Adding more inline comments and add a check for an empty pattern

tests/WP_SQLite_Translator_Tests.php

@@ -119,16 +119,17 @@ public static function regexpOperators() {
 	}
 
 	public function testRegexpReplace() {
+		/* Testing if an actual replacment works correctly */
 		$this->assertQuery(
 			"INSERT INTO _options (option_name, option_value) VALUES ('test-ignore', '1');"
 		);
 		$this->assertQuery(
 			"INSERT INTO _options (option_name, option_value) VALUES ('test-remove', '2');"
 		);
-
 		$this->assertQuery( "SELECT * FROM _options WHERE REGEXP_REPLACE(option_name, '(-ignore|-remove)', '') = 'test'" );
 		$this->assertCount( 2, $this->engine->get_query_results() );
 
+		/* If one of the required parameters is null, the return value is null, copying the MYSQL/MariaDB behavior */
 		$this->assertQuery( "SELECT REGEXP_REPLACE( null, 'a', 'x') as result" );
 		$results = $this->engine->get_query_results();
 		$this->assertEquals( null, $results[0]->result );
@@ -140,6 +141,11 @@ public function testRegexpReplace() {
 		$this->assertQuery( "SELECT REGEXP_REPLACE( 'abc', 'a', null) as result" );
 		$results = $this->engine->get_query_results();
 		$this->assertEquals( null, $results[0]->result );
+
+		/* Providing an empty pattern should produce an error - but we changed that to null to avoid breaking things */
+		$this->assertQuery( "SELECT REGEXP_REPLACE( 'abc', '', 'x') as result" );
+		$results = $this->engine->get_query_results();
+		$this->assertEquals( null, $results[0]->result );
 	}
 
 	public function testInsertDateNow() {

wp-includes/sqlite/class-wp-sqlite-pdo-user-defined-functions.php

@@ -524,6 +524,11 @@ public function regexp_replace( $field, $pattern, $replacement ) {
 			return null;
 		}
 
+		/* Return null if the pattern is empty - this changes MySQL/MariaDB behavior! */
+		if ( empty( $pattern ) ) {
+			return null;
+		}
+
 		if ( "\x00" === $pattern[0] ) {
 			$pattern = substr( $pattern, 1 );
 			$flags   = '';