$ perl -e '"cl_fl_id1_1" =~ /(?:(?:[a-z]{1,}_){2})([a-z0-9]{1,})/ && print "$1\n"'

id1

$

MariaDB [test]> select regexp_substr('cl_fl_id1_1', '(?:(?:[a-z]{1,}_){2})([a-z0-9]{1,})') matched;

+-----------+

| matched   |

+-----------+

| cl_fl_id1 |

+-----------+

1 row in set (0.00 sec)

MariaDB [test]> 

$ mysqld --print-defaults

mysqld would have been started with the following arguments:

--user=mysql --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/run/mysqld/mysqld.sock --port=3306 --basedir=/usr --datadir=/var/lib/mysql --tmpdir=/tmp --lc_messages_dir=/usr/share/mysql --lc_messages=en_US --skip-external-locking --bind-address=0.0.0.0 --max_connections=100 --connect_timeout=5 --wait_timeout=600 --max_allowed_packet=256M --thread_cache_size=128 --sort_buffer_size=4M --bulk_insert_buffer_size=16M --tmp_table_size=32M --max_heap_table_size=32M --myisam_recover_options=BACKUP --key_buffer_size=128M --table_open_cache=400 --myisam_sort_buffer_size=512M --concurrent_insert=2 --read_buffer_size=2M --read_rnd_buffer_size=1M --query_cache_limit=128K --query_cache_size=64M --log_warnings=2 --slow_query_log_file=/var/log/mysql/mariadb-slow.log --long_query_time=10 --log_slow_verbosity=query_plan --sql_mode=STRICT_ALL_TABLES,ONLY_FULL_GROUP_BY --default_storage_engine=InnoDB --default_tmp_storage_engine=Aria --innodb_buffer_pool_size=256M --innodb_log_buffer_size=8M --innodb_file_per_table=1 --innodb_open_files=400 --innodb_io_capacity=400 --innodb_flush_method=O_DIRECT --innodb_flush_log_at_trx_commit=1 --sync_binlog=1 --histogram_size=128 --use_stat_tables=preferably

$

select regexp_substr('cl_fl_id1_1', '(?:(?:[a-z]{1,}_){2})([a-z0-9]{1,})') matched;