How do I pass $1 to s///?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 16:56 UTC
Your $1 gets interpolated too early. What you really need is a templating system, but you can use the following in the meanwhile: `sub foo { (my $text = $_[0]) =~ s/$_[1]/eval $_[2]/e; print $text; } foo("This is test 1\n", qr/test (.*)/, '"fish $1"');` [download] (Insert notice of hazards of using `eval EXPR` here.)	[reply] [d/l] [select]
Re^2: How do I pass $1 to s///? by Anonymous Monk on Jan 06, 2006 at 19:19 UTC
No need to use `eval`, I'd prefer one of... `sub foo { (my $text = $_[0]) =~ s/$_[1]/$_[2]->($1)/e; print $text; } foo("This is test 1\n", qr/test (.)/, sub{"fish $_[0]"});` [download] ...or... `sub foo { (my $text = $_[0]) =~ s/$_[1]/$_[2]->()/e; print $text; } foo("This is test 1\n", qr/test (.)/, sub{"fish $1"});` [download]	[reply] [d/l] [select]
Re^3: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 19:32 UTC
Nice, as long as the parameters of `foo` aren't read a file.	[reply] [d/l]
Re^4: How do I pass $1 to s///? by Anonymous Monk on Jan 06, 2006 at 20:10 UTC
Re^5: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 20:17 UTC
Some notes below your chosen depth have not been shown here
Re^2: How do I pass $1 to s///? by Anonymous Monk on Jan 06, 2006 at 17:19 UTC
Slightly nicer as... `(my $text = $_[0]) =~ s/$_[1]/$_[2]/ee;` [download]	[reply] [d/l]
Re^3: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 17:25 UTC
I call that uglier since it hides the `eval`. `eval EXPR` is something that should be glaring, blinking and alarming.	[reply] [d/l] [select]
Re^2: How do I pass $1 to s///? by Anonymous Monk on Jan 06, 2006 at 17:07 UTC
Ah, thankyou! The only problem is that I want to pass a much more complicated string in than "fish $1"... What do you mean by a templating system? Can you point me at a webpage or something? Thanks for the replies BTW! It's most appreciated! (You monks reply fast too! :)	[reply]
Re^3: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 17:23 UTC
You can pass any Perl source code in the above snippet (which is why it's potentially dangerous). One templating system would be the Template Toolkit. `use Template; sub foo { my $template = Template->new(); (my $text = $_[0]) =~ s/$_[1]/ my $output = ''; my @matches = map { substr($text, $-[$_], $+[$_] - $-[$_]) } 1 .. $#-; $template->process(\$_[2], { matches => \@matches }, \$output); $output /e; print $text; } foo("This is test 1\n", qr/test (.*)/, 'fish [% matches.1 %]');` [download] (Untested)	[reply] [d/l]
Re^3: How do I pass $1 to s///? by Happy-the-monk (Canon) on Jan 06, 2006 at 17:24 UTC
What do you mean by a templating system? Can you point me at a webpage or something? See some tutorials on Template Systems in the network Tutorial section here on Perl Monks. Cheers, Sören	[reply]
Re: How do I pass $1 to s///? by Happy-the-monk (Canon) on Jan 06, 2006 at 16:54 UTC
backreferences in perlre ~~are~~ used to be written `\1`, `\2` ... tweetiepooh informs me that my perldoc used was outdated... and ikegami again corrects that. Update: your programme prints "`This is fish`". What did you expect it to print? "`This is fish 1`"? Further update: see ikegami's solution below. Third and then fourth update - in italics Cheers, Sören	[reply]
Re^2: How do I pass $1 to s///? by ikegami (Patriarch) on Jan 06, 2006 at 17:42 UTC
Still not quite right. Backreferences are \1, \2, etc in the LHS (regexp part) of the substitution. Backreferences are $1, $2, etc in the RHS (substitituion part) of the substitution. Lastest perlre (via `[doc://perlre]`)	[reply] [d/l]
Re^2: How do I pass $1 to s///? by tweetiepooh (Hermit) on Jan 06, 2006 at 17:09 UTC
\1,\2 If warnings are on using these lead to `\1 better written as $1` [download] There is a section in perlre about it.	[reply] [d/l]
Re^2: How do I pass $1 to s///? by Anonymous Monk on Jan 06, 2006 at 17:03 UTC
> your programme prints "This is fish". ...and gives a warning about $1 being uninitialised :( > What did you expect it to print? "This is fish 1"? Yeah. I've tried "fish \1", "fish \\1", "fish \\\1" as well though with no more luck!	[reply]