On Jul 23, Tom Allison said:
($lineExtract, $_) = /(^\w+\.dat\|)(.+)$/o;
(I like to add the regex option 'o' at the end to improve performance.)
This is a common misconception. The purpose and effects of the /o
modifier (as well as the /s and /m modifiers) are unclear to a lot of Perl
programmers out there. I'll make a PSA now and try to clear things up a
bit.
The /o modifier tells the internal regex compiler that, after this regex
has been compiled 'o'nce, it is never to be compiled again. "Well, what
good is that?" you ask. For your average regex, there is absolutely no
difference, no change in performance: /foo/ and /foo/o are identical.
The place where the /o modifier matters is when there are variables inside
the regex, e.g. /^$field: (.*)/. The /o modifier says that, after the
regex has been compiled for the first time -- which means that all the
variables in it have been interpolated -- *that* compiled regex will take
the place of the regex with the variables in it. Any changes to your
variables will be ignored, for the rest of the program. There's no way to
reverse the effect of /o.
The /s modifier often carries the mnemonic "so you can match your string
as though it were a single line" and that all of the sudden makes people
think all sorts of crazy things. The /m modifier is often said to "make
your regex match over multiple lines" and this too makes people do weird
things. I've seen code like this:
while (<FILE>) {
print $1 if /Start(.*)End/ms;
}
The person has a file with "Start" on one line and "End" on another, and
they're not sure why their regex doesn't match the stuff in between. The
reason is because the /m and /s modifiers change the *regex* and nothing
else. In that code above, you're still only reading one physical line of
text from FILE at a time.
So what do /m and /s do? It's very simple. The only thing the /s
modifier does is make the . metacharacter match newlines. That's all.
If there's no . in your regex, there's no need for you to add an /s to
your regex. The /m modifier makes ^ and $ match the beginning and end of
"lines" -- that is, it makes ^ match after any newline in your string (as
well as at the absolute beginning of your string), and it makes $ match
before any newline in your string (as well as at the absolute end of the
string).
So there you have it. I can go into more detail about /o and regex
compilation (and have before, probably on this list), but for now, what
I've told you is all you need to know.
--
Jeff "japhy" Pinyan % How can we ever be the sold short or
RPI Acacia Brother #734 % the cheated, we who for every service
http://japhy.perlmonk.org/ % have long ago been overpaid?
http://www.perlmonks.org/ % -- Meister Eckhart
--
To unsubscribe, e-mail: [EMAIL PROTECTED]
For additional commands, e-mail: [EMAIL PROTECTED]
<http://learn.perl.org/> <http://learn.perl.org/first-response>
