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When perl compiles your program, any constant regular expressions in the code (ie any that don't interpolate any variables) are compiled. Any that do interpolate variables will be compiled once when they are first used, and on further uses are re-compiled only if they have changed since the last use.

So in this code, the regular expression is compiled only once (at compile time):

for (@words) { print if /foo/; }

And also in this code (but at runtime):

my $s = "foo"; for (@words) { print if /$s/; }

In this code, the two regular expressions are each compiled only once:

my($s1, $s2) = qw/ foo bar /; for (@words) { print if /$s1/; print if /$s2/; }

But in this code, because the one regular expression is being used alternately interpolated with "foo" and "bar", it will be recompiled each time:

my @s = qw/ foo bar /; for (@words) { for my $s (@s) { print if /$s/; } }

You can get information about what perl is doing with your regexps using the -Dr switch if perl has been compiled for debugging, or with use re 'debug';. This produces a lot of scary-looking information about both compiling and running regular expressions, but if you ignore that and just look at the "Compiling ..." lines it will show you what is happening.

The last case above is where qr{} comes in: if we fill @s with qr{}'d expressions instead, we can avoid the recompile:

my @s = map qr{$_}, qw/ foo bar /; for (@words) { for my $s (@s) { print if /$s/; } }

Note that this only helps if you are using exactly the compiled regexp: if you interpolate it into something more, even if only to add an anchor, it will go back to recompiling each time.

Hugo

In reply to Re: Compiling Regular Expressions by hv
in thread Compiling Regular Expressions by Anonymous Monk

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    [marinersk]: sub newtest{my $expected_result = &target('foo'); my $actual_result = &target('foo'); if ($actual_result eq $expected_result) { &tdd_success(); } else { &tdd_fail(); } } # Test works after three years!
    [choroba]: or nobody bothered...
    [choroba]: The problem was bigger, as the test tried to call a method that didn't exist anymore
    [marinersk]: :: ducking ::
    [choroba]: because, someone renamed the method, but didn't notice it was used in the test, as the test was skipped
    [marinersk]: Well, if the method doesn't exist, it would be hard to pass the test.
    [choroba]: later, someone removed the new method, as all its usage places were safe, but didn't notice the test still used the old name
    [choroba]: fortunately, it wasn't that hard to replace the method and fix a few remaining failures due to the changes we did to the codebase over the years
    [marinersk]: choroba Sounds like a process improvement opportunity; tests may not all need to be run, but they should all be compiled with perl -c before check-in/promotion happens.
    [choroba]: so, now I have the test, so I can start making changes in the code. Back to the original ticket, yay!

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