\w      word character
\W      non-word character
\s      space character
\S      non-space character
\d      digit character
\D      non-digit character
^       beginning of string
$       end of string
.       any character

[xyz]   character set
|       alternatives

Quantifiers

*       zero or more
+       1 or more
?       0 or 1
{M}     exactly M
{M,N}   between M and N characters

/pattern/i - case insensitive match

$line =~ tr/ABC/XYZ/;        #  replaces all A's with X's,
                                B's with Y's
                                C's with Z's
$line =~ tr/[a..z]/[A..Z]/;  #  capitalizes all letters
$line =~ s/human/HOMO/;      #  replaces the first occurence of
				"human" with "HOMO"
$line =~ s/\s//g;            #  strips off all spaces