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

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}'

Answer 2

Try the following expression. It will return the first occurrence:
SUBSTRING(full_text, 'I [0-9]{1,3}')

Answer 3

You can use regexp_match() In PostgreSQL 10+
select regexp_match('I 56, donkey, moon, I 92', 'I [0-9]{1,3}');
Quote from documentation:
In most cases regexp_matches() should be used with the g flag, since
if you only want the first match, it's easier and more efficient to
use regexp_match(). However, regexp_match() only exists in PostgreSQL
version 10 and up. When working in older versions, a common trick is
to place a regexp_matches() call in a sub-select...