Repetition

x? match preceding token zero times or once

x* match preceding token zero times or more

x+ match preceding token once or more

x{min, max} match preceding token min to max times (inclusive)

Square Brackets

[xyz] match only one out of several characters

[^xyz] match any character that is not listed

References

http://www.regular-expressions.info/repeat.html

http://www.regular-expressions.info/brackets.html

http://www.regular-expressions.info/charclass.html