WordPress: Implement a Content Parser

example.php

<?php
class content_parser {	
	
	public function __construct() {
		// apply parser to all posts
		add_filter( 'the_content', array( __CLASS__, 'parse' ) );
	}
	
	public static function parse( $text ) {
		global $post;
		
		$text = str_replace( '%DATE%', get_the_date(), $text );
		$text = str_replace( '%TITLE%', get_the_title(), $text );
		$text = str_replace( '%AUTHOR%', get_the_author(), $text );
		$text = str_replace( '%TAGS%', get_the_tag_list(), $text );			
		$text = str_replace( '%PERMALINK%', get_permalink(), $text );
		$text = str_replace( '%COMMENTS%', get_comments_number(), $text );
		$text = str_replace( '%CATEGORIES%', get_the_category_list(), $text );
		
		// tags with custom separator
		$text = preg_replace_callback(
			'/%TAGS\|(.*)%/',
			function ( $matches ) {
				return get_the_tag_list( "", $matches[ 1 ], "" );
			},
			$text
		);
		
		// categories with custom separator
		$text = preg_replace_callback(
			'/%CATEGORIES\|(.*)%/',
			function ( $matches ) {
				return get_the_category_list( $matches[ 1 ] );
			},
			$text
		);
		
		// custom post meta
		$text = preg_replace_callback(
			'/%POST_META\|(.*)%/',
			function ( $matches ) {
				global $post;
				return get_post_meta( $post->ID, $matches[ 1 ], true );
			},
			$text
		);
		
		// custom date format
		$text = preg_replace_callback(
			'/%DATE\|(.*)%/',
			function ( $matches ) {
				return get_the_date( $matches[ 1 ] );
			},
		 	$text
		 );
		
		// custom post thumbnails
		$text = preg_replace_callback(
			'/%POST_THUMBNAIL\|(\d+)x(\d+)%/',
			function ( $matches ) {
				global $post;
				$thumb = get_the_post_thumbnail( $post->ID, array( $matches[ 1 ], $matches[ 2 ] ) );
				if ( ! $thumb ) {
					$default_thumb = get_option( "my_default_thumb" );
					if ( $default_thumb ) {
						$thumb = "<img src=\"$default_thumb\" width=\"" . $matches[ 1 ] . "\" height=\"" . $matches[ 2 ] . "\">";
					}
				}

				return $thumb;
			},
			$text
		);
		
		return $text;
	}
}

// parse any text of your choice
$parsed_text = content_parser::parse( $some_custom_text );

filter.php

<?php
class content_parser {	
	
	/**
	 * Parses text.
	 * 
	 * @uses apply_filters() Calls 'content_parser_after' hook on the parsed text
	 *
	 * @param string $text Text to parse
	 * @return string Parsed text
	 */
	public static function parse( $text ) {
		// ... your parsing code
		return apply_filters( 'content_parser_after', $text );
	}
}

oneline.php

<?php
$text = str_replace( '%DATE%', get_the_date(), $text );

tags.php

<?php
// tags with custom separator
$text = preg_replace_callback(
	'/%TAGS\|(.*)%/',
	function ( $matches ) {
		return get_the_tag_list( "", $matches[ 1 ], "" );
	},
	$text
);

thumb.php

<?php
// custom post thumbnails
$text = preg_replace_callback(
	'/%POST_THUMBNAIL\|(\d+)x(\d+)%/',
	function ( $matches ) {
		global $post;
		return get_the_post_thumbnail( $post->ID, array( $matches[ 1 ], $matches[ 2 ] ) );
	},
	$text
);