sort records in postgres using just a part of string

I have several records and one of my field is segment_number and it is as of:

PID_1
PID_2
PID_11
PID_14
PID_6

I want to sort all the records based on segment_number field but as its is a string and the number is at the end of the string I am not able to sort it in ascending order of numbers.

If I use ORDER BY segment_number,I get the order as this which is not what I want:

PID_1
PID_11
PID_14
PID_2
PID_6

What I want:

PID_1
PID_2
PID_6
PID_11
PID_14

Any leads will be appreciated.