escape_json plpgsql function for postgres versions not having json like 9.0 and 9.1 (9.2 and 9.3 have them). Source http://stackoverflow.com/q/4995945/74167

escape_json.sql

-- based on http://stackoverflow.com/q/4995945/74167
create or replace function escape_json(i_text text) returns text as $$
    declare
        idx integer;
        text_len integer;
        cur_char_unicode integer;
        rtn_value text := i_text;
    begin
        text_len = length(rtn_value);
        idx = 1;

        while (idx <= text_len) loop
            cur_char_unicode = ascii(substr(rtn_value, idx, 1));

            if cur_char_unicode > 255 then
                rtn_value = overlay(rtn_value placing (e'\\u' || lpad(upper(to_hex(cur_char_unicode)), 4, '0')) from idx for 1);
                idx = idx + 5;
                text_len = text_len + 5;
            else
                /* is the current character one of the following: " \ / bs ff nl cr tab */
                if cur_char_unicode in (34, 92, 47, 8, 12, 10, 13, 9) then
                    rtn_value = overlay(rtn_value placing (e'\\' || (
                        case cur_char_unicode
                            when 34 then '"'
                            when 92 then e'\\'
                            when 47 then '/'
                            when  8 then 'b'
                            when 12 then 'f'
                            when 10 then 'n'
                            when 13 then 'r'
                            when  9 then 't'
                        end)) from idx for 1);
                    idx = idx + 1;
                    text_len = text_len + 1;
                end if;
            end if;
            idx = idx + 1;
        end loop;
        return rtn_value;
    end;
    $$ language plpgsql;