[ ] will match anything found between the the square bracket in one character space. eg N[ABC] will find NA, NB or NC but nothing else.
To look for RG2 but not RG20, RG21, etc use RG2[ ] so that the space will be matched.
To search for a range of postcodes starting with NK, NG, NP, ST, SG, SL, SP Put the postcode criteria on the right of the screen (Any of:), as in the following examples:
You can have as range as in [A-Z] You can have a NOT as in [^Q] which would mean anything but Q Therefore [A-Z^Q] means anything in the alphabet except Q