The conversation between human and AI assistant.
[|Human|] Construct a regular expression that characterizes a valid string of length 5 containing only numbers.
[|AI|] The regular expression that characterizes a valid string of length 5 containing only numbers is `^\d{5}$`.

Explanation:
- `^` signifies the start of the string
- `\d` signifies a digit (0-9)
- `{5}` signifies that the digit should occur exactly 5 times
- `$` signifies the end of the string.
[|Human|] 
Construct a regular expression that characterizes a valid string of length 5 containing only numbers.