Re: Re: Regex matching on anywhere but the beginning or end of a line

Interestiing. Though I think your conclusion regarding "if the speed difference isn't relevant" is wrong. For instance, your test data had 2 "'s in a short string. By the time this gets to 10 "s the speed advantage swings the other way. Even more interesting is that if the string has 0 "s, using substr to avoid the zwa's is twice as fast.

Why would you run it on a string that had no "s. Well, if you're processing large volumes of data into a database from a flat file then you need to check each line as you go. Some will have embedded "s but many won't. You could make the s/// conditional by checking first with m// or index, but just checking causes another pass to be made and costs more than using the simpler regex on the substr.

As Abigail said (again) recently, and has rightly pulled me up for before, you have to check a range inputs before you can make a judgement.

I love Perl. There's always another nuance to explore.

#! perl -slw
use strict;
use Benchmark qw[timethese cmpthese];

our $s = q["string (12" or 1 foot or 1') into 6" MySQL varchar"];
our $t = q["string (12 or 1 foot or 1') into 6 MySQL varchar"];
our ($sub,$zwa);

my $tests = {
zwax0        => q[ $zwa=$t; $zwa =~ s/(?<=.)"(?=.)/""/g; ],
subx0        => q[ $sub=$t; substr($sub, 1, -1) =~ s/"/""/g; ],
zwax1        => q[ $zwa=$s; $zwa =~ s/(?<=.)"(?=.)/""/g; ],
subx1        => q[ $sub=$s; substr($sub, 1, -1) =~ s/"/""/g; ],
zwax4        => q[ $zwa=$s x4; $zwa =~ s/(?<=.)"(?=.)/""/g; ],
subx4        => q[ $sub=$s x4; substr($sub, 1, -1) =~ s/"/""/g; ],
zwax10        => q[ $zwa=$s x10; $zwa =~ s/(?<=.)"(?=.)/""/g; ],
subx10        => q[ $sub=$s x10; substr($sub, 1, -1) =~ s/"/""/g; ],
};

timethese( 2, $tests );
print 'Results same' if $zwa eq $sub;

cmpthese( 100000, $tests );
__DATA__
Results same
Benchmark: timing 100000 iterations of subx0, subx1, subx10, subx4, zw
+ax0, zwax1, zwax10, zwax4 ...

     subx0:  1 wallclock secs ( 0.60 usr +  0.00 sys =  0.60 CPU) @ 16
+6112.96/s (n=100000)
     subx1:  7 wallclock secs ( 5.71 usr +  0.00 sys =  5.71 CPU) @ 17
+516.20/s (n=100000)
    subx10: 25 wallclock secs (24.95 usr +  0.00 sys = 24.95 CPU) @ 40
+08.66/s (n=100000)
     subx4: 12 wallclock secs (12.77 usr +  0.00 sys = 12.77 CPU) @ 78
+32.08/s (n=100000)
     zwax0:  2 wallclock secs ( 1.34 usr +  0.00 sys =  1.34 CPU) @ 74
+515.65/s (n=100000)
     zwax1:  5 wallclock secs ( 4.64 usr +  0.00 sys =  4.64 CPU) @ 21
+570.32/s (n=100000)
    zwax10: 31 wallclock secs (30.02 usr +  0.00 sys = 30.02 CPU) @ 33
+30.67/s (n=100000)
     zwax4: 15 wallclock secs (13.69 usr +  0.00 sys = 13.69 CPU) @ 73
+04.60/s (n=100000)

           Rate zwax10 subx10  zwax4  subx4  subx1  zwax1  zwax0  subx
+0
zwax10   3331/s     --   -17%   -54%   -57%   -81%   -85%   -96%   -98
+%
subx10   4009/s    20%     --   -45%   -49%   -77%   -81%   -95%   -98
+%
zwax4    7305/s   119%    82%     --    -7%   -58%   -66%   -90%   -96
+%
subx4    7832/s   135%    95%     7%     --   -55%   -64%   -89%   -95
+%
subx1   17516/s   426%   337%   140%   124%     --   -19%   -76%   -89
+%
zwax1   21570/s   548%   438%   195%   175%    23%     --   -71%   -87
+%
zwax0   74516/s  2137%  1759%   920%   851%   325%   245%     --   -55
+%
subx0  166113/s  4887%  4044%  2174%  2021%   848%   670%   123%     -
+-
[download]

..and remember there are a lot of things monks are supposed to be but lazy is not one of them

Examine what is said, not who speaks.

1) When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
2) The only way of discovering the limits of the possible is to venture a little way past them into the impossible
3) Any sufficiently advanced technology is indistinguishable from magic.
Arthur C. Clarke.

Comment on Re: Re: Regex matching on anywhere but the beginning or end of a line Download Code

Replies are listed 'Best First'.
Re: Re: Re: Regex matching on anywhere but the beginning or end of a line by xmath (Hermit) on Feb 23, 2003 at 13:17 UTC
OK, I kinda assumed that he gave a "typical" example of an input string, but you're right ofcourse - I should have tested across a wide range of values. I should actually never have involved benchmarks, since the execution time of the subtitution is likely to be negligable compared to other parts of his application (whatever it may be), and is furthermore dependent on the input strings, on the platform, perl version, and possibly the phase of the moon. But well, too late for that, the thread is now filled with benchmarks.. my fault :-)	[reply]

Replies are listed 'Best First'.

Re: Re: Re: Regex matching on anywhere but the beginning or end of a line
by xmath (Hermit) on Feb 23, 2003 at 13:17 UTC

I should actually never have involved benchmarks, since the execution time of the subtitution is likely to be negligable compared to other parts of his application (whatever it may be), and is furthermore dependent on the input strings, on the platform, perl version, and possibly the phase of the moon.

But well, too late for that, the thread is now filled with benchmarks.. my fault :-)

[reply]