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Assigning a value to pos to control regular expression matching
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Subject Author Date
Assigning a value to pos to control regular expression matching Mark 05-13-2008
Posted by Mark on May 13, 2008, 5:15 pm
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 use strict ;
use warnings ;

my $txt = 'abc Start something' ;
my $startpoint = index($txt,'Start') ;
die "Start point not found" if $startpoint < 0 ;

$_ = $txt ;

pos = $startpoint ;
if (/\G(.*)/g) {
print "matched=$1\n" ; # prints "matched=Start something"
}
else {
print "no match\n" ; }

pos = $startpoint ;
if ($txt =~ /\G(.*)/g) {
print "matched=$1\n" ; # prints "matched=abc Start something"

}
else {
print "no match\n" ;
}

I expected the matched part in both cases to be the same. The second
case doesn't seem to honor the value of pos.

Can someone please explain.

Posted by John W. Krahn on May 13, 2008, 5:31 pm
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 Mark wrote:
> use strict ;
> use warnings ;
>
> my $txt = 'abc Start something' ;
> my $startpoint = index($txt,'Start') ;
> die "Start point not found" if $startpoint < 0 ;
>
> $_ = $txt ;
>
> pos = $startpoint ;
> if (/\G(.*)/g) {
> print "matched=$1\n" ; # prints "matched=Start something"
> }
> else {
> print "no match\n" ; }
>
> pos = $startpoint ;

You are now matching with the $txt variable instead of the $_ variable
so that should be:

pos( $txt ) = $startpoint;

> if ($txt =~ /\G(.*)/g) {
> print "matched=$1\n" ; # prints "matched=abc Start something"
>
> }
> else {
> print "no match\n" ;
> }
>
> I expected the matched part in both cases to be the same. The second
> case doesn't seem to honor the value of pos.
>
> Can someone please explain.


John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall

Posted by Gunnar Hjalmarsson on May 13, 2008, 5:34 pm
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Mark wrote:
> use strict ;
> use warnings ;
>
> my $txt = 'abc Start something' ;
> my $startpoint = index($txt,'Start') ;
> die "Start point not found" if $startpoint < 0 ;
>
> $_ = $txt ;
>
> pos = $startpoint ;
> if (/\G(.*)/g) {
> print "matched=$1\n" ; # prints "matched=Start something"
> }
> else {
> print "no match\n" ; }
>
> pos = $startpoint ;
> if ($txt =~ /\G(.*)/g) {
> print "matched=$1\n" ; # prints "matched=abc Start something"
>
> }
> else {
> print "no match\n" ;
> }
>
> I expected the matched part in both cases to be the same.

Try

pos($txt) = $startpoint;

in the second case.

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl

Posted by xhoster on May 13, 2008, 5:38 pm
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>
> pos = $startpoint ;

Using pos without an argument implicitly uses $_ as the argument.
Thus this resets pos for the next time a regex is done on $_.

Since you want to use $txt, not $_, for the next match,
change that to:

pos $txt = $startpoint;



> if ($txt =~ /\G(.*)/g) {
> print "matched=$1\n" ; # prints "matched=abc Start something"
>
> }
> else {
> print "no match\n" ;
> }
>
> I expected the matched part in both cases to be the same. The second
> case doesn't seem to honor the value of pos.
>
> Can someone please explain.

Xho

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Posted by Mark on May 13, 2008, 5:43 pm
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> use strict ;
> use warnings ;
>
> my $txt =3D 'abc Start something' ;
> my $startpoint =3D index($txt,'Start') ;
> die "Start point not found" if $startpoint < 0 ;
>
> $_ =3D $txt ;
>
> pos =3D $startpoint ;
> if (/\G(.*)/g) {
> =A0 =A0 print "matched=3D$1\n" ; # prints "matched=3DStart something"}
>
> else {
> =A0 =A0 print "no match\n" ; }
>
> pos =3D $startpoint ;
> if ($txt =3D~ /\G(.*)/g) {
> =A0 =A0 print "matched=3D$1\n" ; # prints "matched=3Dabc Start something"
>
> }
>
> else {
> =A0 =A0 print "no match\n" ;
>
> }
>
> I expected the matched part in both cases to be the same. =A0The second
> case doesn't seem to honor the value of pos.
>
> Can someone please explain.

Thank you to all that responded.

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