Problem with pre-compiled regex

saranrsm has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks

Problem: Im trying to pre-compile a couple of patterns (Regular expression) and match it into the given text.

Done so far: I have tried doing this without pre-compiled regex, which work perfectly

$text="I have a problem with pre-compiled regex";
@words=qw(have regex);
for $a(@words)
{
    while ($text =~ m/$a/g)
    {
        print "Found $&\n";
    }
}
[download]

Stuck with: Now when I wanted to pre-compile these patterns (have, regex) and match with $text, I don't get the expected result. I have posted the script which doesn't work, below.

$text="I have a problem with pre-compiled regex";
@words=qw(qr/have/ qr/regex/);
for $a(@words)
{
    while ($text =~ /$a/g)
    {
        print "Found $&\n";
    }
}
[download]

Comment on Problem with pre-compiled regex Select or Download Code

Replies are listed 'Best First'.
Re: Problem with pre-compiled regex by moritz (Cardinal) on Jan 17, 2012 at 14:16 UTC
`@words=qw(qr/have/ qr/regex/);` You probably meant `@words = (qr/have/, qr/regex/);`. Since `qw/ .. /` always returns a list of strings, you are trying to match the text against the string `qr/have/`, not against a precompiled regex. Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: Problem with pre-compiled regex by saranrsm (Acolyte) on Jan 17, 2012 at 14:42 UTC
Dear Monk Thank you very much for your kind assistance and your tweak made it work perfectly but I have this confusion, the scripts that I posted with and without pre-compiled regex takes almost same time to execute but shouldn't the pre-compiled regex run faster because of pre-compilation... P.S. I haven't used the @words and $text mentioned, I tried with a larger set of @words and a bigger $text instead of mere (have, regex) and a single sentence for $text as mentioned in the script	[reply]
Re^3: Problem with pre-compiled regex by moritz (Cardinal) on Jan 17, 2012 at 15:31 UTC
Compiling a short and simple regex that consists of only one string literal is so fast that you'll probably have trouble to come up with a benchmark to measure it. So if there is a difference, it'll be too small to be easily observable. I also think that Perl has an internal cache that avoids recompiling the same regex all over again. Perl 6 - second systems done right	[reply]
Re^4: Problem with pre-compiled regex by ikegami (Patriarch) on Jan 18, 2012 at 07:03 UTC
Re^3: Problem with pre-compiled regex by JavaFan (Canon) on Jan 17, 2012 at 16:45 UTC
shouldn't the pre-compiled regex run faster because of pre-compilation Both your regexp and your program are fairly trivial. Even if there wasn't any caching involved for non-precompiled regexes, I doubt you'd notice the difference. Whether that's the case for your real $text, I do not know. Perl has been caching compiled patterns since the 20th century. (5.004 or 5.005, IIRC). This is why `/o` is usually pointless (if not broken). Often, the compilation time is dwarved by the runtime anyway. You'd have to be careful -- if you interpolate your compiled pattern into a larger one, you're paying the price of compilation twice.	[reply] [d/l]
Re^3: Problem with pre-compiled regex by LanX (Saint) on Jan 17, 2012 at 15:14 UTC
>but shouldn't the pre-compiled regex run faster Not necessarily From perlop `Since Perl may compile the pattern at the moment of execution of qr() operator, using qr() may have speed advantages in some situations,...` And not any more as often as it used to be! `...Perl has many other internal optimizations, ...` (emphasizes added) UPDATE: Furthermore in both your code examples the regexes are just compiled and used once. Cheers Rolf	[reply]
Re^4: Problem with pre-compiled regex by JavaFan (Canon) on Jan 17, 2012 at 16:34 UTC
Re: Problem with pre-compiled regex by Anonymous Monk on Jan 17, 2012 at 15:50 UTC
When both the Perl language and computer systems were considerably slower than they are today, it made a difference. Maybe. But, not anymore and nevermore, I think.	[reply]


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