Building regular expressions on the fly: Interpolation and compilation

• You can use variables in your pattern to help build your regular expression:

  	$threenum = '\d{3}';
  	$areacode = '\(' . $threenum . '\)';
  	$fournum = '\d{4}';
  	$phonenum = "$areacode$threenum-$fournum";
  	if( $input =~ /^$phonenum$/o ) {
  	    ...
  	}

• The /o modifier tells perl that the regular expression pattern will never change, and only needs to be compiled once. Useful for optimizing
• Note that when using /o perl assumes you won't change the variables. If you do, perl will continue as if it was the original pattern.
• You can explicitly compile a regular expression by using qr/pattern/, and use that in a regex:

  	$phonenum = qr/$areacode$threenum-$fournum/;
  	if( $input =~ /^$phonenum$/ ) {
  	    ...
  	}