Hi,
I want to split middle 2 digits from a numeric var, which have 11 / 18 digits. For 11 digits, I want to keep 3,4 digits; for 18 digits, keep 9,10 digits.
I tried to tostring it and use substr, but it didn't work, read: IMAGE_NUM cannot be converted reversibly; no replace
Any possible way to do it? Many thanks!

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double IMAGE_NUM
13940240545
13940240558
13940240558
13961253786
13961253786
13961253786
13961253787
13961253797
13961253797
13961253798
201510149002872670
201510149002872670
201510149002872670
201510149002872670
201510149002872670
201510149002872670
end