Background information

Oracle supports the REGEXP_LIKE function, but PolarDB-O does not.

It is specifically used in Oracle as follows:
SQL> SELECT * FROM xmldemo WHERE REGEXP_LIKE (B, '^f([a-z]+)e$');
         A B
---------- --------------------
       20 firstline

New solution

Using POSIX regular expressions for rewriting ,similar to and '~' can support regular expression matching.
van=> SELECT * FROM xmldemo WHERE b SIMILAR to 'f([a-z]+)e';
 a  |     b 
    ----+-----------
 20 | firstline
(1 row)
 and
van=> SELECT * FROM xmldemo WHERE b ~ 'f([a-z]+)e'; 
a  |     b 
    ----+-----------
 20 | firstline
(1 row)

For more information, see https://www.postgresql.org/docs/11/functions-matching.html#FUNCTIONS-POSIX-REGEXP