The following are some of the special sequences used in regular expressions:
1. A: Check if the string starts with the given character.
2. (Forward Slash) b: Checks if the string starts or ends with the given character. (string)/b checks for the beginning while (backward slash) b (string) checks for the end.
3. B: It is exactly opposite to the b. Checks if the string does not start with the given character.
4. d: Checks for the numerical values in the string.
5. D: Checks for any non-numerical value or character.
6. s: Checks for any whitespace character.
7. S: Checks for any non-whitespace character.
8. w: Checks for any alphanumeric character.
9. W: Checks for any non-alphanumeric character.