Find and replace “tabs” using search and replace in nano

sed -i -E "s/(<username>.+)name(.+<\/username>)/\1something\2/" file.xml

This is, I think, what you're looking for.

Explanation:

• parentheses in the first part define groups (strings in fact) that can be reused in the second part
• \1, \2, etc. in the second part are references to the i-th group captured in the first part (the numbering starts with 1)
• -E enables extended regular expressions (needed for + and grouping).

I'm not sure why Perl isn't acceptable here. On the inputs you provided, this line gives the output you asked for:

perl -0777p -e 's/.* > (.*) joined the channel\.\n(((?!.* \1 (was kicked from channel\.|was banned from channel\.)\n).*\n)+?.*\1 disconnected)/\2/mg' irc.txt

The -e argument is exactly the first argument to your magicregextool except that I added the /mg regex modifier. This may not be "unmodified" but it doesn't seem unreasonable either. If you don't want to type in the whole line, how about this script as magicregextool:

#!/usr/bin/perl -0777p
BEGIN { $::arg = shift @ARGV; }
eval $arg;

Or even:

#!/bin/sh
perl -0777pe $*

Then you just type:

magicregextool 's/.* > (.*) joined the channel\.\n(((?!.* \1 (was kicked from channel\.|was banned from channel\.)\n).*\n)+?.*\1 disconnected)/\2/mg' irc.txt

Which is the same as your sample (again other than adding the /mg modifier).

An additional benefit to this is that if you are running multiple related search/replace operations on each file, you can put them together in the same script:

#!/usr/bin/perl -0777p
s/.* > (.*) joined the channel\.\n(((?!.* \1 (was kicked from channel\.|was banned from channel\.)\n).*\n)+?.*\1 disconnected)/\2/mg;
s/(some other\n)matched text/\1/mg;