abc… Letters
123… Digits
d Any Digit
D Any Non-digit character
. Any Character
. Period
[abc] Only a, b, or c
[^abc] Not a, b, nor c
[a-z] Characters a to z
[0-9] Numbers 0 to 9
w Any Alphanumeric character
W Any Non-alphanumeric character
{m} m Repetitions
{m,n} m to n Repetitions
* Zero or more repetitions
+ One or more repetitions
? Optional character
s Any Whitespace
S Any Non-whitespace character
^…$ Starts and ends
(…) Capture Group
(a(bc)) Capture Sub-group
(.*) Capture all
(ab|cd) Matches ab or cd