diff --git a/wp-includes/blocks.php b/wp-includes/blocks.php
index 2ec3a07e3e..98d8f10d5c 100644
--- a/wp-includes/blocks.php
+++ b/wp-includes/blocks.php
@@ -112,3 +112,40 @@ function get_dynamic_block_names() {
 
 	return $dynamic_block_names;
 }
+
+/**
+ * Parses blocks out of a content string.
+ *
+ * @since 5.0.0
+ *
+ * @param  string $content Post content.
+ * @return array  Array of parsed block objects.
+ */
+function parse_blocks( $content ) {
+	/*
+	 * If there are no blocks in the content, return a single block, rather
+	 * than wasting time trying to parse the string.
+	 */
+	if ( ! has_blocks( $content ) ) {
+		return array(
+			array(
+				'blockName'   => null,
+				'attrs'       => array(),
+				'innerBlocks' => array(),
+				'innerHTML'   => $content,
+			),
+		);
+	}
+
+	/**
+	 * Filter to allow plugins to replace the server-side block parser
+	 *
+	 * @since 5.0.0
+	 *
+	 * @param string $parser_class Name of block parser class
+	 */
+	$parser_class = apply_filters( 'block_parser_class', 'WP_Block_Parser' );
+
+	$parser = new $parser_class();
+	return $parser->parse( $content );
+}
diff --git a/wp-includes/class-wp-block-parser.php b/wp-includes/class-wp-block-parser.php
new file mode 100644
index 0000000000..78b6921787
--- /dev/null
+++ b/wp-includes/class-wp-block-parser.php
@@ -0,0 +1,473 @@
+<?php
+
+/**
+ * Class WP_Block_Parser_Block
+ *
+ * Holds the block structure in memory
+ *
+ * @since 3.8.0
+ */
+class WP_Block_Parser_Block {
+	/**
+	 * Name of block
+	 *
+	 * @example "core/paragraph"
+	 *
+	 * @since 3.8.0
+	 * @var string
+	 */
+	public $blockName;
+
+	/**
+	 * Optional set of attributes from block comment delimiters
+	 *
+	 * @example null
+	 * @example array( 'columns' => 3 )
+	 *
+	 * @since 3.8.0
+	 * @var array|null
+	 */
+	public $attrs;
+
+	/**
+	 * List of inner blocks (of this same class)
+	 *
+	 * @since 3.8.0
+	 * @var WP_Block_Parser_Block[]
+	 */
+	public $innerBlocks;
+
+	/**
+	 * Resultant HTML from inside block comment delimiters
+	 * after removing inner blocks
+	 *
+	 * @example "...Just <!-- wp:test /--> testing..." -> "Just testing..."
+	 *
+	 * @since 3.8.0
+	 * @var string
+	 */
+	public $innerHTML;
+
+	function __construct( $name, $attrs, $innerBlocks, $innerHTML ) {
+		$this->blockName   = $name;
+		$this->attrs       = $attrs;
+		$this->innerBlocks = $innerBlocks;
+		$this->innerHTML   = $innerHTML;
+	}
+}
+
+/**
+ * Class WP_Block_Parser_Frame
+ *
+ * Holds partial blocks in memory while parsing
+ *
+ * @internal
+ * @since 3.8.0
+ */
+class WP_Block_Parser_Frame {
+	/**
+	 * Full or partial block
+	 *
+	 * @since 3.8.0
+	 * @var WP_Block_Parser_Block
+	 */
+	public $block;
+
+	/**
+	 * Byte offset into document for start of parse token
+	 *
+	 * @since 3.8.0
+	 * @var int
+	 */
+	public $token_start;
+
+	/**
+	 * Byte length of entire parse token string
+	 *
+	 * @since 3.8.0
+	 * @var int
+	 */
+	public $token_length;
+
+	/**
+	 * Byte offset into document for after parse token ends
+	 * (used during reconstruction of stack into parse production)
+	 *
+	 * @since 3.8.0
+	 * @var int
+	 */
+	public $prev_offset;
+
+	/**
+	 * Byte offset into document where leading HTML before token starts
+	 *
+	 * @since 3.8.0
+	 * @var int
+	 */
+	public $leading_html_start;
+
+	function __construct( $block, $token_start, $token_length, $prev_offset = null, $leading_html_start = null ) {
+		$this->block              = $block;
+		$this->token_start        = $token_start;
+		$this->token_length       = $token_length;
+		$this->prev_offset        = isset( $prev_offset ) ? $prev_offset : $token_start + $token_length;
+		$this->leading_html_start = $leading_html_start;
+	}
+}
+
+/**
+ * Class WP_Block_Parser
+ *
+ * Parses a document and constructs a list of parsed block objects
+ *
+ * @since 3.8.0
+ * @since 4.0.0 returns arrays not objects, all attributes are arrays
+ */
+class WP_Block_Parser {
+	/**
+	 * Input document being parsed
+	 *
+	 * @example "Pre-text\n<!-- wp:paragraph -->This is inside a block!<!-- /wp:paragraph -->"
+	 *
+	 * @since 3.8.0
+	 * @var string
+	 */
+	public $document;
+
+	/**
+	 * Tracks parsing progress through document
+	 *
+	 * @since 3.8.0
+	 * @var int
+	 */
+	public $offset;
+
+	/**
+	 * List of parsed blocks
+	 *
+	 * @since 3.8.0
+	 * @var WP_Block_Parser_Block[]
+	 */
+	public $output;
+
+	/**
+	 * Stack of partially-parsed structures in memory during parse
+	 *
+	 * @since 3.8.0
+	 * @var WP_Block_Parser_Frame[]
+	 */
+	public $stack;
+
+	/**
+	 * Parses a document and returns a list of block structures
+	 *
+	 * When encountering an invalid parse will return a best-effort
+	 * parse. In contrast to the specification parser this does not
+	 * return an error on invalid inputs.
+	 *
+	 * @since 3.8.0
+	 *
+	 * @param string $document
+	 * @return WP_Block_Parser_Block[]
+	 */
+	function parse( $document ) {
+		$this->document = $document;
+		$this->offset   = 0;
+		$this->output   = array();
+		$this->stack    = array();
+
+		do {
+			// twiddle our thumbs
+		} while ( $this->proceed() );
+
+		return $this->output;
+	}
+
+	/**
+	 * Processes the next token from the input document
+	 * and returns whether to proceed eating more tokens
+	 *
+	 * This is the "next step" function that essentially
+	 * takes a token as its input and decides what to do
+	 * with that token before descending deeper into a
+	 * nested block tree or continuing along the document
+	 * or breaking out of a level of nesting.
+	 *
+	 * @internal
+	 * @since 3.8.0
+	 * @return bool
+	 */
+	function proceed() {
+		$next_token = $this->next_token();
+		list( $token_type, $block_name, $attrs, $start_offset, $token_length ) = $next_token;
+		$stack_depth = count( $this->stack );
+
+		// we may have some HTML soup before the next block
+		$leading_html_start = $start_offset > $this->offset ? $this->offset : null;
+
+		switch ( $token_type ) {
+			case 'no-more-tokens':
+				// if not in a block then flush output
+				if ( 0 === $stack_depth ) {
+					$this->add_freeform();
+					return false;
+				}
+
+				/*
+				 * Otherwise we have a problem
+				 * This is an error
+				 *
+				 * we have options
+				 * - treat it all as freeform text
+				 * - assume an implicit closer (easiest when not nesting)
+				 */
+
+				// for the easy case we'll assume an implicit closer
+				if ( 1 === $stack_depth ) {
+					$this->add_block_from_stack();
+					return false;
+				}
+
+				/*
+				 * for the nested case where it's more difficult we'll
+				 * have to assume that multiple closers are missing
+				 * and so we'll collapse the whole stack piecewise
+				 */
+				while ( 0 < count( $this->stack ) ) {
+					$this->add_block_from_stack();
+				}
+				return false;
+
+			case 'void-block':
+				/*
+				 * easy case is if we stumbled upon a void block
+				 * in the top-level of the document
+				 */
+				if ( 0 === $stack_depth ) {
+					if ( isset( $leading_html_start ) ) {
+						$this->output[] = (array) self::freeform( substr(
+							$this->document,
+							$leading_html_start,
+							$start_offset - $leading_html_start
+						) );
+					}
+
+					$this->output[] = (array) new WP_Block_Parser_Block( $block_name, $attrs, array(), '' );
+					$this->offset = $start_offset + $token_length;
+					return true;
+				}
+
+				// otherwise we found an inner block
+				$this->add_inner_block(
+					new WP_Block_Parser_Block( $block_name, $attrs, array(), '' ),
+					$start_offset,
+					$token_length
+				);
+				$this->offset = $start_offset + $token_length;
+				return true;
+
+			case 'block-opener':
+				// track all newly-opened blocks on the stack
+				array_push( $this->stack, new WP_Block_Parser_Frame(
+					new WP_Block_Parser_Block( $block_name, $attrs, array(), '' ),
+					$start_offset,
+					$token_length,
+					$start_offset + $token_length,
+					$leading_html_start
+				) );
+				$this->offset = $start_offset + $token_length;
+				return true;
+
+			case 'block-closer':
+				/*
+				 * if we're missing an opener we're in trouble
+				 * This is an error
+				 */
+				if ( 0 === $stack_depth ) {
+					/*
+					 * we have options
+					 * - assume an implicit opener
+					 * - assume _this_ is the opener
+					 * - give up and close out the document
+					 */
+					$this->add_freeform();
+					return false;
+				}
+
+				// if we're not nesting then this is easy - close the block
+				if ( 1 === $stack_depth ) {
+					$this->add_block_from_stack( $start_offset );
+					$this->offset = $start_offset + $token_length;
+					return true;
+				}
+
+				/*
+				 * otherwise we're nested and we have to close out the current
+				 * block and add it as a new innerBlock to the parent
+				 */
+				$stack_top = array_pop( $this->stack );
+				$stack_top->block->innerHTML .= substr( $this->document, $stack_top->prev_offset, $start_offset - $stack_top->prev_offset );
+				$stack_top->prev_offset = $start_offset + $token_length;
+
+				$this->add_inner_block(
+					$stack_top->block,
+					$stack_top->token_start,
+					$stack_top->token_length,
+					$start_offset + $token_length
+				);
+				$this->offset = $start_offset + $token_length;
+				return true;
+
+			default:
+				// This is an error
+				$this->add_freeform();
+				return false;
+		}
+	}
+
+	/**
+	 * Scans the document from where we last left off
+	 * and finds the next valid token to parse if it exists
+	 *
+	 * Returns the type of the find: kind of find, block information, attributes
+	 *
+	 * @internal
+	 * @since 3.8.0
+	 * @return array
+	 */
+	function next_token() {
+		$matches = null;
+
+		/*
+		 * aye the magic
+		 * we're using a single RegExp to tokenize the block comment delimiters
+		 * we're also using a trick here because the only difference between a
+		 * block opener and a block closer is the leading `/` before `wp:` (and
+		 * a closer has no attributes). we can trap them both and process the
+		 * match back in PHP to see which one it was.
+		 */
+		$has_match = preg_match(
+			'/<!--\s+(?<closer>\/)?wp:(?<namespace>[a-z][a-z0-9_-]*\/)?(?<name>[a-z][a-z0-9_-]*)\s+(?<attrs>{(?:(?!}\s+-->).)+?}\s+)?(?<void>\/)?-->/s',
+			$this->document,
+			$matches,
+			PREG_OFFSET_CAPTURE,
+			$this->offset
+		);
+
+		// we have no more tokens
+		if ( 0 === $has_match ) {
+			return array( 'no-more-tokens', null, null, null, null );
+		}
+
+		list( $match, $started_at ) = $matches[ 0 ];
+
+		$length    = strlen( $match );
+		$is_closer = isset( $matches[ 'closer' ] ) && -1 !== $matches[ 'closer' ][ 1 ];
+		$is_void   = isset( $matches[ 'void' ] ) && -1 !== $matches[ 'void' ][ 1 ];
+		$namespace = $matches[ 'namespace' ];
+		$namespace = ( isset( $namespace ) && -1 !== $namespace[ 1 ] ) ? $namespace[ 0 ] : 'core/';
+		$name      = $namespace . $matches[ 'name' ][ 0 ];
+		$has_attrs = isset( $matches[ 'attrs' ] ) && -1 !== $matches[ 'attrs' ][ 1 ];
+
+		/*
+		 * Fun fact! It's not trivial in PHP to create "an empty associative array" since all arrays
+		 * are associative arrays. If we use `array()` we get a JSON `[]`
+		 */
+		$attrs = $has_attrs
+			? json_decode( $matches[ 'attrs' ][ 0 ], /* as-associative */ true )
+			: json_decode( '{}', /* don't ask why, just verify in PHP */ false );
+
+		/*
+		 * This state isn't allowed
+		 * This is an error
+		 */
+		if ( $is_closer && ( $is_void || $has_attrs ) ) {
+			// we can ignore them since they don't hurt anything
+		}
+
+		if ( $is_void ) {
+			return array( 'void-block', $name, $attrs, $started_at, $length );
+		}
+
+		if ( $is_closer ) {
+			return array( 'block-closer', $name, null, $started_at, $length );
+		}
+
+		return array( 'block-opener', $name, $attrs, $started_at, $length );
+	}
+
+	/**
+	 * Returns a new block object for freeform HTML
+	 *
+	 * @internal
+	 * @since 3.9.0
+	 *
+	 * @param string $innerHTML HTML content of block
+	 * @return WP_Block_Parser_Block freeform block object
+	 */
+	static function freeform( $innerHTML ) {
+		return new WP_Block_Parser_Block( null, array(), array(), $innerHTML );
+	}
+
+	/**
+	 * Pushes a length of text from the input document
+	 * to the output list as a freeform block
+	 *
+	 * @internal
+	 * @since 3.8.0
+	 * @param null $length how many bytes of document text to output
+	 */
+	function add_freeform( $length = null ) {
+		$length = $length ? $length : strlen( $this->document ) - $this->offset;
+
+		if ( 0 === $length ) {
+			return;
+		}
+
+		$this->output[] = (array) self::freeform( substr( $this->document, $this->offset, $length ) );
+	}
+
+	/**
+	 * Given a block structure from memory pushes
+	 * a new block to the output list
+	 *
+	 * @internal
+	 * @since 3.8.0
+	 * @param WP_Block_Parser_Block $block the block to add to the output
+	 * @param int $token_start byte offset into the document where the first token for the block starts
+	 * @param int $token_length byte length of entire block from start of opening token to end of closing token
+	 * @param int|null $last_offset last byte offset into document if continuing form earlier output
+	 */
+	function add_inner_block( WP_Block_Parser_Block $block, $token_start, $token_length, $last_offset = null ) {
+		$parent = $this->stack[ count( $this->stack ) - 1 ];
+		$parent->block->innerBlocks[] = $block;
+		$parent->block->innerHTML .= substr( $this->document, $parent->prev_offset, $token_start - $parent->prev_offset );
+		$parent->prev_offset = $last_offset ? $last_offset : $token_start + $token_length;
+	}
+
+	/**
+	 * Pushes the top block from the parsing stack to the output list
+	 *
+	 * @internal
+	 * @since 3.8.0
+	 * @param int|null $end_offset byte offset into document for where we should stop sending text output as HTML
+	 */
+	function add_block_from_stack( $end_offset = null ) {
+		$stack_top   = array_pop( $this->stack );
+		$prev_offset = $stack_top->prev_offset;
+
+		$stack_top->block->innerHTML .= isset( $end_offset )
+			? substr( $this->document, $prev_offset, $end_offset - $prev_offset )
+			: substr( $this->document, $prev_offset );
+
+		if ( isset( $stack_top->leading_html_start ) ) {
+			$this->output[] = (array) self::freeform( substr(
+				$this->document,
+				$stack_top->leading_html_start,
+				$stack_top->token_start - $stack_top->leading_html_start
+			) );
+		}
+
+		$this->output[] = (array) $stack_top->block;
+	}
+}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index fcc845d3f4..00e26a52e0 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44115';
+$wp_version = '5.1-alpha-44116';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-settings.php b/wp-settings.php
index 4ae30df9ca..fbd231d45a 100644
--- a/wp-settings.php
+++ b/wp-settings.php
@@ -246,6 +246,7 @@ require( ABSPATH . WPINC . '/rest-api/search/class-wp-rest-search-handler.php' )
 require( ABSPATH . WPINC . '/rest-api/search/class-wp-rest-post-search-handler.php' );
 require( ABSPATH . WPINC . '/class-wp-block-type.php' );
 require( ABSPATH . WPINC . '/class-wp-block-type-registry.php' );
+require( ABSPATH . WPINC . '/class-wp-block-parser.php' );
 require( ABSPATH . WPINC . '/blocks.php' );
 
 $GLOBALS['wp_embed'] = new WP_Embed();