.=match any character
^=beginning of a line
$=end of a line
[[:<:]]=beginning of a word
[[:>:]]=end of a word
xy|z=match xy OR z
x?=match zero or one x character
x*=match zero or more x characters
x+=match one or more x characters
x{3}=match x three times
x{3,}=match x three or more times
x{3,5}=match x three to five times
[xyz]=match x, y OR z
[^xyz]=exclude x, y AND z
[:class:]=match a character class, i.e.:
[:alpha:] for letters
[:digit:] for numbers/digits
[:space:] for white space
[:punct:] for punctuation
[:upper:] for upper-case letters