For openSUSE Leap 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/openSUSE_Leap_15.3/devel:languages:ruby.repo
zypper refresh
zypper install oniguruma
For SLE 15 SP4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/SLE_15_SP4/devel:languages:ruby.repo
zypper refresh
zypper install oniguruma
For SLE 15 SP3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/SLE_15_SP3/devel:languages:ruby.repo
zypper refresh
zypper install oniguruma
For SLE 15 SP2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/SLE_15_SP2/devel:languages:ruby.repo
zypper refresh
zypper install oniguruma
For SLE 12 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:ruby/SLE_12/devel:languages:ruby.repo
zypper refresh
zypper install oniguruma