Add support for double-quoted search results in WordPress

double-quoted-search.php

<?php
/**
 * Add support for double-quoted search results.
 *
 * Can handle multiple, double-quoted search results as well.
 */
add_action( 'pre_get_posts', function( $q ) {
	if ( ! $q->get( 's' ) ) {
		return;
	}

	$q->set( 's', wp_unslash( $q->get( 's' ) ) );

	// Remove breaklines. Same as WP_Query::parse_search().
	$q->set( 's', str_replace( array( "\r", "\n" ), '', $q->get( 's' ) ) );

	// Use same regex as WP_Query::parse_search().
	preg_match_all( '/".*?("|$)|((?<=[\t ",+])|^)[^\t ",+]+/', $q->get( 's' ), $matches );
	if ( empty( $matches[0] ) ) {
		return;
	}

	$match_quotations = [];

	// Save the current search so we can reset it later.
	$unmodified_search = $q->get( 's' );

	/*
	 * Remove double-quoted search terms from 's' search field.
	 *
	 * We will later append these terms to the end of the SQL query.
	 */
	foreach ( $matches[0] as $match ) {
		if ( 0 === strpos( $match, '"' ) && '"' === substr( $match, -1 ) ) {
			$q->set( 's', str_replace( $match, '', $q->get( 's' ) ) );

			// Save our quoted search for later.
			$match_quotations[] = trim( $match, '"' );
		}
	}

	$q->set( 's', trim( $q->get( 's' ) ) );

	// No double-quoted search terms, so bail.
	if ( empty( $match_quotations ) ) {
		return;
	}

	/*
	 * Append our quoted search terms to existing search.
	 *
	 * This is mostly a reinterpretation of parse_search() in WP_Query.
	 */
	$quote_filter = function( $retval, $q ) use ( $match_quotations, $unmodified_search ) {
		global $wpdb;

		$searchand = ' AND ';
		$like_op   = 'LIKE';
		$andor_op  = 'OR';

		foreach ( $match_quotations as $term ) {
			$like = '%' . $wpdb->esc_like( $term ) . '%';

			/*
			 * Only match against post title and post content.
			 *
			 * WP includes post_excerpt in search, but that isn't useful for us.
			 */
			$retval .= $wpdb->prepare( "{$searchand}(({$wpdb->posts}.post_title $like_op %s) $andor_op ({$wpdb->posts}.post_content $like_op %s))", $like, $like );
		}

		// Reset 's' field to the original search term.
		$q->set( 's', $unmodified_search );

		return $retval;
	};

	// Add our filter.
	add_filter( 'posts_search', $quote_filter, 10, 2 );

	// Remove our filter afterwards.
	add_action( 'posts_selection', function() use ( $quote_filter ) {
		remove_filter( 'posts_search', $quote_filter );
	} );
} );