Drupal 8 Webform : Regular expression in conditions to check specific day of the week
I doubt that you can achieve this without some JS involved.

You can use JS RegExp.

You could try Form API #states and try to hide the desired select field if some other field's value matches your conditions.

$form['my_select_element'] = [
  '#type' =&gt; 'select',
  '#states' =&gt; [
    // Hide this field when the 'weekday' radio has 'tuesday' or 'thursday' selected.
    'invisible' =&gt; [
      ':input[name="field_weekday[und]"]' =&gt; ['value' =&gt; 'tuesday'],
      'or',
      ':input[name="field_weekday[und]"]' =&gt; ['value' =&gt; 'thursday'],
    ],
  ],
];


(#states with multiple conditions, Source.)