title: promql regex with label_replace returns blank value when regex contain \w
body: <p>I have this metric being returned</p>
  <pre><code>my_sqldatabase{aggregation=&quot;maximum&quot;, instance=&quot;sql-metrics-exporter&quot;, interval=&quot;PT5M&quot;, job=&quot;metrics-sqldatabase&quot;, id=&quot;/sub/1291-3432432-e432/mydatabase&quot;}
  </code></pre>
  <p>I want to extract the last bit after / (i.e. mydatabase) for a new label called db and have tried the below</p>
  <pre><code>label_replace(my_sqldatabase{metric=&quot;storage_percent&quot;, aggregation=&quot;maximum&quot;}, &quot;db&quot;, &quot;$1&quot;, &quot;id&quot;, &quot;([\\w]+$)&quot;)
  </code></pre>
  <p>I always end up with a blank value db like so or an error</p>
  <pre><code>my_sqldatabase{aggregation=&quot;maximum&quot;, db=&quot;&quot; instance=&quot;sql-metrics-exporter&quot;, interval=&quot;PT5M&quot;, job=&quot;metrics-sqldatabase&quot;, id=&quot;/sub/1291-3432432-e432/mydatabase&quot;}
  </code></pre>
  <p>Any ideas where I'm going wrong? I've tried, ([\w]+$), [\w]+$, [\w]+$, ([\\w]+$) and either get an error or blank value. It seems to be <code>\</code> that's causing the issue but I can't figure out why or how to get around it.</p>