Automatically Bold (or Style) the First X Words of Content

lede-words.php

<?php
/**
 * Lede words
 * 
 */
add_filter( 'the_content', 'ac_the_content_lede_words' );

function ac_the_content_lede_words( $content ) {

	// Get number of lede words
	$lede_words = (int) get_post_meta( get_the_ID(), '_lede_words', true ); // Create metabox for this or manually save custom field value, and adjust key if necessary

	// Bail: Zero lede words
	if ( 0 === $lede_words ) {
		return $content;
	}
	
	// Include trailing comma
	$trailing_comma = true; // Set false to exclude trailing comma from lede words
	$comma = ( false === $trailing_comma ) ? ',' : '';

	// Expressions
	$first = $second = $third = '';

	switch ( $lede_words ) {
		case 1:
			$first  = '[^\s<' . $comma . ']+';
			break;
		case 2:
			$first  = '[^\s<]+';
			$third  = '\s+[^\s' . $comma . ']+';
			break;
		case 3:	
		default:
			$first  = '[^\s<]+';
			$second = '\s+[^\s]+';
			$third  = '\s+[^\s' . $comma . ']+';
	}

	// More than three lede words
	while ( $lede_words > 3 ) {
		// Repeat the second expression
		$second .= '\s+[^\s]+';
		$lede_words--;
	}

	// Replace
	$content = preg_replace( '/<p>(' . $first . $second . $third . ')/', '<p><span class="lede-words">$1</span>', $content, 1 );

	return $content;

}