Question on regex substitution using variables...

[Ian]

Hi,

Hopefully a simple question but my brain is hurting...

I want to make a regex substitution, using search and replace
patterns contained in variables. What I want to do is:

$f = "fred.abc";
$f =~ s/(.*)\.abc/$1.def/;
print "$f\n";

but where the two parts of the substitution are variables:

my $to_pattern = "(.*)\\.abc ";
my $from_pattern = "\$1.def";
$f =~ s/$to_pattern/$from_pattern/;
print "$f\n";

Unfortunately this doesn't seem to work, where the first example
correctly prints out "fred.def" the second one doesn't seem to
do the back-substitution, and just prints "$1.def".

Is this possible, and if so, what quoting magic do I need to make
it work???

TIA,
Ian.



#!/usr/bin/perl -w

use strict;

my $f;

$f = "fred.abc";
$f =~ s/(.*)\.abc/$1.def/;

print "$f\n"; # prints "fred.def" (correct)


$f = "joe.abc";

my $to_pattern = "(.*)\\.abc ";
my $from_pattern = "\$1.def";

$f =~ s/$to_pattern/$from_pattern/;

print "$f\n"; # prints "$1.def"

# end


--
Ian

"Tamahome!! !" - "Miaka!!!"

[Ben Bacarisse]

Re: Question on regex substitution using variables...

On Thu, 02 Feb 2006 17:06:09 +0000, Ian wrote:
[color=blue]
> but where the two parts of the substitution are variables:
>
> my $to_pattern = "(.*)\\.abc ";
> my $from_pattern = "\$1.def";
> $f =~ s/$to_pattern/$from_pattern/;
> print "$f\n";
>
> Unfortunately this doesn't seem to work, where the first example correctly
> prints out "fred.def" the second one doesn't seem to do the
> back-substitution, and just prints "$1.def".[/color]

That is because the second part of the s/// construct is an ordinary
double quoted string. Once you have put a literal $ into a string, it is
not re-interpreted whenever the string is substituted. (What would you
expect

my $a = "X"; my $b = "\$a"; print "$b"

to print?)
[color=blue]
> Is this possible, and if so, what quoting magic do I need to make it
> work???[/color]

You need, in some sence, less quoting. One way is to re-write the
substitution as a little code block (in a string) and have it evaluated:

my $from_pattern = "\$1.'.def' ";
$f =~ s/$to_pattern/eval($from_patt ern)/e;

This being Perl, I await news of the 1867 other ways to do it... :-)

--
Ben.

[Ian]

Re: Question on regex substitution using variables...

On 2006-02-02, Ben Bacarisse <ben.usenet@bsb .me.uk> wrote:

[snip - understood]
[color=blue]
> You need, in some sence, less quoting. One way is to re-write the
> substitution as a little code block (in a string) and have it evaluated:
>
> my $from_pattern = "\$1.'.def' ";
> $f =~ s/$to_pattern/eval($from_patt ern)/e;[/color]

Ta, this gives the effect I'm after. I was contemplating doing the whole
thing in an eval() but this is neater.
[color=blue]
> This being Perl, I await news of the 1867 other ways to do it... :-)[/color]

At least!

--
Ian

"Tamahome!! !" - "Miaka!!!"

[Jim Gibson]

Re: Question on regex substitution using variables...

In article <pan.2006.02.02 .18.23.39.19133 4@bsb.me.uk>, Ben Bacarisse
<ben.usenet@bsb .me.uk> wrote:
[color=blue]
> On Thu, 02 Feb 2006 17:06:09 +0000, Ian wrote:
>[color=green]
> > but where the two parts of the substitution are variables:
> >
> > my $to_pattern = "(.*)\\.abc ";
> > my $from_pattern = "\$1.def";
> > $f =~ s/$to_pattern/$from_pattern/;
> > print "$f\n";
> >
> > Unfortunately this doesn't seem to work, where the first example correctly
> > prints out "fred.def" the second one doesn't seem to do the
> > back-substitution, and just prints "$1.def".[/color][/color]
....[color=blue]
> You need, in some sence, less quoting. One way is to re-write the
> substitution as a little code block (in a string) and have it evaluated:
>
> my $from_pattern = "\$1.'.def' ";
> $f =~ s/$to_pattern/eval($from_patt ern)/e;[/color]

A second e modifier will force a second round of evaluation:

$f =~ s/$to_pattern/$from_pattern/ee;
[color=blue]
>
> This being Perl, I await news of the 1867 other ways to do it... :-)[/color]

(1866 to go :)

FYI: this newsgroup is defunct. Try comp.lang.perl. misc in the future.

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[Ben Bacarisse]

Re: Question on regex substitution using variables...

On Thu, 02 Feb 2006 12:26:21 -0800, Jim Gibson wrote:
[color=blue]
> In article <pan.2006.02.02 .18.23.39.19133 4@bsb.me.uk>, Ben Bacarisse
> <ben.usenet@bsb .me.uk> wrote:
>[color=green]
>> On Thu, 02 Feb 2006 17:06:09 +0000, Ian wrote:
>>[color=darkred]
>> > but where the two parts of the substitution are variables:
>> >
>> > my $to_pattern = "(.*)\\.abc ";
>> > my $from_pattern = "\$1.def";
>> > $f =~ s/$to_pattern/$from_pattern/;
>> > print "$f\n";
>> >
>> > Unfortunately this doesn't seem to work, where the first example
>> > correctly prints out "fred.def" the second one doesn't seem to do the
>> > back-substitution, and just prints "$1.def".[/color][/color]
> ...[color=green]
>> You need, in some sence, less quoting. One way is to re-write the
>> substitution as a little code block (in a string) and have it evaluated:
>>
>> my $from_pattern = "\$1.'.def' ";
>> $f =~ s/$to_pattern/eval($from_patt ern)/e;[/color]
>
> A second e modifier will force a second round of evaluation:
>
> $f =~ s/$to_pattern/$from_pattern/ee;[/color]

Lovely! I did not know that.
[color=blue][color=green]
>> This being Perl, I await news of the 1867 other ways to do it... :-)[/color]
>
> (1866 to go :)[/color]

No, that's the one as far as I'm concerned!
[color=blue]
> FYI: this newsgroup is defunct. Try comp.lang.perl. misc in the future.[/color]

Thanks for the heads up.

--
Ben.