CI 3: query builder - php error - preg_match(): Compilation failed

Code:
//select

$this->db->select('pc_workload_execution.id, pc_workload_execution.pc_study_class_executors_id as executor, pc_studies.id as studies_id, pc_studies.study_finance_code, pc_studies.title_hr as study_title, pc_studies.study_level_execution_type, hr_employee.id as hr_employee_id, hr_employee.firstname, hr_employee.lastname, hr_employee.old_sys_id, hr_employee.hr_employment_status_id, val_pc_katedre.print_title, val_pc_katedre.katedra_finance_code, val_pc_katedre.id as katedra_id, pc_classes_workload_semester.class_type_id as class_type, val_class_type.code_hr as class_type_code, pc_classes_workload_semester.finance_coef as finance_coef');



//join

 $this->db->join('pc_study_class_executors', 'pc_study_class_executors.id = pc_workload_execution.pc_study_class_executors_id');

 $this->db->join('pc_classes_workload_semester', 'pc_classes_workload_semester.id = pc_study_class_executors.pc_study_class_workload_semester_id');

 $this->db->join('pc_study_mandatory_classes', 'pc_study_mandatory_classes.id = pc_classes_workload_semester.pc_study_mandatory_classes_id');

 $this->db->join('pc_studies', 'pc_studies.id = pc_study_mandatory_classes.pc_studies_id');

 $this->db->join('pc_classes', 'pc_classes.id = pc_study_mandatory_classes.pc_classes_id');

 $this->db->join('val_pc_katedre', 'val_pc_katedre.id = pc_classes.katedra_holder_id');

 $this->db->join('val_class_type', 'val_class_type.id = pc_classes_workload_semester.class_type_id');

 $this->db->join('hr_employee', 'hr_employee.id = pc_study_class_executors.hr_employee_id');

 

//where

 $this->db->where('pc_studies.id IN ('.$studies.')');

 $this->db->where('pc_classes.katedra_holder_id IN ('.$katedre.')');

 $this->db->where('pc_workload_execution.id NOT IN ('.$ignore_m_executions.')');

 // $this->db->where('hr_employment_status_id IN ('.$hr_employment_statuses.')');



//order by

 $this->db->order_by('studies_id', 'asc');



 $data_mandatory = $this->db->get('pc_workload_execution');