or without ?

I want a regex2 that returns ABC123?A, 1ABCA?, ?2ABCD but not ABC123, ABC.?1D1, ABC(a)?1d
on mysql, I did that and it works:

select *
from (
select * from norm_prod.skill_patterns
where pattern REGEXP '^[A-Za-z0-9?]+$') AS XXX
where XXX.pattern not REGEXP '^[A-Za-z0-9]+$'

>Solution :

How about something like this:

^(?=.*\?)[a-zA-Z0-9\?]+$

As you can see here at regex101.com

Explanation

The (?=.*\?) is a positive lookahead that tells the regex that the start of the match should be followed by 0 or more characters and then a ? – i.e., there should be a ? somewhere in the match.
The [a-zA-Z0-9\?]+ matches one-or-more occurrences of the characters given in the character class i.e. a-z, A-Z and digits from 0-9, and the question mark ?.

Altogether, the regex first checks if there is a question mark somewhere in the string to be matched. If yes, then it matches the characters mentioned above. If either the ? is not present, or there is some foreign character, then the string is not matched.