Regular expressions (regexes).........

# Match something that looks like an IP address
/^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/
/^(\d{1,3}\.){3}\d{1,3}$/

# Test if a number is in range 0-255
/^([0-9]|[0-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])$/

# Match an IP address
my $ip_part = qr|([0-9]|[0-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])|;
if ($ip =~ /^($ip_part\.){3}$ip_part$/) {
 say "valid ip";
}

# Check if the string looks like an email address
/\S+@\S+\.\S+/

# Check if the string is a decimal number
/^\d+$/
/^[+-]?\d+$/
/^[+-]?\d+\.?\d*$/

# Check if the string is a hexadecimal number
/^0x[0-9a-f]+$/i

# Check if the string is an octal number
/^0[0-7]+$/

# Check if the string is binary
/^[01]+$/

# Check if a word appears twice in the string
/(word).*\1/

# Increase all numbers by one in the string
$str =~ s/(\d+)/$1+1/ge

# Extract HTTP User-Agent string from the HTTP headers
/^User-Agent: (.+)$/

# Match printable ASCII characters
/[ -~]/

# Match unprintable ASCII characters
/[^ -~]/

# Match text between two HTML tags
m|<strong>([^<]*)</strong>|
m|<strong>(.*?)</strong>|

# Replace all <b> tags with <strong>
$html =~ s|<(/)?b>|<$1strong>|g

# Extract all matches from a regular expression
my @matches = $text =~ /regex/g;