Automatically add the Bootstrap 4 "img-fluid" class to any uploaded image through the WordPress dashboard. Add to "functions.php"

bs4-image-fluid-class-auto-wp.php

<?php

//Wordpress Fluid Images Bootstrap 4.0.0-alpha.4
function bootstrap_fluid_images( $html ){
  $classes = 'img-fluid'; // Bootstrap 4.0.0-alpha.4
  // check if there are already classes assigned to the anchor
  if ( preg_match('/<img.*? class="/', $html) ) {
    $html = preg_replace('/(<img.*? class=".*?)(".*?\/>)/', '$1 ' . $classes . ' $2', $html);
  } else {
    $html = preg_replace('/(<img.*?)(\/>)/', '$1 class="' . $classes . '" $2', $html);
  }
  // remove dimensions from images,, does not need it!
  $html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );
  return $html;
}
add_filter( 'the_content','bootstrap_fluid_images',10 );
add_filter( 'post_thumbnail_html', 'bootstrap_fluid_images', 10 );