When you have variables in a regex, Perl examines the contents of those variables to see if the overall representation of the regex has changed:

for ("ab", "cd") {
  if ($str =~ /$_/) { ... }
}
[download]

In the above code, the regex 'ab' is compiled and executed, and then the regex 'cd' is compiled and executed. Compare that with:

($x, $y) = ("ab", "c");
for (1, 2) {
  if ($str =~ /$x$y/) { ... }
  ($x, $y) = ("a", "bc");
}
[download]

Here, even though $x and $y change, the ACTUAL regex ('abc') does not change, so the regex is compiled only once. The process that Perl does internally is this:

1. take regex at this opcode
2. interpolate any variables
3. compare with previous value of the regex at this opcode
4. compile if different
5. execute this regex

When you use the /o modifier, it tells Perl that after it has compiled the regex, it should SKIP steps 2-4 of this process, meaning that the regex at this opcode will NEVER change.

So what is qr// good for? Consider this:

my @strings = make_10_strings();
for (@strings) {
  for my $p ('x+', 'yz?y', 'xz+y') {
    if ($_ =~ $p) { handle($_) }
  }
}
[download]

This code compiles a grand total of 30 regexes. Why? Because for each string in @strings we've got three patterns to execute, and because each time the $_ =~ $p is encountered the contents of $p has changed, the regex is compared and recompiled each time. Now sure, you could reverse the order of the loops, but that will result in the calls to handle() happening in a different order.

So enter the qr//.

When Perl sees a regex comprised solely of a single variable, Perl checks to see if that variable is a Regexp object (what qr// returns). If it is, Perl knows that the regex has already been compiled, so it simply uses the compiled regex in the place of the regex. That means doing:

my @strings = make_10_strings();
for (@strings) {
  for my $p (qr/x+/, qr/yz?y/, qr/xz+y/) {
    if ($_ =~ $p) { handle($_) }
  }
}
[download]

is considerably faster. There is no additional compilation happening. It's probably even better to move the qr// values into an array, but that might be moot since they're made of constant strings in this example. The point is, the use of qr// in a looping construct is the primary benefit it offers. Yes, it helps break a regex up into pieces too, but that's just a matter of convenience.

Be warned that the benefit of qr// objects is lost if there is additional text in the pattern match. I mean that $foo =~ /^$rx_obj$/ suffers from the same problem as $foo =~ /^$text$/.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.