Does the 'o' modifier for Perl regular expressions still provide any benefit?

Question

It used to be considered beneficial to include the 'o' modifier at the end of Perl regular expressions. The current Perl documentation does not even seem to list it, certainly not at the modifiers section of perlre.

Does it provide any benefit now?

It is still accepted, for reasons of backwards compatibility if nothing else.

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As noted by J A Faucett and brian d foy, the 'o' modifier is still documented, if you find the right places to look (one of which is not the perlre documentation). It is mentioned in the perlop pages. It is also found in the perlreref pages.

As noted by Alan M in the accepted answer, the better modern technique is usually to use the qr// (quoted regex) operator.

Answer 1

This is an optimization in the case that the regex includes a variable reference. It indicates that the regex does not change even though it has a variable within it. This allows for optimizations that would not be possible otherwise.

Answer 2

I'm sure it's still supported, but it's pretty much obsolete. If you want the regex to be compiled only once, you're better off using a regex object, like so:

my $reg = qr/foo$bar/;

The interpolation of $bar is done when the variable is initialized, so you will always be using the cached, compiled regex from then on within the enclosing scope. But sometimes you want the regex to be recompiled, because you want it to use the variable's new value. Here's the example Friedl used in The Book:

sub CheckLogfileForToday()
{
  my $today = (qw<Sun Mon Tue Wed Thu Fri Sat>)[(localtime)[6]];

  my $today_regex = qr/^$today:/i; # compiles once per function call

  while (<LOGFILE>) {
    if ($_ =~ $today_regex) {
      ...
    }
  }
}

Within the scope of the function, the value of $today_regex stays the same. But the next time the function is called, the regex will be recompiled with the new value of $today. If he had just used

if ($_ =~ m/^$today:/io)

...the regex would never be updated. So, with the object form you have the efficiency of /o without sacrificing flexibility.

Answer 3

The /o modifier is in the perlop documentation instead of the perlre documentation since it is a quote-like modifier rather than a regex modifier. That has always seemed odd to me, but that's how it is.

Before Perl 5.6, Perl would recompile the regex even if the variable had not changed. You don't need to do that anymore. You could use /o to compile the regex once despite further changes to the variable, but as the other answers noted, qr// is better for that.