Editting Part of Regex

mt2k has asked for the wisdom of the Perl Monks concerning the following question:

How would I go about changing just a small part of a regex?

Example:

$string = "
hello
<tag>
code
goes
here
</tag>
more text
";
[download]

How would I go about changing $string so that only the \n's between the <TAG> and </TAG> tags become <BR> tags. Meaning that in the end, $string would hold:

$string = "
hello
<tag><br>code<br>goes here<br></tag>
more text
";
[download]

My attempt would be something like:

$string =~ s/<tag>(.*?)<\/tag>/$1 =~ s/\n/<br>/gs;/ieg;
[download]

but I know that $1, $2, etc. are read-only.... Help!

Comment on Editting Part of Regex Select or Download Code

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Re: Editting Part of Regex by chipmunk (Parson) on Mar 08, 2001 at 19:44 UTC
You're on the right track... You can copy $1 to another variable in order to do the substitution. Also, you'll need to use different delimiters for the nested substitutions. `$string =~ s{<tag>(.*?)</tag>} {(my $x = $1) =~ s/\n/<br>/g; "<tag>$x</tag>"}gise;` [download] By using something other than / as a delimiter for the outer substitution, I avoided having to backslash the literal /s.	[reply] [d/l]
Re: Editting Part of Regex by mirod (Canon) on Mar 08, 2001 at 19:55 UTC
`/e` is your friend but don't get too friendly: `#!/bin/perl -w use strict; my $string="hello <tag> code goes here </tag> "; $string=~ s{(<tag>.?</tag>)}{replace( $1)}egs; print $string; sub replace { my $text_in_tag= shift; $text_in_tag=~ s{\n}{<br />}g; return $text_in_tag; }` [download] The e modifier means that the replacement expression is `eval`-ed instead of being treated as a string. Hence $1 is replaced by replace( $1) You can also use the old function-in-a-string trick: `$string=~ s{(<tag>.?</tag>)}{@{[replace( $1)]}}gs;`	[reply] [d/l] [select]
(sacked: substr) Re: Editting Part of Regex by sacked (Hermit) on Mar 08, 2001 at 22:25 UTC
Here is another, perhaps less pretty, way to do it: `substr($string, index($string, '<tag>'), rindex($string, '</tag>')) =~ + s/\n(?!$)/<br>/g;` [download] Update: I was using the return value of rindex as the length parameter to substr. Oops! Here is a correct, albeit even less pretty, version of the above: `substr($string, index($string, '<tag>'), rindex($string, '</tag>') - i +ndex($string, '<tag>')) =~ s/\n/<br>/g;` [download] Yech! It gets rid of the negative look-ahead assertion (since the length is corrected), but it's ugly, and duplicates some function calls. A better version (but uses temp vars) is: `my $st = index($string, '<tag>'); my $en = rindex($string, '</tag>') - $st; substr($string, $st, $en) =~ s/\n/<br>/g;` [download] Much better. Though I'm still not sure if it's an improvement over the other suggestions above. --sacked	[reply] [d/l] [select]