A regular expression (or regex)

[anakin30]

dear monks,

i have a A regular expression(or regex) in my perl script. 

this is the code snippet, 

 if ($this_in =~ /^dn: /gio)

i know what does the "g" and "i" means, as below. 
g - DO globally pattern matching.
i - Do case-insensitive pattern matching.

can somebody please explain to me whats the "o"  means. 

i would like to thank you guys in advanced for helping..

anakin30.....

[suhailck]

from, perlretut


If $pattern won't be changing over the lifetime of the script, we can 
+add the "//o" modifier, which directs Perl to
       only perform variable substitutions once:

           #!/usr/bin/perl
           #    Improved simple_grep
           $regexp = shift;
           while (<>) {
               print if /$regexp/o;  # a good deal faster
           }

[marto]

Did you not find what you where looking for in perlop and perlre as previously advised?

[JavaFan]

To me, it says the author of the regexp is considering Perl programming to be magic he doesn't fully understand.

Neither the /g nor the /o are actually useful here. And I wouldn't be surprised if the /i isn't either.

[kennethk]

This is answered in Optimizing pattern evaluation in perlretut:

If $pattern won't be changing over the lifetime of the script, we can add the //o modifier, which directs Perl to only perform variable substitutions once

It wouldn't make sense to use this optimization in the context you've shown - even if it were embedded in a loop, there is no variable being interpolated.

A point on formatting posts - please do not use <pre> tags. Rather, write normal text, separate paragraphs with <p> tags and wrap your code in <code> tags. See How do I post a question effectively?.

[ikegami]

i know what does the "g" and "i" means, as below.

Apparently not. Not only does if (/.../g) make no sense, it's practically always a bug. (if (/.../gc) might be seen in some advanced code.)

[ikegami]

can somebody please explain to me whats the "o" means.

In interpolating patterns, /o will only interpolate once, the first time the regex is evaluated.

for my $x (qw( foo bar )) {
   /$x/o;
}

is the same as

/foo/;
/foo/;

It's used for optimising, but it's made rather useless by qr//.

[SuicideJunkie]

See: http://perldoc.perl.org/perlop.html#Regexp-Quote-Like-Operators

PS: When posting use <p>Text</p> tags for paragraph text and <c>my $code</c> for code.

There are lots of links right below the box you type into explaining how to format your post; they're quite handy.