re = /(<li><p><a href="https:\/\/stat.ethz.ch\/(m.*r-sig-)(.*)">.*)/
str = '
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-mac"><code>R-SIG-Mac</code></a>: R Special Interest Group on Mac ports of R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-db"><code>R-SIG-DB</code></a>: R SIG on Database Interfaces</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-debian"><code>R-SIG-Debian</code></a>: R Special Interest Group for Debian ports of R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-dynamic-models"><code>R-SIG-dynamic-models</code></a>: Special Interest Group for Dynamic Simulation Models in R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-ecology"><code>R-SIG-ecology</code></a>: Using R in ecological data analysis</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-epi"><code>R-SIG-Epi</code></a>: R for epidemiological data analysis</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-fedora"><code>R-SIG-Fedora</code></a>: R Special Interest Group for Fedora and Redhat ports of R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-finance"><code>R-SIG-Finance</code></a>: Special Interest Group for ‘R in Finance’</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-geo"><code>R-SIG-Geo</code></a>: R Special Interest Group on using Geographical data and Mapping</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-gr"><code>R-SIG-gR</code></a>: R SIG on gRaphical models</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-gui"><code>R-SIG-GUI</code></a>: R Special Interest Group on GUI Development</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-hpc"><code>R-SIG-HPC</code></a>: R SIG on High-Performance Computing</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-insurance"><code>R-SIG-Insurance</code></a>: Special Interest Group on using R in actuarial science and insurance</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-jobs"><code>R-SIG-Jobs</code></a>: R SIG List for Announcements of Jobs where R is used</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-mediawiki"><code>R-SIG-mediawiki</code></a>: R SIG on the R Extension for Mediawiki</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models"><code>R-SIG-mixed-models</code></a>: R SIG on Mixed Effect Models, notably lmer() related</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-networks"><code>R-SIG-networks</code></a>: R SIG for users and developers of network- or graph-related software within R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-phylo"><code>R-SIG-phylo</code></a>: R SIG on phylogenetic and comparative methods and analyses</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-qa"><code>R-SIG-QA</code></a>: R SIG on Quality Assurance & Validation</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-robust"><code>R-SIG-Robust</code></a>: R SIG on Robust Statistics</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-teaching"><code>R-SIG-teaching</code></a>: SIG on Teaching Statistics (and more) using R</p></li>
<li><p><a href="https://stat.ethz.ch/mailman/listinfo/r-sig-wiki"><code>R-SIG-Wiki</code></a>: SIG on the Development of an “R Wiki”</p></li>
</ul>'
subst = ''
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html