$ seq 10 20 | sed 's/1**//'
sed: -e expression #1, char 7: Invalid preceding regular expression
and only allows this syntax when ERE syntax is enabled
$ seq 10 20 | sed -E 's/1**//'
0
2
3
4
5
6
7
8
9
20
grep, which also implements this GNU extension, allows
** in BRE syntax
$ seq 10 20 | grep -o '1**'
1
11
1
1
1
1
1
1
1
1
$ seq 10 20 | grep -oE '1**'
1
11
1
1
1
1
1
1
1
1
Additionally, the documentation for the `--posix` flag states
> disable all GNU extensions.
** is specifically documented as a GNU extension, but `--posix` doesn't disable the use of
**
$ seq 10 20 | sed --posix -E 's/1**//'
0
2
3
4
5
6
7
8
9
20