Extract first X sentences from WordPress post content for post excerpt

split_text_to_sentences.php

<?php
	/**
	 * Extract out first X sentences from uncleaned raw text
	 * @param string $text Raw text
	 * @param int $num_sentences Optional. Number of sentences
	 * @return string
	 */
	function extract_sentences_from_text($raw_text, $num_sentences=3) {
		$text = $raw_text;
		$text = strip_tags($text);
		
		if(function_exists("strip_shortcodes")) {
			$text = strip_shortcodes($text);
		}
		
		$text = preg_replace("#https?\://([^\s*]+)#si", " ", $text);
		$text = str_replace("\r", "", $text);
		$text = str_replace("\n", " ", $text);
		$text = preg_replace("#\s{2,}#si", " ", $text);
		$text = trim($text);
		
		if("" === $text) {
			return $text;
		}
		
		$lines = preg_split("#(\"?(?:\.|\!|\?)\"?\s+\"?[A-Z])#si", $text, null, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
		
		if(!empty($lines)) {
			$raw_sentences = array();
			$last_letter = "";
			
			foreach($lines as $key => $line) {
				if(($key % 2) == 1) {
					$raw_sentences[] = $last_letter . $lines[ ($key - 1) ] . trim(substr($line, 0, -1));
					
					$last_letter = substr($line, -1);
				}
				
				if(count($raw_sentences) >= $num_sentences) {
					break;
				}
			}
			
			if(!empty($raw_sentences)) {
				$sentences = array_slice($raw_sentences, 0, $num_sentences);
				
				return implode(" ", $sentences);
			}
		}
		
		return "";
	}
	
	// excerpt filter
	add_filter('get_the_excerpt', function($excerpt) {
		global $wpdb;
		
		if(get_the_ID() && function_exists("extract_sentences_from_text") && ("" !== ($first_sentences = extract_sentences_from_text(get_the_content(), 3)))) {
			return $first_sentences;
		}
		
		return $excerpt;
	}, 1, 1);