Salesforce Process Builder: Generate formula code for matching multiple values

pb.html

<form>
  <label for="fieldName">Field name (include object):</label>
  <input id="fieldName" name="fieldName" type="text" size="100" onkeydown="if (event.keyCode == 13) false">
</form>

<p>
  <input type="checkbox" id="isPicklist" name="isPicklist" value="picklist">
  <label for="isPicklist">Picklist?</label>
  <input type="checkbox" id="isNot" name="isNot" value="not">
  <label for="isNot">NOT?</label>
</p>

<label for="matchingValues">Comma-separated list of values to match (a single space is allowed after each comma):</label>
<textarea id="matchingValues" name="matchingValues" rows="4" cols="80">Foo,Bar,Baz</textarea>

<p id="createBtn" style="color: blue; text-decoration: underline">Create</p>

<p id="output"></p>

<p id="clearBtn" style="color: blue; text-decoration: underline">Clear</p>

pb.js

var createBtn = document.getElementById("createBtn");
var clearBtn = document.getElementById("clearBtn");

// Defaults
document.getElementById("fieldName").value = "[Contact].SomeField__c";

// Handlers
createBtn.onclick = function() {
  var fieldName = document.getElementById("fieldName");
	var isPicklist = document.getElementById("isPicklist");
  var isNot = document.getElementById("isNot");
  var matchingValues = document.getElementById("matchingValues");

	var fname = isPicklist.checked ?
  	"TEXT(" + fieldName.value + ")" :
    fieldName.value;
    
  var regexOr = matchingValues.value.trim().replace(/,\s?/g,"|");
  
  // Return a string "FALSE" because the Process Builder needs to make a comparison to the target field name's value, which will be a string (or a picklist coerced to string via TEXT). If we use a boolean here, process builder will reject it as it won't compare a boolean to a string. Note that if a literal "FALSE" is a valid option for your field, then you will need to change FALSE below to something else.
  var result = isNot.checked ?
      ('"FALSE", ' + fname) :
      (fname + ', "FALSE"');
  
  var formula = "IF(" +
    "REGEX(" + fname + ', "' + regexOr + '"), ' +
    result +
    ")";

	document.getElementById("output").textContent = formula;
}

clearBtn.onclick = function() {
	document.getElementById("fieldName").value = "";
  document.getElementById("isPicklist").checked = false;
  
	["matchingValues", "output"].forEach(function (id) {
  	document.getElementById(id).textContent = "";
  });
}

Also, note that Process Builder still has SOQL limits that you can run into, which it apparently does not optimize for. My process was complex enough that I had to just write Apex code anyway. Yay Salesforce!