Validate Hex Color Code

Check if a string is a valid 3 or 6-digit hexadecimal color code with optional #.

regex validation strings

Write a function `is_valid_hex_color(s: str) -> bool` that returns `True` if the input string `s` is a valid hex color code, and `False` otherwise.

A valid hex color code must:
- Optionally start with a single `#` character.
- Contain either exactly 3 or exactly 6 hexadecimal digits (0-9, a-f, A-F).
- Have no other characters, no spaces, and no extra `#`.

The function should be case-insensitive regarding the hex digits.

Examples:
- `"#ABC"` and `"#abc"` are valid.
- `"#A1B2C3"` and `"#a1b2c3"` are valid.
- `"123456"` and `"FFF"` are valid (without #).
- `"#ABCD"` (4 digits), `"#12"`, `"#12 345"`, `"##123"`, and `"#GGG"` are invalid.

Implement the function using a regular expression.

Constraints

Input string length will be between 0 and 20. Use `re.fullmatch` or anchor the pattern to match the entire string.

Example

>>> is_valid_hex_color('#A1B2C3')
True
>>> is_valid_hex_color('ABC')
True
>>> is_valid_hex_color('123456')
True
>>> is_valid_hex_color('#ABCD')
False
>>> is_valid_hex_color('')
False

10 points ~10 min

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Hints

Use the `re.fullmatch()` function to ensure the entire string matches.

The pattern should allow an optional `#` at the start: `#?`.

For hex digits, use `[0-9a-fA-F]` with `{3}` or `{6}`.

Combine the 3 and 6 digit alternatives with `|` and group them.

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