Extracting your matches: Capturing

• Pattern fragments enclosed within () are captured and stored inside numbered variables, ($1, $2, etc.) in the order that the captured fragments are found
• These captured strings can be referenced later in your code:

  	if( $input =~ /\bx(\d{4})\b/ ) {	# Can we match a phone extension?
  	    $phone_ext = $1;
  	}

  Or even within the same match:

  	/(.*) \1/	# Matches anything occurring twice with a space inbetween
  			# like "aniMAL MALaise" or "It was THAT THAT did it in."

  Or substitution:

  	s/\b(\w+) (\w+)\b/$2 $1/	# Reverses two words

• The numbered variables are completed based on the left to right order of the closing parentheses:

  	"An example:" =~ /^\w+( (.{4}).*)$/;	# Sets $1 to the inner "exam"
  						# and $2 to the outer " example:"

• Note that the numbered variables aren't available until after the initial match completes. If you want to match something captured earlier in the same match, use \1, \2, etc:

  	/(\w+) \1/	# Matches double words like "To see that that succeeds"