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Regex for "at start of line OR preceded by space".
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Regex for "at start of line OR preceded by space". Robbie Hatley 04-27-2008
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Posted by A. Sinan Unur on April 27, 2008, 7:46 pm
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 1h4nVnZ2dnUVZ_vOdnZ2d@giganews.com:

> I needed a regex that says "either at the start of a line, OR
> preceded by some whitespace".

The only difference between this criterion and "preceded by whitespace"
can occur at the beginning of the string. Therefore:

#!/usr/bin/perl

my $x = <<EOSTR;
Test1 Test2
Test3 Test4 Test5
Test6
Test7 Test8
Test9
Test0a

EOSTR

print "$1\n" while $x =~ /(?:\A|\s+)(\S+)/g ;



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Posted by dummy on April 28, 2008, 4:06 am
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 On Sun, 27 Apr 2008 23:46:31 GMT, "A. Sinan Unur"

>1h4nVnZ2dnUVZ_vOdnZ2d@giganews.com:
>
>> I needed a regex that says "either at the start of a line, OR
>> preceded by some whitespace".
>
>The only difference between this criterion and "preceded by whitespace"
>can occur at the beginning of the string. Therefore:
>
>#!/usr/bin/perl
>
>my $x = <<EOSTR;
>Test1 Test2
> Test3 Test4 Test5
> Test6
>Test7 Test8
>Test9
> Test0a
>
>EOSTR
>
>print "$1\n" while $x =~ /(?:\A|\s+)(\S+)/g ;

On my XP machine that produces:
Test1
Test2
Test3
Test4
Test5
Test6
Test7
Test8
Test9
Test0a

But this:

use strict; use warnings;
while (<DATA>) {
print "$1\n" if /^\s*(\S+)(?:\s|$)/;
}
__DATA__
Test1 Test2
Test3 Test4 Test5
Test6
Test7 Test8
Test9
Test0a

Gives:
Test1
Test3
Test6
Test7
Test9
Test0a

which I think is better?

Posted by A. Sinan Unur on April 28, 2008, 4:50 am
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dummy@phony.info wrote in

> On Sun, 27 Apr 2008 23:46:31 GMT, "A. Sinan Unur"
>
>>1h4nVnZ2dnUVZ_vOdnZ2d@giganews.com:
>>
>>> I needed a regex that says "either at the start of a line, OR
>>> preceded by some whitespace".
>>
>>The only difference between this criterion and "preceded by
>>whitespace" can occur at the beginning of the string. Therefore:
>>
>>#!/usr/bin/perl
>>
>>my $x = <<EOSTR;
>>Test1 Test2
>> Test3 Test4 Test5
>> Test6
>>Test7 Test8
>>Test9
>> Test0a
>>
>>EOSTR
>>
>>print "$1\n" while $x =~ /(?:\A|\s+)(\S+)/g ;
>
> On my XP machine that produces:
> Test1
> Test2
> Test3
> Test4
> Test5
> Test6
> Test7
> Test8
> Test9
> Test0a
>
> But this:
>
> use strict; use warnings;
> while (<DATA>) {
> print "$1\n" if /^\s*(\S+)(?:\s|$)/;
> }
> __DATA__
> Test1 Test2
> Test3 Test4 Test5
> Test6
> Test7 Test8
> Test9
> Test0a
>
> Gives:
> Test1
> Test3
> Test6
> Test7
> Test9
> Test0a
>
> which I think is better?

How can that be better?

Read the OP's criterion again:

>>> I needed a regex that says "either at the start of a line, OR
>>> preceded by some whitespace".

Yours misses Test2, Test4, Test5 and Test8 which are all preceded by
whitespace.

Sinan

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comp.lang.perl.misc guidelines on the WWW:
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Posted by dummy on April 29, 2008, 1:28 am
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On Mon, 28 Apr 2008 08:50:53 GMT, "A. Sinan Unur"

>dummy@phony.info wrote in
>
>> On Sun, 27 Apr 2008 23:46:31 GMT, "A. Sinan Unur"
>>
>>>1h4nVnZ2dnUVZ_vOdnZ2d@giganews.com:
>>>
>>>> I needed a regex that says "either at the start of a line, OR
>>>> preceded by some whitespace".
>>>
>>>The only difference between this criterion and "preceded by
>>>whitespace" can occur at the beginning of the string. Therefore:
>>>
>>>#!/usr/bin/perl
>>>
>>>my $x = <<EOSTR;
>>>Test1 Test2
>>> Test3 Test4 Test5
>>> Test6
>>>Test7 Test8
>>>Test9
>>> Test0a
>>>
>>>EOSTR
>>>
>>>print "$1\n" while $x =~ /(?:\A|\s+)(\S+)/g ;
>>
>> On my XP machine that produces:
>> Test1
>> Test2
>> Test3
>> Test4
>> Test5
>> Test6
>> Test7
>> Test8
>> Test9
>> Test0a
>>
>> But this:
>>
>> use strict; use warnings;
>> while (<DATA>) {
>> print "$1\n" if /^\s*(\S+)(?:\s|$)/;
>> }
>> __DATA__
>> Test1 Test2
>> Test3 Test4 Test5
>> Test6
>> Test7 Test8
>> Test9
>> Test0a
>>
>> Gives:
>> Test1
>> Test3
>> Test6
>> Test7
>> Test9
>> Test0a
>>
>> which I think is better?
>
>How can that be better?
>
>Read the OP's criterion again:
>
>>>> I needed a regex that says "either at the start of a line, OR
>>>> preceded by some whitespace".
>
>Yours misses Test2, Test4, Test5 and Test8 which are all preceded by
>whitespace.
>
>Sinan

OOPS!

Posted by Robbie Hatley on May 3, 2008, 1:46 am
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"A. Sinan Unur" wrote:

> "Robbie Hatley" wrote:
>
> > I needed a regex that says "either at the start of a line, OR
> > preceded by some whitespace".
>
> The only difference between this criterion and "preceded by whitespace"
> can occur at the beginning of the string. Therefore:
>
> #!/usr/bin/perl
>
> my $x = <<EOSTR;
> Test1 Test2
> Test3 Test4 Test5
> Test6
> Test7 Test8
> Test9
> Test0a
>
> EOSTR
>
> print "$1\n" while $x =~ /(?:\A|\s+)(\S+)/g ;

Close! But ultimately, no cigar. The preceding space
(if any) must not be part of the match, because this is
actually being used in a s/// command, so i don't
want to strip/alter/replace the leading spaces.

Get rid of the "+" and add in a lookbehind, and it works:

$x =~ s/(?:\A|(?<=\s))$Regex1/Prefix$&/g ;

(Can't use "\s+" in a look-behind, because look-behinds
must always be fixed-width.)

The "^" assertion can also be used instead of "\A":

$x =~ s/(?:^|(?<=\s))$Regex1/Prefix$&/g ;

(Also see Dr. Ruud's post, and my reply to it.)

--
Cheers,
Robbie Hatley
lonewolf aatt well dott com
www dott well dott com slant user slant lonewolf slant



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