FAQ 6.9 What is "/o" really for?

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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;


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