Relative performance of comparable regular expressions
Hi,
I have a question about relative performance of comparable regular expressions.
I have large log files that start with three letters month names (non-unicode).
Which would give better performance, matching with ^[a-zA-Z]{3}, or with
^\S{3} ?
Also, which is better (if different at all): \d\d or \d{2} ?
Also, would matching . be different (performance-wise) than matching the
actual character, e.g. matching : ?
And lastly, at the end of a line, is there any performance difference between
(.+)$ and (.+)
Thanks,
Ron.
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Re: Relative performance of comparable regular expressions
On Jan 13, 7:24 pm, Barak, Ron ron.ba...@lsi.com wrote:
Hi,
I have a question about relative performance of comparable regular
expressions.
I have large log files that start with three letters month names
(non-unicode).
Which would give better performance, matching with ^[a-zA-Z]{3}, or with
^\S{3} ?
(1) If you want to match at the start of a line, use re.match()
*without* the pointless ^. Don't use re.search with a pattern
starting with ^ -- it won't be any faster than and it could be a lot
worse; re.search doesn't know to stop if the first match fails:
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.match(text)
100 loops, best of 3: 1.15 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.search(text)
10 loops, best of 3: 4.47 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*1000 rx.search(text)
1 loops, best of 3: 34.1 usec per loop
(2) I think you mean ^\s{3} not ^\S{3}
(3) Now that you've seen how to do timings, over to you :-)
Also, which is better (if different at all): \d\d or \d{2} ?
Also, would matching . be different (performance-wise) than matching the
actual character, e.g. matching : ?
And lastly, at the end of a line, is there any performance difference between
(.+)$ and (.+)
Cheers,
John
--
http://mail.python.org/mailman/listinfo/python-list
Re: Relative performance of comparable regular expressions
John Machin wrote:
On Jan 13, 7:24 pm, Barak, Ron ron.ba...@lsi.com wrote:
Hi,
I have a question about relative performance of comparable regular
expressions.
I have large log files that start with three letters month names
(non-unicode).
Which would give better performance, matching with ^[a-zA-Z]{3}, or with
^\S{3} ?
(1) If you want to match at the start of a line, use re.match()
*without* the pointless ^. Don't use re.search with a pattern
starting with ^ -- it won't be any faster than and it could be a lot
worse; re.search doesn't know to stop if the first match fails:
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.match(text)
100 loops, best of 3: 1.15 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.search(text)
10 loops, best of 3: 4.47 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*1000 rx.search(text)
1 loops, best of 3: 34.1 usec per loop
(2) I think you mean ^\s{3} not ^\S{3}
(3) Now that you've seen how to do timings, over to you :-)
Also, which is better (if different at all): \d\d or \d{2} ?
Also, would matching . be different (performance-wise) than matching the
actual character, e.g. matching : ?
And lastly, at the end of a line, is there any performance difference
between (.+)$ and (.+)
Of course if the log strings all begin with a string like Dec 12 2009
then you don't need regular expressions at all - just pull the
characters out using their positions and slicing. The month would be
string[0:3] and so on.
regards
Steve
--
Steve Holden+1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/
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RE: Relative performance of comparable regular expressions
Hi John,
Thanks for the below - teaching me how to fish ( instead of just giving me
a fish :-)
Now I could definitely get the answers for myself, and also be a bit more
enlightened.
As for your (2) remark below (on my question: Which would give better
performance, matching with ^[a-zA-Z]{3}, or with ^\S{3} ?):
(2) I think you mean ^\s{3} not ^\S{3},
I actually did meant to use \S, namely - a character that is not a white-space.
Bye,
Ron.
-Original Message-
From: John Machin []
Sent: Tuesday, January 13, 2009 11:15
To: python-list@python.org
Subject: Re: Relative performance of comparable regular expressions
On Jan 13, 7:24 pm, Barak, Ron ron.ba...@lsi.com wrote:
Hi,
I have a question about relative performance of comparable regular
expressions.
I have large log files that start with three letters month names
(non-unicode).
Which would give better performance, matching with ^[a-zA-Z]{3}, or with
^\S{3} ?
(1) If you want to match at the start of a line, use re.match()
*without* the pointless ^. Don't use re.search with a pattern starting with
^ -- it won't be any faster than and it could be a lot worse; re.search
doesn't know to stop if the first match fails:
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.match(text)
100 loops, best of 3: 1.15 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.search(text)
10 loops, best of 3: 4.47 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*1000 rx.search(text)
1 loops, best of 3: 34.1 usec per loop
(2) I think you mean ^\s{3} not ^\S{3}
(3) Now that you've seen how to do timings, over to you :-)
Also, which is better (if different at all): \d\d or \d{2} ?
Also, would matching . be different (performance-wise) than matching the
actual character, e.g. matching : ?
And lastly, at the end of a line, is there any performance difference between
(.+)$ and (.+)
Cheers,
John
--
http://mail.python.org/mailman/listinfo/python-list
Re: Relative performance of comparable regular expressions
On Tue, Jan 13, 2009 at 6:16 AM, Barak, Ron ron.ba...@lsi.com wrote:
Hi John,
Thanks for the below - teaching me how to fish ( instead of just giving
me a fish :-)
Now I could definitely get the answers for myself, and also be a bit more
enlightened.
As for your (2) remark below (on my question: Which would give better
performance, matching with ^[a-zA-Z]{3}, or with ^\S{3} ?):
(2) I think you mean ^\s{3} not ^\S{3},
I actually did meant to use \S, namely - a character that is not a
white-space.
(A) Please don't top-post, it makes replying to you more awkward and
makes it harder for readers to follow the conversation.
(B) But ^[a-zA-Z]{3}, and ^\S{3} aren't even equivalent! \S allows
*digits* and *punctuation* too, whereas the former *only* matches
letters.
Cheers,
Chris
--
Follow the path of the Iguana...
http://rebertia.com
-Original Message-
From: John Machin [
]
Sent: Tuesday, January 13, 2009 11:15
To: python-list@python.org
Subject: Re: Relative performance of comparable regular expressions
On Jan 13, 7:24 pm, Barak, Ron ron.ba...@lsi.com wrote:
Hi,
I have a question about relative performance of comparable regular
expressions.
I have large log files that start with three letters month names
(non-unicode).
Which would give better performance, matching with ^[a-zA-Z]{3}, or
with ^\S{3} ?
(1) If you want to match at the start of a line, use re.match()
*without* the pointless ^. Don't use re.search with a pattern starting
with ^ -- it won't be any faster than and it could be a lot worse;
re.search doesn't know to stop if the first match fails:
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.match(text)
100 loops, best of 3: 1.15 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*100 rx.search(text)
10 loops, best of 3: 4.47 usec per loop
command-prompt\python26\python -m timeit -simport re;rx=re.compile
('^AB')
;text='Z'*1000 rx.search(text)
1 loops, best of 3: 34.1 usec per loop
(2) I think you mean ^\s{3} not ^\S{3}
(3) Now that you've seen how to do timings, over to you :-)
Also, which is better (if different at all): \d\d or \d{2} ?
Also, would matching . be different (performance-wise) than matching the
actual character, e.g. matching : ?
And lastly, at the end of a line, is there any performance difference
between (.+)$ and (.+)
Cheers,
John
--
http://mail.python.org/mailman/listinfo/python-list
