regex

# Anchors
^   Start of string
$   End of string

# Ranges
[A-Za-z]    any letter
[0-9]       any digit
[349]       matches 3, 4 or 9       
[^5]        any character except 5 (negation)

# Boundaries
\s  whitespace
\S  non-whitespace
\b  word boundary
\B  non-word boundary

# Quantifiers
*     zero or more times
?     zero or one time
+     one or more times
{n}   exactly n times