Postgresql - How do I extract the first occurence of a substring in a string using a regular expression pattern?

Question

I am trying to extract a substring from a text column using a regular expression, but in some cases, there are multiple instances of that substring in the string.

In those cases, I am finding that the query does not return the first occurrence of the substring. Does anyone know what I am doing wrong?

For example:

If I have this data:

create table data1
(full_text text, name text);

insert into data1 (full_text)
values ('I 56, donkey, moon, I 92')

I am using

UPDATE data1
SET name = substring(full_text from '%#"I ([0-9]{1,3})#"%' for '#')

and I want to get 'I 56' not 'I 92'

Answer 1

You can use regexp_matches() instead:

update data1
  set full_text = (regexp_matches(full_text, 'I [0-9]{1,3}'))[1];

As no additional flag is passed, regexp_matches() only returns the first match - but it returns an array so you need to pick the first (and only) element from the result (that's the [1] part)

It is probably a good idea to limit the update to only rows that would match the regex in the first place:

update data1
  set full_text = (regexp_matches(full_text, 'I [0-9]{1,3}'))[1]
where full_text ~ 'I [0-9]{1,3}'