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Home: Perl Programming Help: Frequently Asked Questions:
What is /o really for?



Mar 15, 2001, 6:01 AM

Post #1 of 1 (25050 views)
What is /o really for? Can't Post

What is /o really for?

Using a variable in a regular expression match forces a re-evaluation (and perhaps recompilation) each time through. The /o modifier locks in the regexp 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 regexp engine will neither know nor care whether the variables change after the pattern is evaluated the very first time.

/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 regexp 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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