Replace specific capture group instead of entire regex in Perl

Question

I've got a regular expression with capture groups that matches what I want in a broader context. I then take capture group $1 and use it for my needs. That's easy.

But how to use capture groups with s/// when I just want to replace the content of $1, not the entire regex, with my replacement?

For instance, if I do:

$str =~ s/prefix (something) suffix/42/

prefix and suffix are removed. Instead, I would like something to be replaced by 42, while keeping prefix and suffix intact.

Answer 1

If you only need to replace one capture then using @LAST_MATCH_START and @LAST_MATCH_END (with use English; see perldoc perlvar) together with substr might be a viable choice:

use English qw(-no_match_vars);
$your_string =~ m/aaa (bbb) ccc/;
substr $your_string, $LAST_MATCH_START[1], $LAST_MATCH_END[1] - $LAST_MATCH_START[1], "new content";
# replaces "bbb" with "new content"

Answer 2

As I understand, you can use look-ahead or look-behind that don't consume characters. Or save data in groups and only remove what you are looking for. Examples:

With look-ahead:

s/your_text(?=ahead_text)//;

Grouping data:

s/(your_text)(ahead_text)/$2/;

Answer 3

This is an old question but I found the below easier for replacing lines that start with >something to >something_else. Good for changing the headers for fasta sequences

  while ($filelines=~ />(.*)\s/g){
        unless ($1 =~ /else/i){
                $filelines =~ s/($1)/$1\_else/;
        }

  }