How to extract all the strings before the second colon in r?

I have a time data looks like this:
2024-01-26 19:24:40.0

I want to extract everything before the second colon like this: 2024-01-26 19:24

I tried the code as below:

a="2024-01-26 19:24:40.0"
b=sub("(^([^:]+:){2}).*$", "\\1",a)

But the result was like this: 2024-01-26 19:24:

So how to write the code in the right way?

>Solution :

Option 1

> sub("(.*\\d+:\\d+):.*", "\\1", a)
[1] "2024-01-26 19:24"

Option 2

> sub(":\\d+\\.\\d+", "", a)
[1] "2024-01-26 19:24"