wordpress the_content with teaser (more tag), add to functions.php under wordpress theme

article-teaser.php

<?php

add_filter( 'the_content', function( $content ) {
  $content = preg_replace_callback( "@(^.*)(<p>\s*(<span\s*id=\"more-\d+\">\s*</span>|<!--\s*more\s*-->)\s*</p>)@s", function ( $m ) {
    return '<div class="article-teaser">' . $m[1] . '</div><!--/.article-teaser-->' . PHP_EOL . $m[2];
  }, $content );

  return $content;
});

article-teaser2.php

<?php
  
  add_filter( 'the_content', function( $content ) {
		if ( preg_match( '@^(?<begin>.*)(?<more><!--\s?more(.*?)?-->|<span\s*id="more-\d+">\s*</span>)@s', $content, $m ) ) {
			$more      = $m['more'];
			$ret       = $m['begin'];
			$tag_stack = [];
			$offset    = 0;

			while ( preg_match( '@<(?<ending>/)?(?<tag>\w+)(?<attr>\s[^>]+)?>@', $m['begin'], $m2, PREG_OFFSET_CAPTURE, $offset ) ) {
				$offset = $m2[0][1] + 1;
				if ( $m2['ending'][0] ) {
					array_pop( $tag_stack );
				} else {
					$tag = $m2[0][0];
					// remove the id.
					$tag         = preg_replace( '@\s+id="[^"]+"@', '', $tag );
					$tag         = preg_replace( "@\s+id='[^']+'@", '', $tag );
					$tag_stack[] = $tag;
				}
			}
			$tag_stack2 = array_reverse( $tag_stack );

			foreach ( $tag_stack2 as $tag ) {
				$ret .= str_replace( '<', '</', $tag );
			}
			$ret = '<div class="article-teaser">' . PHP_EOL . $ret . PHP_EOL . '</div><!--/.article-teaser-->' . PHP_EOL . $more . PHP_EOL;
			foreach ( $tag_stack as $tag ) {
				$ret .= $tag;
			}
			$ret     .= substr( $content, strlen( $m[0] ) );
			$content = $ret;
		}

		return $content;
	});