What is question mark in regex?

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Tags

1. #expression
2. #using
3. #preceding
4. #matches
5. #parenthesis
6. #question
7. #colou
8. #token
9. #tokens
10. #closing
11. #color
12. #together
13. #called
14. #makes
15. #parentheses
16. #placing
17. #optional
18. #colour
19. #grouping
20. #several
21. #quantifier
22. #after
23. #regular