. - Any character
* - Zero or more
+ - One or more
? - Zero or one
^ - Start of line
$ - End of line

[abc] - a, b, or c
[a-z] - Any lowercase
[A-Z] - Any uppercase
[0-9] - Any digit
\d - Digit
\w - Word character

\d+ - Numbers
\w+@\w+\.\w+ - Email
https?://\S+ - URLs
\b\w{5}\b - 5-letter words
^\s* - Leading spaces
\s+$ - Trailing spaces