Hi,

I wanted to show to my classmate the comportment of a RegEX using sed to capture the subgroup in a pattern match.

But, when trying with sed -E, the pattern matched even though it should not have.

Can someone explain to me why if this comportment turns out to be normal ?

View snippet : 

$ echo abc | grep -E '(...).+' -c

0

$ echo abc | sed -E 's/(...).+/\1/'

abc

$ sed --version 

sed (GNU sed) 4.9

Packaged by Debian

Copyright (C) 2022 Free Software Foundation, Inc.

License GPLv3+: GNU GPL version 3 or later <https://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,

Paolo Bonzini, Jim Meyering, and Assaf Gordon.

This sed program was built with SELinux support.

SELinux is disabled on this system.

GNU sed home page: <https://www.gnu.org/software/sed/>.

General help using GNU software: <https://www.gnu.org/gethelp/>.

E-mail bug reports to: <bug-sed@gnu.org>.

Regards,

-Alexandre.