Cheatsheet:
[a-zA-Z] Any single character in the range a-z or A-Z
^ Start of line
$ End of line
. Any single character
\s Any whitespace character
\S Any non-whitespace character
\d Any digit
\D Any non-digit
\w Any word character (letter, number, underscore)
\b Any word boundary
(...) Capture everything enclosed
(a|b) a or b
a? Zero or one of a
a* Zero or more of a
a+ One or more of a