Self-contained copy of wpautop() to include in migration scripts etc.

wpautop.inc.php

<?php

function _autop_newline_preservation_helper( $matches ) {
    return str_replace( "\n", '<WPPreserveNewline />', $matches[0] );
}

function get_html_split_regex() {
    static $regex;

    if ( ! isset( $regex ) ) {
        // phpcs:disable Squiz.Strings.ConcatenationSpacing.PaddingFound -- don't remove regex indentation
        $comments =
            '!'             // Start of comment, after the <.
            . '(?:'         // Unroll the loop: Consume everything until --> is found.
            .     '-(?!->)' // Dash not followed by end of comment.
            .     '[^\-]*+' // Consume non-dashes.
            . ')*+'         // Loop possessively.
            . '(?:-->)?';   // End of comment. If not found, match all input.

        $cdata =
            '!\[CDATA\['    // Start of comment, after the <.
            . '[^\]]*+'     // Consume non-].
            . '(?:'         // Unroll the loop: Consume everything until ]]> is found.
            .     '](?!]>)' // One ] not followed by end of comment.
            .     '[^\]]*+' // Consume non-].
            . ')*+'         // Loop possessively.
            . '(?:]]>)?';   // End of comment. If not found, match all input.

        $escaped =
            '(?='             // Is the element escaped?
            .    '!--'
            . '|'
            .    '!\[CDATA\['
            . ')'
            . '(?(?=!-)'      // If yes, which type?
            .     $comments
            . '|'
            .     $cdata
            . ')';

        $regex =
            '/('                // Capture the entire match.
            .     '<'           // Find start of element.
            .     '(?'          // Conditional expression follows.
            .         $escaped  // Find end of escaped element.
            .     '|'           // ...else...
            .         '[^>]*>?' // Find end of normal element.
            .     ')'
            . ')/';
        // phpcs:enable
    }

    return $regex;
}

function wp_html_split( $input ) {
    return preg_split( get_html_split_regex(), $input, -1, PREG_SPLIT_DELIM_CAPTURE );
}

function wp_replace_in_html_tags( $haystack, $replace_pairs ) {
    // Find all elements.
    $textarr = wp_html_split( $haystack );
    $changed = false;

    // Optimize when searching for one item.
    if ( 1 === count( $replace_pairs ) ) {
        // Extract $needle and $replace.
        foreach ( $replace_pairs as $needle => $replace ) {
        }

        // Loop through delimiters (elements) only.
        for ( $i = 1, $c = count( $textarr ); $i < $c; $i += 2 ) {
            if ( false !== strpos( $textarr[ $i ], $needle ) ) {
                $textarr[ $i ] = str_replace( $needle, $replace, $textarr[ $i ] );
                $changed       = true;
            }
        }
    } else {
        // Extract all $needles.
        $needles = array_keys( $replace_pairs );

        // Loop through delimiters (elements) only.
        for ( $i = 1, $c = count( $textarr ); $i < $c; $i += 2 ) {
            foreach ( $needles as $needle ) {
                if ( false !== strpos( $textarr[ $i ], $needle ) ) {
                    $textarr[ $i ] = strtr( $textarr[ $i ], $replace_pairs );
                    $changed       = true;
                    // After one strtr() break out of the foreach loop and look at next element.
                    break;
                }
            }
        }
    }

    if ( $changed ) {
        $haystack = implode( $textarr );
    }

    return $haystack;
}

function wpautop( $text, $br = true ) {
    $pre_tags = array();

    if ( trim( $text ) === '' ) {
        return '';
    }

    // Just to make things a little easier, pad the end.
    $text = $text . "\n";

    /*
     * Pre tags shouldn't be touched by autop.
     * Replace pre tags with placeholders and bring them back after autop.
     */
    if ( strpos( $text, '<pre' ) !== false ) {
        $text_parts = explode( '</pre>', $text );
        $last_part  = array_pop( $text_parts );
        $text       = '';
        $i          = 0;

        foreach ( $text_parts as $text_part ) {
            $start = strpos( $text_part, '<pre' );

            // Malformed HTML?
            if ( false === $start ) {
                $text .= $text_part;
                continue;
            }

            $name              = "<pre wp-pre-tag-$i></pre>";
            $pre_tags[ $name ] = substr( $text_part, $start ) . '</pre>';

            $text .= substr( $text_part, 0, $start ) . $name;
            $i++;
        }

        $text .= $last_part;
    }
    // Change multiple <br>'s into two line breaks, which will turn into paragraphs.
    $text = preg_replace( '|<br\s*/?>\s*<br\s*/?>|', "\n\n", $text );

    $allblocks = '(?:table|thead|tfoot|caption|col|colgroup|tbody|tr|td|th|div|dl|dd|dt|ul|ol|li|pre|form|map|area|blockquote|address|math|style|p|h[1-6]|hr|fieldset|legend|section|article|aside|hgroup|header|footer|nav|figure|figcaption|details|menu|summary)';

    // Add a double line break above block-level opening tags.
    $text = preg_replace( '!(<' . $allblocks . '[\s/>])!', "\n\n$1", $text );

    // Add a double line break below block-level closing tags.
    $text = preg_replace( '!(</' . $allblocks . '>)!', "$1\n\n", $text );

    // Add a double line break after hr tags, which are self closing.
    $text = preg_replace( '!(<hr\s*?/?>)!', "$1\n\n", $text );

    // Standardize newline characters to "\n".
    $text = str_replace( array( "\r\n", "\r" ), "\n", $text );

    // Find newlines in all elements and add placeholders.
    $text = wp_replace_in_html_tags( $text, array( "\n" => ' <!-- wpnl --> ' ) );

    // Collapse line breaks before and after <option> elements so they don't get autop'd.
    if ( strpos( $text, '<option' ) !== false ) {
        $text = preg_replace( '|\s*<option|', '<option', $text );
        $text = preg_replace( '|</option>\s*|', '</option>', $text );
    }

    /*
     * Collapse line breaks inside <object> elements, before <param> and <embed> elements
     * so they don't get autop'd.
     */
    if ( strpos( $text, '</object>' ) !== false ) {
        $text = preg_replace( '|(<object[^>]*>)\s*|', '$1', $text );
        $text = preg_replace( '|\s*</object>|', '</object>', $text );
        $text = preg_replace( '%\s*(</?(?:param|embed)[^>]*>)\s*%', '$1', $text );
    }

    /*
     * Collapse line breaks inside <audio> and <video> elements,
     * before and after <source> and <track> elements.
     */
    if ( strpos( $text, '<source' ) !== false || strpos( $text, '<track' ) !== false ) {
        $text = preg_replace( '%([<\[](?:audio|video)[^>\]]*[>\]])\s*%', '$1', $text );
        $text = preg_replace( '%\s*([<\[]/(?:audio|video)[>\]])%', '$1', $text );
        $text = preg_replace( '%\s*(<(?:source|track)[^>]*>)\s*%', '$1', $text );
    }

    // Collapse line breaks before and after <figcaption> elements.
    if ( strpos( $text, '<figcaption' ) !== false ) {
        $text = preg_replace( '|\s*(<figcaption[^>]*>)|', '$1', $text );
        $text = preg_replace( '|</figcaption>\s*|', '</figcaption>', $text );
    }

    // Remove more than two contiguous line breaks.
    $text = preg_replace( "/\n\n+/", "\n\n", $text );

    // Split up the contents into an array of strings, separated by double line breaks.
    $paragraphs = preg_split( '/\n\s*\n/', $text, -1, PREG_SPLIT_NO_EMPTY );

    // Reset $text prior to rebuilding.
    $text = '';

    // Rebuild the content as a string, wrapping every bit with a <p>.
    foreach ( $paragraphs as $paragraph ) {
        $text .= '<p>' . trim( $paragraph, "\n" ) . "</p>\n";
    }

    // Under certain strange conditions it could create a P of entirely whitespace.
    $text = preg_replace( '|<p>\s*</p>|', '', $text );

    // Add a closing <p> inside <div>, <address>, or <form> tag if missing.
    $text = preg_replace( '!<p>([^<]+)</(div|address|form)>!', '<p>$1</p></$2>', $text );

    // If an opening or closing block element tag is wrapped in a <p>, unwrap it.
    $text = preg_replace( '!<p>\s*(</?' . $allblocks . '[^>]*>)\s*</p>!', '$1', $text );

    // In some cases <li> may get wrapped in <p>, fix them.
    $text = preg_replace( '|<p>(<li.+?)</p>|', '$1', $text );

    // If a <blockquote> is wrapped with a <p>, move it inside the <blockquote>.
    $text = preg_replace( '|<p><blockquote([^>]*)>|i', '<blockquote$1><p>', $text );
    $text = str_replace( '</blockquote></p>', '</p></blockquote>', $text );

    // If an opening or closing block element tag is preceded by an opening <p> tag, remove it.
    $text = preg_replace( '!<p>\s*(</?' . $allblocks . '[^>]*>)!', '$1', $text );

    // If an opening or closing block element tag is followed by a closing <p> tag, remove it.
    $text = preg_replace( '!(</?' . $allblocks . '[^>]*>)\s*</p>!', '$1', $text );

    // Optionally insert line breaks.
    if ( $br ) {
        // Replace newlines that shouldn't be touched with a placeholder.
        $text = preg_replace_callback( '/<(script|style|svg).*?<\/\\1>/s', '_autop_newline_preservation_helper', $text );

        // Normalize <br>
        $text = str_replace( array( '<br>', '<br/>' ), '<br />', $text );

        // Replace any new line characters that aren't preceded by a <br /> with a <br />.
        $text = preg_replace( '|(?<!<br />)\s*\n|', "<br />\n", $text );

        // Replace newline placeholders with newlines.
        $text = str_replace( '<WPPreserveNewline />', "\n", $text );
    }

    // If a <br /> tag is after an opening or closing block tag, remove it.
    $text = preg_replace( '!(</?' . $allblocks . '[^>]*>)\s*<br />!', '$1', $text );

    // If a <br /> tag is before a subset of opening or closing block tags, remove it.
    $text = preg_replace( '!<br />(\s*</?(?:p|li|div|dl|dd|dt|th|pre|td|ul|ol)[^>]*>)!', '$1', $text );
    $text = preg_replace( "|\n</p>$|", '</p>', $text );

    // Replace placeholder <pre> tags with their original content.
    if ( ! empty( $pre_tags ) ) {
        $text = str_replace( array_keys( $pre_tags ), array_values( $pre_tags ), $text );
    }

    // Restore newlines in all elements.
    if ( false !== strpos( $text, '<!-- wpnl -->' ) ) {
        $text = str_replace( array( ' <!-- wpnl --> ', '<!-- wpnl -->' ), "\n", $text );
    }

    return $text;
}