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perl: reg.expr: combine starting and ending removal in one exprecion

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# 1  
Old 08-26-2009
perl: reg.expr: combine starting and ending removal in one exprecion
Hello,
I am new in perl and in regular exprecion; so I am looking for help (or an experienced advise.)

The target is a triming spaces from a string: i.e., remove spases from begining and from end of a string.
One of main point of a searched solution is performance: for current task it is very important.
Therefore simple loop character by character seems to me ineffective.
I guess the reg.exp. engine should be pretty sufficient.
So, I've come out with:
Code:
 # $str = "   some text    "
  $str=~s/^[ ]*//;
  $str=~s/[ ]*$//;

This works fine,but,
first of all: I could not combine both patern into one exprecion.
Would you suggest how it could be done?
second: What about time? What expected to be longer : one more complicated regexp or twice, but simple? Who know how that task is processed inside of perl?

And last question: is there simpler way to do such simple task in perl?
# 2  
Old 08-26-2009
this is probably the most efficient way:

Code:
$str = "   some text    ";
$str =~ s/^\s+//;
$str =~ s/\s+$//;


You can do it with one regexp but it might be a little slower, only testing would tell for sure:

Code:
$str = "   some text    ";
$str =~ s/^\s+|\s+$//g;

As far as a simpler way, I don't think so.

---------- Post updated at 11:45 AM ---------- Previous update was at 11:30 AM ----------

Run this on the machine that will run the code to see which is best for that machine:

Code:
#!/usr/bin/perl

use warnings;
use strict;

use Benchmark qw(cmpthese timethese);

sub double_star {
  my $string = shift;
  $string =~ s/^\s*//;
  $string =~ s/\s*$//;
  return $string;
}

sub double_plus {
  my $string = shift;
  $string =~ s/^\s+//;
  $string =~ s/\s+$//;
  return $string;
}

sub single_or {
  my $string = shift;
  $string =~ s/^\s+|\s+$//g;
  return $string;
}

sub replace {
  my $string = shift;
  $string =~ s/^\s*(\S*(?:\s+\S+)*)\s*$/$1/;
  return $string;
}

sub for_star {
  my $string = shift;
  for ($string) { s/^\s+//; s/\s+$//; }
  return $string;
}

sub for_plus {
  my $string = shift;
  for ($string) { s/^\s*//; s/\s*$//; }
  return $string;
}

sub regex_or {
  my $string = shift;
  $string =~ s/(?:^ +)||(?: +$)//g;
  return $string;
}

cmpthese(
  -1,
  {
    'single_or'   => q|single_or(  '    Mary had a little lamb.   ');|,
    'double_star' => q|double_star('    Mary had a little lamb.   ');|,
    'double_plus' => q|double_plus('    Mary had a little lamb.   ');|,
    'replace'     => q|replace(    '    Mary had a little lamb.   ');|,
    'for_star'    => q|for_star(   '    Mary had a little lamb.   ');|,
    'for_plus'    => q|for_plus(   '    Mary had a little lamb.   ');|,
    'regex_or'    => q|regex_or(   '    Mary had a little lamb.   ');|,
  }
);

Results might vary machine to machine and perl version to perl version.
Last edited by KevinADC; 08-26-2009 at 01:37 PM.. Reason: added "g" to the last regexp
# 3  
Old 08-27-2009
KevinADC
Very BIG thank you!!
So much appreciate so informative and complete answer!!!
For sure I own you at least beer! Smilie
You have showed many different way to do this with reg.exp., plus you discovered for me the Benchmark!!!
Very appreciate!!

You are right: double \s+ is most efficient!
Code:
                Rate regex_or replace single_or for_plus double_star for_star double_plus
regex_or     22791/s       --    -26%      -49%     -53%        -63%     -67%        -78%
replace      30919/s      36%      --      -31%     -36%        -50%     -56%        -70%
single_or    44823/s      97%     45%        --      -7%        -28%     -36%        -57%
for_plus     48188/s     111%     56%        8%       --        -22%     -31%        -53%
double_star  61837/s     171%    100%       38%      28%          --     -11%        -40%
for_star     69591/s     205%    125%       55%      44%         13%       --        -33%
double_plus 103385/s     354%    234%      131%     115%         67%      49%          --

(It is strange that the '|' - 'or' in reg exp. does not work in debugin. But in a sub it works! In 'perl -d -e 0' :
Code:
  DB<387> $str="    some val    "; $str=~s/^\s+||\s+$//;print ">$str<";
>some val    <
  DB<388> $str="    some val    "; $str=~s/^\s*||\s*$//;print ">$str<";
>some val    <
  DB<389> sub single_or {my $string = shift;$string =~ s/^\s+|\s+$//g;return $string;}

  DB<390> print ">".single_or ($str)."<";
>some val<
  DB<384> $str="    some val    "
  DB<385> $str=~s/^\s*(\S*(?:\s+\S+)*)\s*$/\1/; print ">$str<";
>some val<

Once again: THANK YOU!
Last edited by alex_5161; 08-27-2009 at 12:59 PM..
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