const pattern = /<\s*tag\b[^>]*?\battribute1\s*=\s*"([^"]*)"[^>]*>/g;

// When the document is ready, attach the event handlers.
document.addEventListener("DOMContentLoaded", function () {
  const input = document.getElementById("input");
  const searchButton = document.getElementById("search");
 
  searchButton.addEventListener('click', (event) => {
    console.log(
      [...input.value.matchAll(pattern)]
    );
    
    event.preventDefault();
    return false;
  });
});

*, *::before, *::after {
  box-sizing: border-box;
}

form {
  /* Display the form in two columns. */
  display: flex;
  flex-direction: row;
  column-gap: 1em;
}

form > div:first-child {
  width: calc(100% - 6em); /* 100% - gap - right column. */
  min-width: 20em; /* Enough for the textarea content. */
}

form > div:last-child {
  width: 5em; /* Enough for the button. */
}

textarea {
  width: 100%; /* Override cols="80". */
  font-size: .85em;
}

.as-console-row-code {
  font-size: .85em !important; /* Instead of 13px. */
}

<form action="#">
  <div>
    <textarea name="input" id="input"
              cols="80" rows="13"
              placeholder="Enter your HTML here">&lt;tag attribute1=&quot;'text123'&quot; /&gt;
&lt;tag attribute2=&quot;'true'&quot; 
     attribute1=&quot;'text1'&quot;/&gt;
&lt;tag attribute2=&quot;'true'&quot;/&gt;
&lt;!-- This one should not match --&gt;
&lt;tagger attribute1=&quot;something&quot;&gt;
   &lt;!-- But this one yes --&gt;
   &lt;tag data-id=&quot;6735&quot;
        attribute1=&quot;'some more text'&quot;
        required &gt;
      Whatever in here
   &lt;/tag&gt;
&lt;/tagger&gt;</textarea>
  </div>
  <div>
    <button id="search"
       title="Search for &lt;tag ... attribute1 ...&gt;">
       Search
    </button>
  </div>
</form>