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% sed --version
sed (GNU sed) 4.2.2
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Jay Fenlason, Tom Lord, Ken Pizzini,
and Paolo Bonzini.
GNU sed home page: <http://www.gnu.org/software/sed/>.
General help using GNU software: <http://www.gnu.org/gethelp/>.
E-mail bug reports to: <[email protected]>.
Be sure to include the word ``sed'' somewhere in the ``Subject:'' field.

% sed -E 's_[a-zA-Z0-9_]+.*_\1_' /etc/passwd
sed: -e expression #1, char 21: invalid reference \1 on `s' command's RHS

% sed -E 's_([a-zA-Z0-9_]+).*_\1_' /etc/passwd
root
bin
daemon
adm
lp
sync
shutdown
halt
mail
operator
games
ftp
nobody
systemd
dbus
polkitd
sshd
postfix
chrony
rpc
ntp
tss
apache
rpcuser
nfsnobody
abrt
libstoragemgmt
tcpdump
stap
uuidd
unbound