The block_parser_class WordPress PHP filter allows plugins to replace the server-side block parser.
Usage
add_filter('block_parser_class', 'your_custom_parser_class');
function your_custom_parser_class($parser_class) {
    // your custom code here
    return $parser_class;
}
Parameters
- $parser_class: string. Name of block parser class.
More information
See WordPress Developer Resources: block_parser_class
Examples
Replacing the default block parser with a custom parser
This example replaces the default block parser with a custom parser called My_Custom_Block_Parser.
add_filter('block_parser_class', 'use_my_custom_block_parser');
function use_my_custom_block_parser($parser_class) {
    return 'My_Custom_Block_Parser';
}
Using a namespaced custom block parser
This example demonstrates how to use a namespaced custom block parser.
add_filter('block_parser_class', 'use_namespaced_custom_parser');
function use_namespaced_custom_parser($parser_class) {
    return 'MyNamespace\\CustomBlockParser';
}
Conditionally replacing the block parser
This example shows how to conditionally replace the block parser based on a certain condition (e.g., a user role).
add_filter('block_parser_class', 'conditionally_replace_block_parser');
function conditionally_replace_block_parser($parser_class) {
    if (current_user_can('edit_posts')) {
        return 'My_Editors_Block_Parser';
    }
    return $parser_class;
}
Replacing the block parser for a specific post type
This example demonstrates how to replace the block parser only for a specific post type.
add_filter('block_parser_class', 'replace_block_parser_for_post_type', 10, 2);
function replace_block_parser_for_post_type($parser_class, $post_type) {
    if ($post_type == 'my_custom_post_type') {
        return 'My_Custom_Post_Type_Block_Parser';
    }
    return $parser_class;
}
Using an external class as the block parser
This example shows how to use an external class from a third-party library as the block parser.
add_filter('block_parser_class', 'use_external_block_parser');
function use_external_block_parser($parser_class) {
    return 'ThirdPartyLibrary\\ExternalBlockParser';
}