Escape function for regular expression or LIKE patterns

how about trying something like this, substituting var_name for my hard-coded 'John Bernard':

create table my_table(name text primary key);
insert into my_table(name) values ('John Bernard'), 
                                  ('John Bernard (1)'), 
                                  ('John Bernard (2)'), 
                                  ('John Bernard (3)');


select max(regexp_replace(substring(name, 13), ' |\(|\)', '', 'g')::integer+1) 
from my_table 
where substring(name, 1, 12)='John Bernard' 
      and substring(name, 13)~'^ \([1-9][0-9]*\)$';

 max
-----
   4
(1 row)

one caveat: I am assuming single-user access to the database while this process is running (and so are you in your approach). If that is not the case then the max(n)+1 approach will not be a good one.

To address the question at the top:

Assuming standard_conforming_strings = on, like it's default since Postgres 9.1.

Regular expression escape function

Let's start with a complete list of characters with special meaning in regular expression patterns:

!$()*+.:<=>?[\]^{|}-

Wrapped in a bracket expression most of them lose their special meaning - with a few exceptions:

• - needs to be first or last or it signifies a range of characters.
• ] and \ have to be escaped with \ (in the replacement, too).

After adding capturing parentheses for the back reference below we get this regexp pattern:

([!$()*+.:<=>?[\\\]^{|}-])

Using it, this function escapes all special characters with a backslash (\) - thereby removing the special meaning:

CREATE OR REPLACE FUNCTION f_regexp_escape(text)
  RETURNS text
  LANGUAGE sql IMMUTABLE STRICT PARALLEL SAFE AS
$func$
SELECT regexp_replace($1, '([!$()*+.:<=>?[\\\]^{|}-])', '\\\1', 'g')
$func$;

Add PARALLEL SAFE (because it is) in Postgres 10 or later to allow parallelism for queries using it.

Demo

SELECT f_regexp_escape('test(1) > Foo*');

Returns:

test\(1\) \> Foo\*

And while:

SELECT 'test(1) > Foo*' ~ 'test(1) > Foo*';

returns FALSE, which may come as a surprise to naive users,

SELECT 'test(1) > Foo*' ~ f_regexp_escape('test(1) > Foo*');

Returns TRUE as it should now.

LIKE escape function

For completeness, the pendant for LIKE patterns, where only three characters are special:

\%_

The manual:

The default escape character is the backslash but a different one can be selected by using the ESCAPE clause.

This function assumes the default:

CREATE OR REPLACE FUNCTION f_like_escape(text)
  RETURNS text
  LANGUAGE sql IMMUTABLE STRICT PARALLEL SAFE AS
$func$
SELECT replace(replace(replace($1
         , '\', '\\')  -- must come 1st
         , '%', '\%')
         , '_', '\_');
$func$;

We could use the more elegant regexp_replace() here, too, but for the few characters, a cascade of replace() functions is faster.

Again, PARALLEL SAFE in Postgres 10 or later.

Demo

SELECT f_like_escape('20% \ 50% low_prices');

Returns:

20\% \\ 50\% low\_prices