I want to find the first and the last occurrences of a specific character inside a string. As an examp开发者_Python百科le, consider a string named "2010-####-3434", and suppose the character to be searched for is "#". The first occurrence of hash inside the string is at 6-th position, and the last occurrence is at 9-th position.

Well...

Select position('#' in '2010-####-3434');

will give you the first. If you want the last, just run that again with the reverse of your string. A pl/pgsql string reverse can be found here.

Select length('2010-####-3434') - position('#' in reverse_string('2010-####-3434')) + 1;

My example:

reverse(substr(reverse(newvalue),0,strpos(reverse(newvalue),',')))

1. Reverse all string
2. Substring string
3. Reverse result

In the case where char = '.', an escape is needed. So the function can be written:

CREATE OR REPLACE FUNCTION last_post(text,char) 
RETURNS integer LANGUAGE SQL AS $$  
select length($1)- length(regexp_replace($1, E'.*\\' || $2,''));  
$$;

9.5+ with array_positions

Using basic PostgreSQL array functions we call string_to_array(), and then feed that to array_positions() like this array_positions(string_to_array(str,null), c)

SELECT
  arrpos[array_lower(arrpos,1)] AS first,
  arrpos[array_upper(arrpos,1)] AS last
FROM ( VALUES
  ('2010-####-3434', '#')
) AS t(str,c)
CROSS JOIN LATERAL array_positions(string_to_array(str,null), c)
  AS arrpos;

I do not know how to do that, but the regular expression functions like regexp_matches, regexp_replace, and regexp_split_to_array may be an alternative route to solving your problem.

This pure SQL function will provide the last position of a char inside the string, counting from 1. It returns 0 if not found ... But (big disclaimer) it breaks if the character is some regex metacharacter ( .$^()[]*+ )

CREATE FUNCTION last_post(text,char) RETURNS integer AS $$ 
     select length($1)- length(regexp_replace($1, '.*' || $2,''));
$$ LANGUAGE SQL IMMUTABLE;

test=# select last_post('hi#-#-#byte','#');
 last_post
-----------
         7

test=# select last_post('hi#-#-#byte','a');
 last_post
-----------
         0

A more robust solution would involve pl/pgSQL, as rfusca's answer.

Another way to count last position is to slit string to array by delimeter equals to needed character and then substract length of characters for the last element from the length of whole string

CREATE FUNCTION last_pos(char, text) RETURNS INTEGER AS
$$
select length($2) - length(a.arr[array_length(a.arr,1)]) 
from (select string_to_array($2, $1) as arr) as a
$$ LANGUAGE SQL;

For the first position it is easier to use

select position('#' in '2010-####-3434');