Replace a string with another string from a list d

Question

Let's say I have this string: '2015/4/21 (Tuesday)'. I want to replace 'Tuesday' with another string, for example: 'cat'. The result should be: '2015/4/21 (cat)'.

But I also want it to be dynamic, if it's 'Tuesday', then 'cat'. If it's 'Monday', then it's dog, etc.

How do I do this in PostgreSQL 8.4?

There is a similar post: postgresql - replace all instances of a string within text field

But mine needs to replace something dynamic depending on the day and that post replaces a known value.

Answer 1

You should be able to do this with nested replace():

select replace(replace(str, 'Tuesday', 'cat'), 'Monday', 'dog')

If the value is not in the string, nothing happens.

Answer 2

For a couple of mutually exclusive replacements, nested replace statements are the simplest and fastest way. Just like @Gordon suggests.

But that does not scale well for more than a few replacements and there are pitfalls:

Substrings

It becomes ambiguous when strings can be substrings of each other. Consider these two expressions:

SELECT replace((replace('2015 (Monday)', 'day', 'snake'), 'Monday', 'dog')
     , replace((replace('2015 (Monday)', 'Monday', 'dog'), 'day', 'snake');

The result depends on the sequence of replacements. You have to define priorities. Typically you would replace longer strings first.

Chains

Then there is also the possibility that one replacement might create a match for the next:

SELECT replace((replace('2015 (Sunday)', 'Sun', 'Mon'), 'Monday', 'dog')
     , replace((replace('2015 (Sunday)', 'Monday', 'dog'), 'Sun', 'Mon');

Again, you have to define priorities.
Every replacement possibly influences the next. With more than a few replacements, this becomes murky and error prone quickly. Also very hard to maintain if replacements can change.

As I said, with just the days of the week, nested replace() statements are fine. That's not actually "dynamic". If weekdays were just to illustrate the problem and you actually have to deal with more cases or truly dynamic strings, I would consider a different approach. Find completely dynamic solutions in this related answer:

• How to replace all subsets of characters based on values of other tables in pl/pgsql?

Answer 3

Another option is a recursive function:

CREATE OR REPLACE FUNCTION replace_recursive(search text, from_to text[][])
RETURNS TEXT LANGUAGE plpgsql AS $$
BEGIN
    IF (array_length(from_to,1) > 1) THEN
        RETURN replace_recursive(
            replace(search, from_to[1][1], from_to[1][2]),
            from_to[2:array_upper(from_to,1)]
        );
    ELSE
        RETURN replace(search, from_to[1][1], from_to[1][2]);
    END IF;
END;$$;

Result:

select replace_recursive('from1, from2, from3', array[['from1', 'to1'], ['from2', 'to2']]);

┌───────────────────┐
│ replace_recursive │
├───────────────────┤
│ to1, to2, from3   │
└───────────────────┘