FAQ 6.9: What is "/o" really for?

Do you have a question? Post it now! No Registration Necessary.  Now with pictures!

This message is one of several periodic postings to comp.lang.perl.misc
intended to make it easier for perl programmers to find answers to
common questions. The core of this message represents an excerpt
from the documentation provided with Perl.


6.9: What is "/o" really for?

    Using a variable in a regular expression match forces a re-evaluation
    (and perhaps recompilation) each time the regular expression is
    encountered. The "/o" modifier locks in the regex the first time it's
    used. This always happens in a constant regular expression, and in fact,
    the pattern was compiled into the internal format at the same time your
    entire program was.

    Use of "/o" is irrelevant unless variable interpolation is used in the
    pattern, and if so, the regex engine will neither know nor care whether
    the variables change after the pattern is evaluated the *very first*

    "/o" is often used to gain an extra measure of efficiency by not
    performing subsequent evaluations when you know it won't matter (because
    you know the variables won't change), or more rarely, when you don't
    want the regex to notice if they do.

    For example, here's a "paragrep" program:

        $/ = '';  # paragraph mode
        $pat = shift;
        while (<>) {
            print if /$pat/o;


Documents such as this have been called "Answers to Frequently
Asked Questions" or FAQ for short.  They represent an important
part of the Usenet tradition.  They serve to reduce the volume of
redundant traffic on a news group by providing quality answers to
questions that keep coming up.

If you are some how irritated by seeing these postings you are free
to ignore them or add the sender to your killfile.  If you find
errors or other problems with these postings please send corrections
or comments to the posting email address or to the maintainers as
directed in the perlfaq manual page.

Note that the FAQ text posted by this server may have been modified
from that distributed in the stable Perl release.  It may have been
edited to reflect the additions, changes and corrections provided
by respondents, reviewers, and critics to previous postings of
these FAQ. Complete text of these FAQ are available on request.

The perlfaq manual page contains the following copyright notice.


    Copyright (c) 1997-2002 Tom Christiansen and Nathan
    Torkington, and other contributors as noted. All rights

This posting is provided in the hope that it will be useful but
does not represent a commitment or contract of any kind on the part
of the contributers, authors or their agents.

Site Timeline