manipulating capture groups in s///

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

I am sure this has been asked before, but I couldn't find the answer on my own.

Very simply, I want to do a substitution where the string being substituted in, is based on what was matched; in other (clearer) words, it would look like: s/(bar)/&foo($1)/g;.

The best I've been able to come up with is: s/((bar)(?{$b=foo($2)}))/$b/g;

I am a little wary of this, as I am not entirely certain that I understand exactly how it's doing what it's doing (additionaly the docs seem to suggest that (?{}) is liable to open the gates of hell and unleash all sorts of despair on humanity). Additionally, as I understand it, this requires a global variable (the use of $b is quite intentional here).

So my question is: (a) am I going to run into trouble with the above? and (b) is there a better way?

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Re: manipulating capture groups in s/// by diotalevi (Canon) on Nov 11, 2003 at 19:19 UTC
Yes, you'll run into trouble, you should have used the /e flag. It causes the right side of your expression to be executed and its return value is used as the replacement. `s((bar))(foo($1))ge`	[reply] [d/l]
Re^2: manipulating capture groups in s/// by Aristotle (Chancellor) on Nov 11, 2003 at 19:57 UTC
Neat way to parenthesize. I need to remember this for obfuscations. :) Makeshifts last the longest.	[reply]
Re: manipulating capture groups in s/// by Anonymous Monk on Nov 11, 2003 at 19:26 UTC
If the replacement rules can be formulated in a look up table just use a hash: `my %lookup = ( bar => 'thing1', foo => 'thing2'); $_ = "this has bar and foo\n"; s/(bar\|foo)/$lookup{$1}/g; print; #=> this has thing1 and thing2` [download] If the replacement requires computation, then use the /e modifier which evaluates the replacement side of the s/// operator as Perl code: `sub foo { return reverse uc shift } $_ = "this has bar and foo\n"; s/(bar\|foo)/foo($1)/eg; print; #=> this has RAB and OOF` [download]	[reply] [d/l] [select]
Re: manipulating capture groups in s/// by ptkdb (Monk) on Nov 11, 2003 at 20:17 UTC
if you're so concerned about using (?{}), why not simply do your substitution in two steps? `my($b) = /(bar)/ ; # step 1 find the string my($substitute) = foo($b) ; # Step 2 make the substitution s/bar/$substitute/ ; # plug it in` [download]	[reply] [d/l]


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