Regex to find a number in a string

Perhaps

\d{4,5}

?

\d{4,5} will also find strings with 6 digit numbers in - I don't know whether that's a problem or not. You might want to do something like this:

([^\d]+|^)\d{4,5}[^\d]

The foolproof one to avoid longer numbers would be:

([^\d]|^)\d{4,5}([^\d]|$)

I'm assuming you don't want to allow for a comma after the thousands digit? If you do then:

([^\d]|^)\d{1,2},\d{3}([^\d]|$)