What does colon mean in regex?

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Tags

1. #define
2. #times
3. #regular
4. #actually
5. #character
6. #brackets
7. #followed
8. #means
9. #matches
10. #characters
11. #meaning
12. #expressions
13. #square
14. #colon
15. #special
16. #needed
17. #literal
18. #group
19. #match